Jump to content
Holidays
Local
Radar
Snow?
Sign in to follow this  
DiagonalRedLine

The Model Dictionary

Recommended Posts

I haven't read all of this but it's a stirling effort to put this together. 

 

Some comments:

 

Retrogressing: actually means to move westwards (against the mean flow), not necessarily north-westwards.

 

Undercutting: can mean something different to what is mentioned here. Talking about in the vertical, having cold air undercut warm moist air. I would discourage use of "underneath" in your description because it isn't clear whether you mean "underneath" in the true sense (vertically) or "underneath" how it appears on a map - that is, to the south. 

 

Upper temperatures: My only real bugbear. I would strongly discourage use of the term "uppers" for 850hPa temperatures, although it's a little too late for this as it's become entrenched on NW. If you were to say "uppers" to a meteorologist (without any further clarification), he/she would automatically think you are talking about 500hPa or 250hPa, and most likely the geopotential height, not the temperature. After that, 500hPa temperatures or 250hPa temperatures. 850hPa temperatures? Not so likely! Uppers != 850hPa temperatures.

 

I don't know how NW forums corrupted the term because people used to just say "the 850s". Which doesn't clarify what field you're talking about, but at least gives you the height! "Uppers" is really not logical and is a step backwards. 

Edited by forecaster

Share this post


Link to post
Share on other sites

Thanks, that's handy to know. I see what you mean about the term 'uppers' and the good point you made about it not being that specific. Will change/modify some of those terms while I've got time. :)

(Edit: will sort it out later instead when I should have even more time to do it. Will have to do it as a separate post as no longer have option to edit my first post, sadly).

Edited by DiagonalRedLine

Share this post


Link to post
Share on other sites

I haven't read all of this but it's a stirling effort to put this together. 

 

Some comments:

 

Retrogressing: actually means to move westwards (against the mean flow), not necessarily north-westwards.

 

Undercutting: can mean something different to what is mentioned here. Talking about in the vertical, having cold air undercut warm moist air. I would discourage use of "underneath" in your description because it isn't clear whether you mean "underneath" in the true sense (vertically) or "underneath" how it appears on a map - that is, to the south. 

 

Upper temperatures: My only real bugbear. I would strongly discourage use of the term "uppers" for 850hPa temperatures, although it's a little too late for this as it's become entrenched on NW. If you were to say "uppers" to a meteorologist (without any further clarification), he/she would automatically think you are talking about 500hPa or 250hPa, and most likely the geopotential height, not the temperature. After that, 500hPa temperatures or 250hPa temperatures. 850hPa temperatures? Not so likely! Uppers != 850hPa temperatures.

 

I don't know how NW forums corrupted the term because people used to just say "the 850s". Which doesn't clarify what field you're talking about, but at least gives you the height! "Uppers" is really not logical and is a step backwards. 

 

got to agree with your comments especially so about 'uppers'.

Share this post


Link to post
Share on other sites

got to agree with your comments especially so about 'uppers'.

Yeah but the lay people on here understand what it means, so :-)

Share this post


Link to post
Share on other sites

Yeah but the lay people on here understand what it means, so :-)

 

Yes, so it should probably specify both its common usage on the forums so that newcomers can understand what is being said and its more precise usage with that being made clear, excatly how a dictionary would.

 

Excellent effort on the post though. Would have saved me much time when I was trying to sort out what people were talking about. For weeks I guessed what "FI" meant but couldn't work out what it stood for until someone wrote the full words. 

Share this post


Link to post
Share on other sites

Hi John/forecaster what would they refer the 850's as out of interest?

 

I would say FI begins where the large amount of scatter begins with the ensembles. We've seen large amount of scatter down to T72hrs and in extreme cases as little as 48hrs.

Edited by The PIT

Share this post


Link to post
Share on other sites

By the way I think the Jetstream is at around 30,000 ft not metres isn't it? ie 10,000 m.

Share this post


Link to post
Share on other sites

Jet stream is between 29500ft to 52500ft according to met and km to ft converter.

Share this post


Link to post
Share on other sites

For weeks I guessed what "FI" meant but couldn't work out what it stood for until someone wrote the full words. 

I've always thought it was an abbreviation for ( Forget It ).

I've seen some call it (Fantasy Island) more so in winter

when the charts are showing a very wintry outlook Posted Image

Edited by Dancerwithwings

Share this post


Link to post
Share on other sites

Not really surprising it crops more in winter as the atmosphere is more dynamic so more variation in the possible outcomes.

  • Like 1

Share this post


Link to post
Share on other sites

Hi John/forecaster what would they refer the 850's as out of interest?

 

I would say FI begins where the large amount of scatter begins with the ensembles. We've seen large amount of scatter down to T72hrs and in extreme cases as little as 48hrs.

uppers is not correct

850's is not

neither explain what anyone is talking about, yes it is accepted short hand on here but newcomers/visitors may be mystified what is being referred to. 850mb temperature is the long correct version.

 

re FI

who knows it is another creation of web weather folk

Share this post


Link to post
Share on other sites

Hi John/forecaster what would they refer the 850's as out of interest?

 

I would say FI begins where the large amount of scatter begins with the ensembles. We've seen large amount of scatter down to T72hrs and in extreme cases as little as 48hrs.

 

"850 temps/temperatures" is the only way I've heard them referenced. Wet bulb potential temperature at 850hPa is used more frequently though for airmass indications, and that would be called "850 theta w". 

Share this post


Link to post
Share on other sites

a site well worth using for definitions if Net Wx Guides do not show, and they do explain most things, is to go to 

http://weatherfaqs.org.uk/

 

copied from their site for the Polar jet which is what blows in our part of the world

The Polar Front Jet

As its name implies, this jet stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 deg N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet. 

  • Like 3

Share this post


Link to post
Share on other sites

The following terms have been updated and/or corrected in the Model Dictionary: 850 hPa temperatures, Retrogressing High, Undercutting and also added Johnholmes handy info regarding the 'Polar Jet'. Also made slight tweaks to the Fantasy Island description. The modified terms have '(Updated)' next to them, while new terms have '(New)' next to them.

 

Thanks for all the feedback so far. Posted Image

A

 

Amplification - When amplification occurs within the weather pattern. It is a bumpy looking pattern on the models' pressure charts where the general airflow becomes all wavy, usually thanks to pronounced ridges of High Pressure systems and notable *troughing of Low Pressure systems between the ridging High Pressure cells (*see 'Troughs' for more information). For example, you might get a long flat Westerly airflow over the UK that becomes more and more wavier as the pattern tries to amplify and High Pressure in the Atlantic tries to send some of its ridging Northwards to the West of us. Fairly potent periods of cool/cold North-Westerly winds can occur that spill down along the High Pressure system's Eastern side towards the UK. The amplification of the Jet Stream also helps along with a pressure pattern becoming all 'bumpy'. You can also get some portions of the pattern that's amplified and some portions that are flatter. Amplified patterns can, but not always, lead to Atlantic and/or Northern blocking further down a line, such as a High Pressure system trying to ridge in above us from the South.

 

An example of an amplified pattern:

Posted ImageECMWF amplified example.png

 

The black arrows shows the bumpiness of the pattern and the general airflow(s).

 

Atlantic Blocking - A High Pressure system that sets itself up either over us, to the West of us, or to East of us and stops the normal travel of the West to East progression of Atlantic Low Pressure systems/depressions. Effectively, the High Pressure system acts as a 'block' deflecting the Lows away from the UK.

 

Azores High - An area of High Pressure that hangs about to our South West in the Atlantic Ocean. It contributes to some of the weather in the UK where it sometimes extends some of its ridging towards us bringing warm or hot spells during the Summer.

 

 

B

 

Bartlett High - Cold and snow fans worst nightmare during the Winter. Mwha ha ha ha haa! It is an area of heights that sit stubbornly to our South-East. The High Pressure system can be very hard to budge. You can go weeks and weeks with winds coming from a mild South-Westerly direction with approaching Low Pressure systems from our West forced to track North-Eastwards. Spells of rain can occur fairly frequently as well, most especially to the North-West where the Low Pressure is more influential. Occasional wintry outbreaks of sleet and snow can happen (mostly for high-ground, but not necessarily so) via brief occurrences of winds swinging to a cool/cold Westerly or North-Westerly direction. The Bartlett High was named after the weather forecaster, Paul Bartlett.

 

The below chart shows a Bartlett High to our South and South-East, with a long draw of winds from the South-West. The Southern part of the UK would see the most mildest conditions in this setup, while much further North, it would be cooler:

Posted ImageBartlett High.gif

 

BI - Stands for British Isles. The shortened form of this is sometimes used in the model output thread.

 

Blocking - Where High Pressure systems set themselves up in places that interrupts the normal flow of weather patterns. Essentially, the High Pressure systems behave like a 'block'. (see 'Atlantic blocking' and 'Northern blocking' for examples).

 

 

C

 

Channel Low - Pretty much what the words refer to - A Low Pressure system over the Channel. In Winter, a channel Low often often brings spells of sleet and snow on its Northern side over the Southern UK as it engages with cold air to the East/North-East. A number of times, though, the very far South of the UK will just experience rain instead with milder air from the South being more influential.

 

Black circle showing a Channel Low.

Posted ImageChannel Low.png

 

CFS - Stands for 'Coupled Forecasting System'. It belongs to the same organization that produces the GFS model - NCEP. Generating charts up to 9 months ahead (yep, you heard that right), it is a long range model. But to use it effectively, the model should only really be used for trend spotting. It also produces anomaly charts up to several months ahead and this can also be useful for trend spotting. For example, if the anomaly charts have been continually producing a cold, blocked outlook for Christmas with higher than average heights to the North and lower than average heights to the South, then it could very well be the route we would head in. While charts for several months ahead will understandably change a huge amount, the CFS has been known to be close to perfect with some of its outlooks that have trended in the past.

 

However, if you were ever interested in long range weather forecasting, this site: http://www.weatherweb.net/wxwebchartscfshowtouse.php provides some useful tips on how to use charts like this effectively...

 

"1. Take a long view - step back from the charts and see how the weather is affecting large geographical areas, such as the whole of Europe.

2. Spot extremes - Look for any extremes of weather, heavy rain, high temperatures.

3. Visit daily - Take a look at the forecasts each day. This helps builds a picture of consistency; the more consistent the model from day to day, the more confidence you can have in it.

4. Don't take it literally - The CFS can only give guidance, not a forecast! Just because it says a particular day in 3 weeks may be warm, that doesn't mean that it will be, it should be taken that there is a chance of warmer weather during that period.

5. Look at each of the members and look for consistency between each.

One should exercise caution when using long range forecasting models such as the GFS. They should be used for guidance only, but the experience of using these is that they are better than nothing at all!"

 

D

 

E

 

ECMWF - Stands for 'European Centre for Medium-Range Weather Forecasts'. A European weather model. Some people use the term 'ECM' as its shortened version. It is regarded as one of the best performing models out their. As a general rule, the ECMWF is supposed to be better than the GFS at handling blocking patterns to the East, while the GFS is supposed to be better than the ECMWF (though not always) at handling developments to the North-West of us. And sometimes this can have a huge impact as to what happens over the UK. It is one reason why the GFS tends to pick out Northerly setups (with cool/cold air flowing down from the North) earlier than the European model.

 

This model updates twice a day and goes up to 240 hours (10 days) ahead. Made up of the 00Z and the 12Z runs.

 

Ensembles - What happens here is that an operational model run (main model run) is re-produced again at a lower resolution. This is then referred to as the 'control run'. Variations of this run are produced, each of which with different starting conditions. Along with the main operational model run, the results are then recorded as a line graph - each of these lines can be referred to as 'ensemble members'. They show various possibilities that could occur in regards to the weather. This is supposed to increase accuracy and help determine whether the particular operational model run is worth taking seriously or not. Most times you will find that the ensembles tell the bigger story of what will happen, than what the main model run shows. You can view various ensemble graphs, such as...

  • [*]
Air-Pressure [*]850 hPa Upper Temperatures [*]Precipitation amounts [*]Average Temperatures

You can also select what the ensembles show for your nearby region or town. How ever close the lines are on the graph reflects the confidence the model and its ensemble members has on a particular weather outlook. Underneath is an example of the Sea Level Pressure for London using GFS's ensembles:

 

Posted ImageLondon Ensemble memeber example.png

 

(GFS's ensemble set is also known as the 'GEFS ensembles')

 

The X axis shows the days ahead, while the Y axis shows the numbers relating to the type of weather, climate or atmospheric condition it is. The further along the graph you go, the more the ensemble members' confidence reduces, and thus the lines on the graph begin to scatter. The period at where the lines start to scatter depends on how well the operational model and its ensemble members are handling particular weather patterns. As you would likely expect, you can see that the lines for the first few days ahead are close together and reflects the confidence the GFS and its members has on the atmospheric conditions. The ensembles generally agree to a slight pressure drop around the 4th October (the dip between the 3rd October and the 5th October), with the pressure rising after that. Except for the odd ensemble member, the period at where the lines start to scatter becomes noticeable past the 7th October, with various solutions on offer. The red line (the ensemble mean) is the mean average of all of the ensemble members' outcomes. You can view the model's ensemble mean as an actual graphical model chart and see how it compares to the operational graphical model chart.

 

An example using the ECMWF's 00Z '500 hPa and Sea Level Pressure' ensemble mean chart (with the operational ECMWF 00Z '500 hPa and Sea Level Pressure' chart below it). Both of these showing the pressure conditions at 240 hours ahead:

Posted Imageecmwf ensemble mean.gif

Posted Imageecmwf operational run.gif

 

Since the ensemble mean is just an mean average of what the ensemble members show, and since the operational model run and its ensemble members' confidence drop the further ahead they predict, this will explain why the ensemble mean chart becomes less detailed the further ahead you're viewing it. It sort of works out a middle-ground-like solution of what its ensemble members show regarding what the weather conditions might be like several days ahead. The first few time-frames of the ensemble mean tends to be as detailed as the actual operational model run, and both will follow each other very closely, because of their high confidence in regards to the weather for the next few days. On the above example, however, the 00Z ECMWF ensemble mean expects that the High Pressure to the North-East of the UK on the operational model run will be more concentrated towards the East of the UK. Nor does the ensemble mean show the High Pressure to the East linking up with High Pressure over Greenland, like it does on the operational model run. This could mean that the operational ECMWF run has probably over-estimated the Northwards placement of the High Pressure to the North-East, and that it should be slightly further South/South-Eastwards. And if this was the case, then the operational run has probably also over-estimated how easily its High Pressure system to the North-East links up with the Greenland High Pressure system. For tend-spotting purposes, keeping an eye on future ensemble means and their ensembles from some of the models, can be handy to see where they keep heading in. Comparing a different operational model to an ensemble mean chart is probably a good idea as well, to see how close their outlook relates to each other. Should the operational outlook from a different model follow the ensemble mean closely all the way through, that could provide extra reassurance that the ensemble mean is heading in the right direction.

 

It can actually sometimes be much more reliable to follow the ensemble mean, rather than the operational model run, especially since the operational run only represents one line on an ensemble graph (the green one on the GFS's ensemble graph, for example), and can easily become an big 'outlier' in parts in relation to the ensemble mean, or even in relation to the other lines on the graph. In fact, you can see that the GFS operational run shows the biggest pressure drop around the 15th October on its ensemble graph, and doesn't follow the ensemble mean closely at all around that period, so its chances of coming off are almost virtually zero (unless I suppose another operational model run just happens to show very Low Pressure over the London area for 15th October, in which case its chances of coming off might be a little higher). This is why, however, if you're viewing the main model run, its worth checking the ensembles to see how well they follow the operational model run. If there is not much support for that run at all, then it's a good reason to be particularly cautious of what the operational model shows for a fair amount of days ahead. It's true that since a model's outlook several days ahead is liable to change a lot from one run to another that we shouldn't really trust what any run shows beyond a particular period. But it's still worth checking the main model run against the ensembles to see how confident the operational model run really could be and to see if any trends can be spotted. If the ensemble mean, let's say, continues to show a slight downward trend in pressure, with the general scatter of the other lines on the graph shaping out a slight downward trend, it could be a good sign to where we could head in the future in regards to the air-pressure.

 

For the sake of it, though, it is a good idea to also see what other main models runs are producing in relation to the one you're viewing, because even if a GFS operational model run may have strong ensemble support, it may not always mean too much if the other main operational models (such as the ECMWF, UKMO) completely disagree with GFS's weather scenarios. Either, it could mean the GFS and its ensembles are heading the wrong way, or that the other models are just not picking out the solutions the GFS and its ensembles show. What I will say, though, is even if their is notable large differences between the various models, which ever one has the strongest ensemble support is the one that is likely to lead the way in regards to its outlook. This could be particularly true if the GFS, on its last few runs, has continued to stick to a particular solution with its ensembles continuing to support it (John Holmes, though, does recommend comparing only the 00Z and 12Z GFS runs with each other, rather than all four of the GFS's daily runs when looking for trends). Also, if the other models have continued to offer solutions that vary widely from one run to another, then it's a good chance that they are not quite getting to grips with the weather outlook for the future. This doesn't always last, though, as models (and even the ensembles, especially if they're trending in the wrong way) do go through phrases where they won't perform as well as they could do.

 

 

F

 

(Updated) Fantasy Island - 'FI' for short. A term used to refer to a time-frame in the models where the accuracy in their predictions severely reduces. Usually around the 144 / 168 hour period and beyond in the models' outlooks - the sort of period where their outlooks become fantasy and dreamland-like, hence the term 'Fantasy Island'. However, if the models are having a tough time handling a particular pattern, people may use the 'FI' term for a time-frame in the models earlier than 144 hours. If, let's say, their are some fairly huge differences between the GFS, ECMWF and UKMO models as early as 96 hours, then this could count as Fantasy Island due to the uncertainty of the models' outcomes within that period. But even then, Fantasy Island could occur as early as 48 hours, if the models' ensembles show scatter on their graphs extremely early on.  It should be noted that some people use phrases such as "this model disagrees with another model" to refer to the differences in the evolutions the models show. Although a chart's FI outlook is unlikely to come off, occasionally it can happen. This is especially true if a model has picked up a trend regarding a particular pattern in its last few updates, and also if it has ensemble support.    

 

FAX Charts - White in colour, they are hand drawn by the Met Office, and are similar to the Pressure charts shown on the main models such as the GFS, ECMWF, UKMO and so on, but only without all the pretty colours in the background (although this is only true if you're viewing the '500 hPa Sea Level Pressure' charts from the GFS, UKMO etc, which have the rainbowy colours in it, rather than the just the 'Sea Level Pressure' charts - see example below).

 

('500 hPa and Sea Level Pressure' chart - has the colours of the rainbow in it)

Posted Imageh500 pressure chart.png

 

('Sea Level Pressure' chart without the 500 hPa's - doesn't have the colours of the rainbow in it)

Posted Imagenon-h500hpa slp chart.png

 

One advantage of FAX charts, over other kinds of charts, is because of the human input involved with this chart, they can be rather accurate - even up to 120 hours ahead. When the Met Office generate these charts, they look at the data from various models from the likes of their own UKMO models, the ECMWF, the JMA, the GFS and other data from the Met Office's super-computers. Due to some accuracy issues of the GFS model, the Met Office may not always include this model when basing their conclusions from the outlooks shown on various charts. As such, you will likely find that Met Office's FAX charts are very reminiscent of the pressure charts from the UKMO and/or ECMWF models.

 

FAX charts contain various symbols and features on them:

 

Posted Imagefax chart.png

  • [*]
Low Pressure systems are marked by an 'L', while High Pressure systems are marked by an 'H'. [*]The bold black line with the semi-circles on it represents a 'warm front'. [*]The bold black line with the pointy triangles on it represents a 'cold front'. [*]The bold black line with both the semi-circles and the triangles on it is an 'occluded front'. [*]The bold black line with no triangles or semi-circles on it represents an 'upper trough'

 

Which ever way the semi-circles and triangles are pointing in shows the direction of travel of the weather fronts.

 

The FAX charts are viewable up to 120 hours ahead.

 

 

G

 

GFS - Stands for 'Global Forecasting System'. An American Weather Model. Created by NCEP. It contains billions of different types of charts, such as the standard pressure charts, convection potential, precipitation and its types, temperature charts and many more. This model, however, does come with a few issues: Whilst no model is entirely perfect, the GFS has the tendency to be over eager with the progression Low Pressure systems make from West to East, hence the term "the GFS is being too progressive" may be used in the model output thread. It can also have the habit of over-deepening Low Pressure systems (although it is not necessarily the only model that does this). NCEP often make adjustments to the model to further improve its performance.

 

This model updates four times a day and goes up to 384 hours (16 days) ahead. It is made up of the runs: 00Z, 06Z, 12Z and 18Z.

 

GEM - Stands for 'Global Enviromental Multiscale' model. It can be a suitable model for medium range forecasting and it has fairly good performance overall. The ECMWF and GFS are its only real competitors in that they are the only two other models that go up to ten days ahead and beyond (bar the ECM 32 day model and CFS long range model).

 

The GEM updates twice a day and is made up of the following runs: 00Z and 12Z.

 

 

H

 

High Pressure - 'HP' for short. Areas of stronger than average pressure - usually around 1016 hPa/millibars or higher - that fill up the gaps in the atmosphere between and around Low Pressure systems. Broadly speaking, they are not quite as circular as Low Pressure systems (though not all Lows are round). They tend to be oval or sausage-shaped and will often bring settled, calm conditions. How sunny it will be depends on the orientation and positioning of a High Pressure system. As an example for the UK, if you want warm, sunny weather, ideally the High Pressure needs to hang about just to our East with Southerly/South-Easterly winds drawing up warm, dry air from the continent. Wind tends not to be an issue with High Pressure systems - the isobars are usually well spaced out. In fact, if you're stuck under the centre of a High Pressure system, their may not be any wind at all. Winds, will, however, circulate clockwise around High Pressure cells. 'Anticyclone' is an alternative word people use to refer to areas of High Pressure.

 

Black circle showing an area of High Pressure

Posted ImageHigh Pressure.jpg

 

 

I

 

J

 

(Updated) Jet Stream - Fairly narrow passages of fast high-level winds - around 29,500 to 52,500 ft above the Earth's surface (thanks to 'The PIT' for the correction Posted Image). You could almost think of it as various pathways that direct Low Pressure systems along it, and can be one of the main controllers of our weather. Depending on the positioning of Jet Stream and where passages of high-levels winds flow along, it can have an affect on where High Pressure systems migrate to, and how pronounced their ridging is. You often get one of the main streams of the Jet flowing Eastwards through from America, all the way through to the UK, or just to the North of it. In Summer, the normal position of the main stream of the Jet is to the North of us, although in some of the recent Summers, with the exception of Summer 2013, it has been behaving very strangely - the main stream of the Jet going much further South than usual steering constant Lows over, or to the South of us. Where you find both the cold air to the North and the hot air to the South meet is where the main stream of the Jet tends to flow along (or at least close to this boundary). When ever heights build to our North, this can force the main area of the Jet Stream to go South, although there has also been debates about whether the arctic sea-ice melt has affected the normal flow and positioning of the Jet Stream. When the Jet Stream becomes weak, or fragmented, with streams of the Jet split into pieces, Atlantic or Northern blocking may occur, just like what a very amplified pattern could cause.

 

Some Jet Stream examples underneath:

 

Example 1:

Posted ImageJet Stream examples.jpg

 

In this situation, you have one of the main areas of the Jet Stream flowing to the South of us attempting to bring a Low Pressure system to the South-West of us. The cyclone collides against the High Pressure to the North-East of us thanks to the slight West-South-West alignment of the Jet stream to the South-West of us. And this is kind of helping to steer the Low Pressure up against the High to our North-East. The Low does not quite slide underneath the High, despite one of the main areas of the Jet Stream being to the South/South-West of the UK.

 

Example 2:

Posted ImageJet Stream examples 2.jpg

 

In this example, you have an area of a stream of the Jet flowing on a South-West to North-East direction to the North-West of the UK. The stream of the Jet then curves back down Southwards through the North-Sea to the East of the UK. But the curvy Jet to the North of the UK is still far enough North to allow a High from the South or South-West to build some of its heights North-Eastwards over the UK. A South-West to North-East alignment of the Jet with it staying out to the North-West of the UK often allows High Pressure systems from the South or South-West to ridge North-Eastwards towards us.

 

One other aspect about the Jet Stream is that it can sometimes split up into two main streams - one going to our South, and another going to our North. In this type of setup, the GFS apparently has a slight bias of overdoing the power of the main stream of the Jet going to the North of us.

 

JMA - Stands for 'Japan Meteorological Agency'. One of the slightly less well-known models, although it runs a MSM (Meso-Scale Model - these are short range models) to provide small forecasts in Japan. The JMA model can be a useful asset to weather operations across the globe. It is highly regarded by the UKMO and can occasionally pick out certain weather patterns earlier than some of the other models. 

 

This model updates twice a day and goes up to 192 hours ahead. It is made up of the 00Z and 12Z runs.

 

 

K

 

L

 

Low Pressure - 'LP' for short. Often circular or elongated in shape, they are areas of lower than average pressure - usually around 1012 hPa/millibars and below - that fill up the gaps between and around High Pressure systems in the atmosphere. They bring with them unsettled and changeable weather such as frontal bands of rain or snow, sunshine and showers... and even storms! The weather can be windy too, but the tighter the spaces between the isobars (white or black lines on the charts that mark out the Low and High Pressure cells) the windier the weather is likely to be, although the Jet Stream can have an impact on this as well. The winds spin anti-clockwise around Low Pressure systems. Low Pressure systems can also be referred to as 'Cyclones'. Another aspect about Low Pressure systems is that they come in different intensities: Deep Lows have a central pressure area lower than 980 hPa, while moderate Lows are those with a centre of around 980 to 1000 hPa. Shallow Lows are those that are around 1000 hPa, or higher. Furthermore, Lows can have a habit of 'filling out' if the centre of the cyclone becomes less and less deeper.

 

Black circle showing an area of Low Pressure:

Posted ImageLow Pressure.jpg

 

 

M

 

Model Posers - People, especially ladies, who pose as 'models' in magazines, newspapers, TV shows etc. Avoid talking about this type of model in the Model Output thread as it is off-topic and the Netweather moderators might banish you from the forum. Forever! (although if you're lucky, Chinomaniac and his moderating crew might give you a second chance). ;-)

Posted ImageModel Posers.png

 

 

N

 

NAVGEM - Stands for 'Navy Global Environmental Model'. There's quite an interesting tale behind this model. It never used to be the NAVGEM model. The former model, NOGAPS, used to run all the time. That model became discontinued and the NAVGEM replaced it. And it's very unlikely the NOGAPS model will be brought back to life. Knowing the NOGAPS was gone brought tears to the eyes of some of those who liked that model so much. Posted Image

 

But it's not all bad as the NAVGEM has supposedly had improvements done to it to increase its performance potential. Just like the expired NOGAPS, this model updates four times a day. It is made up of the following runs: 00Z, 06Z, 12Z and 18Z.

 

NMM - A Meso-scale Model which runs at a very high resolution of 12 kilometers. According to Paul, "it is high enough resolution to pick out small more localised weather features that lower resolution models will miss." (http://forum.netweather.tv/blog/189/entry-3452-new-nmm-hi-res-forecast-model-for-the-uk/). This means it can pick out even more details than the GFS model and even the ECMWF model - both of which are run at lower resolutions. (You can find more about the models' resolutions here: http://forum.netweather.tv/topic/72326-model-resolution/ - credit goes to lorenzo).

 

The NMM is a short range model and it updates twice a day, but you need a Netweather Extra subscription to view it.

 

Northern Blocking - Cold and snow fans number one setup in Winter. It is where blocking occurs to the North of us, and/or over the Artic region via High Pressure building to the North of us. Ideally, over the UK, you do not want High Pressure system(s) to the North of us to be too far North, otherwise Lows out West may not track far enough South to deliver cold South-Easterly, Easterly, or North-Easterly winds on their Northern side. When the Northern blocking is significant, such as a strong Greenland High Pressure system linking up with a Scandinavian High Pressure system (and with it being very close by to the North of the UK), this forces the Jet Stream to send a fair amount of its energy to the South of us and we can become locked into a prolonged cold spell with winds constantly coming in from an Easterly direction.

 

An example during late November 2010, where a High Pressure system to our North provided the UK with some chilly Easterly/North-Easterly winds:

Posted ImageNorthern Blocking.gif

 

At times, shortwaves, troughs or small Lows, usually over the Norway (or Scandinavia) area, will try to separate the link up of a High Pressure system to the North-West of us and a High Pressure system to the North-East of us into two, like so:

Posted ImageTrough trying to poke Northwards.jpg

 

But in the case of the 1947 example above, it might get itself trapped inside the High Pressure systems...

Posted ImageTrapped Low.jpg

 

An aspect you have to be careful of is that the Northern blocking isn't very weak as Low Pressure systems from below and above could easily attack the High Pressure system(s) destroying the blocking to the North. And, as such, we could easily revert back to a milder pattern with Westerly or South-Westerly winds. And this would be especially true if the Jet Steam tries to power back through over us from the West without it no longer staying to the South of us. Northern blocking can also become eradicated, if one of the streams of the Jet powers back up to the North and sends most of its energy and associated Low Pressure systems over the top of the blocking High(s) to our North and forces the High Pressure system(s) to sink Southwards over us.

 

Northerly Toppler - A short lasting period where cool or cold winds flow down from the North. High Pressure from the West is forced to 'spill' over us cutting off the Northerly flow, with Westerly or South-Westerly winds returning from the West. But sometimes, approaching Lows from the West might just flatten out an amplified High Pressure system to our West (but with the Northerly airflow still destroyed). Northerly topplers often last for around 2 days; perhaps longer, if a High Pressure system to our West is particularly strong and can withstand the power of the West to East track of the Lows for a few extra days. Despite this, one reason why Northerly airflows and Northerly topplers don't last too long is not only because they 'topple' or get broken down easily, but because of a strong stream of the Jet is always keen to try and blast through a block to the West of us. The Jet Stream is an enemy of the Northerly! Northerlies via a Greenland High, though, will tend to last longer and the chances of them toppling so easily is much less likely.

 

The charts below show the process for one of the types of 'Northerly Topplers' that happen. Example used from the period ' 26th January to 29th January 1935':

 

The Northerly Toppler part 1 - The Northerly airflow beings to get established with Low Pressure to our North-East, and an amplified High Pressure to our West. (dark blue arrows showing cold air draining down from the North)

Posted ImageNortherly toppler part 1.jpg

 

The Northerly Toppler part 2 - The whole of the UK gets affected by a direct Northerly, although warmer South-Westerly winds (indicated by the red arrows with Low Pressure to the North) try to blow over the top of the High Pressure, and it slowly begins to get knocked Eastwards/South-Eastwards towards us. (the black arrows show it slowly beginning to get knocked over towards our direction)

Posted ImageNortherly Toppler part 2.jpg

 

The Northerly Toppler part 3 - The Northerly has almost toppled completely, with only the South-Eastern UK holding onto cold air, which is now coming from the North-East, rather than directly from the North. This is thanks to Low Pressure to the North-West of the UK knocking a portion of the High Pressure system over us, and the far North of the UK begins to get affected by milder Westerly or South-Westerly winds (shown by the red arrows).

Posted ImageNortherly Toppler Part 3.jpg

 

The Northerly Toppler part 4 - It's game over for the Northerly! And the cold North-Easterly airflow now becomes concentrated over France and also areas further South-East, East, and North-East of it. The Low Pressure system which knocked a portion of the High Pressure over us, slowly makes a getaway North-Eastwards, and the orientation of the airflow over the UK is now a less-cold slack North-Westerly airflow.

Posted ImageNortherly Toppler part 4.jpg

 

 

O

 

Outlier - Where either a model's operational run, control run, or a variation of its control run is out of place amongst the other runs on the ensemble graph. The line representing the 'outlier' on a ensemble graph is very spaced out in relation to other lines on the graph, and is a good indication that the ensemble member's outlook is very unlikely to materialize.

 

Posted ImageOulier.png

 

On the London ensemble graph example, the black circle shows how one of the lines looks out of place amongst the other lines, although it does start meeting up with other lines towards the end of the graph. As such, an ensemble member won't be an outlier all the way through - especially at the beginning, where the ensemble members' confidence are high with the tight clustering of the lines. You can get the odd time, where one or two of the 'outliers' on a ensemble graph may have detected a new trend, and the other ensemble members (including the operational and control run), may follow suit on future runs.

 

 

P

 

Performance Statistics - Various checks are carried out on the models. And their performance in regards to how they've handled pressure patterns, and other atmospheric conditions, are recorded in numbers and graphs. You can view how accurate the models have been handling various atmospheric weather patterns here (although this is where it starts to get rather technical: http://www.emc.ncep.noaa.gov/gmb/STATS/STATS.html / http://www.emc.ncep.noaa.gov/gmb/STATS/html/new_acz6.html)

 

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifVerfification Stats (500mb charts NH).jpg

 

As an example, the Performance Statistics graph posted above shows the anomaly corrections of how well the various models have handled the 500 hPa and air pressure patterns in the Northern hemisphere in the last 31 days. The numbers on the Y axis on the graph relates to how well the models have handled the pressure patterns six days ahead. '1' means that a model handled the pressure patterns entirely perfectly with no errors whatsoever, while '0.2' means that a model was way off in the handling of their pressure patterns. Currently, the ECMWF model - the one in red - is performing the strongest with it receiving the highest average score of 0.808. The UKMO model, however - the one in orange - is close behind with an average score of 0.787. You can also see that the red line on the graph (representing the ECMWF) has generally been a touch higher than all the other lines on the graph (representing the other models) in the last 31 days. But their have been a few occasional brief periods within the last 31 days where the UKMO (representing the orange line) has been higher than the ECMWF's line.

 

One other aspect about this type of graph is if it has the 'NHX' part in its name, it's for the Northern hemisphere, and if it has the 'SHX' part in its name, it's for the Southern hemisphere.

 

(New!) Polar Front Jet - As its name implies, this Jet Stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 degree N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet.

 

(Suggested by: Johnholmes)

 

Polar Maritime - 'Pm' for short. An airflow that comes from a cool/cold region from the North-West, usually with a fairly amplified High Pressure system just to the West or South-West of us. It's a type of North-Westerly airflow that is fairly common over the UK and can usually bring a sunshine and shower setup if the air is unstable enough. Cool or cold 850 hPa upper temperatures usually help with the instability. In Winter, if the cold Upper Temperatures coming down from the North-West haven't been moderated enough by the warmish Atlantic seas, some of the showers can fall as snow, most especially in the North-West of the UK and over high ground. But in Summer, when daylight heating from the Sun plays a big role, the temperatures over the land become warmer than that of the ocean temperatures, and you will find that the heaviest showers become concentrated more towards the Eastern side of Britain.

 

Underneath is an example of a Polar Maritime airmass with Low Pressure to the North bringing cold air down from the North-West through the UK. (dark blue arrows showing direction of the airflow's travel).

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifPM airmass.png

 

Polar Vortex - Blobs of dark blues and purples shown on the '500hPa Thickness and Sea Level Pressure' charts from ECMWF, GFS, UKMO, JMA, GEM, GME, NAVGEM etc. Some people will refer to the Polar Vortex as the 'PV'.  But it is essentially like an area of Low Pressure systems that, in our part of the world, dominate the Northern Hemisphere with High Pressure systems surrounding it. The Polar Vortex occupies the Stratosphere and upper Troposphere levels and contains lots of very cold air. During the Winter where it gets very strong, Low Pressure systems become more powerful and increase in size as the Polar Vortex adds 'fuel' to them. This is thanks to lots of very cold air getting mixed in with warm air further South resulting in some very powerful Lows. But as the affects of the Polar Vortex weakens significantly during Summer, Low Pressure systems tend to be much smaller in size and less powerful.

 

The '500 hPa and Sea Level Pressure' chart below [which I nicked from an old Model Output Discussion thread ;-)] shows the whole of the Northern Hemisphere and the areas of dark blues and purples, (the Polar Vortex), that dominates some of it.

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifPolar Vortex.png

 

If wanting a prolonged cold setup, you never want the Polar Vortex to be too invasive to the North-West of us, and especially over Greenland and Iceland, as it can make it hard for Northern blocking to form. The exceptions would be if a 'SSW', or other form of Stratospheric Warming, event occurs (and even then, it would have to be in the right place), or if we get some notable 'WAA' going up into Greenland or Iceland. (see 'Sudden Stratospheric Warming' and 'WAA' for more details). (There's probably likely to be other methods that I've admittedly not thought about, or know about, that could put the Polar Vortex under threat).

 

 

Q

 

R

 

(Updated) Retrogressing High / Retrogression - When a pressure feature, such as a High Pressure cell, moves Westwards against the mean flow. For example, a High over the UK can sometimes retrogress towards Greenland. Whether a High retrogresses or not depends on various aspects such the dominance of the Polar Vortex (during Winter) and how obtrusive the Jet Stream is; if a powerful stream of the Jet is flowing over the top of the High, then it could have a hard time retrogressing (unless the main area of the Jet Stream suddenly heads South, or if the pattern significantly amplifies). It could also depend on other factors such as whether a Low Pressure system can pump up some 'Warm Air Advection' Northwards on the High Pressure system's Western side and help encourage heights to build further Northwards. Although if heights build Northwards only, rather than back Westwards or North-Westwards, then it doesn't really count as Retrogression.

 

Ridge / Ridging - A curvy or pointy area of a High Pressure system. It can be quite elongated. A High Pressure system will, at times, send some of its ridging from one area to another. This idea of 'ridging' means a High Pressure system is trying to extend its heights to other places.

 

An example of a ridge of High Pressure:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifRidge of High Pressure.png

 

 

S

 

Shortwave - It is essentially like a little Low Pressure system that hasn't quite become a proper closed-off circular Low (although they can sometimes develop into circular Low Pressure systems depending on the conditions within the atmosphere). Shortwaves are marked by a small(ish) curvy-shaped wave on the Pressure charts and are often a part of big Low Pressure systems and Longwave Troughs. Cold air above can also often be a part of Shortwave features.

 

Some examples of shortwaves on the chart below (circled in black):

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifShortwave examples.png

 

Slack airflow - Where the isobars on the pressure charts are very spaced out. A slack airflow is associated by gentle winds, or a gentle breeze. Virtually, the weather is non-windy. 

 

Sudden Stratospheric Warming - 'SSW' may also be used. A sudden warming event occurs within the Stratosphere and, if notable enough, can separate the Polar Vortex (and its blue/purple blobs) into separate segments. Or even just make it less stronger in general. Where ever an 'SSW' event occurs, heights will likely to keenly rise in that area. As such, if you wanted a cold setup with a strong, sturdy, Greenland High, a notable 'SSW' event would be ideal around that area to help split the Polar Vortex in that area into two, with a segment of it getting knocked further West and with a segment of it being knocked Eastwards towards Scandinavia. In fact, getting a segment of the Polar Vortex to drop down to the North East of us can help promote Scandinavian Troughs and, if close enough, the UK can become influenced by a very cold Northerly flow from a Trough/Low over Scandinavia.

 

 

T

 

Trough - An area of curvy Low heights away from a main Low Pressure system's centre. Weather fronts and convective weather can be a part of troughs. Troughs are also a part of stretchy Low Pressure systems, and they can by quite pointy shaped. In the United Kingdom, a trough will be marked by a solid line on the Fax charts. There are, nevertheless, different kind of troughs such as Upper-Air Troughs, Surface Troughs, Shortwave Troughs and Longwave Troughs.

 

The black circle on this chart shows an example of a trough tracking towards the UK:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifTrough Example.jpg

 

T+??? - To refer to a time-frame within the models. For example, 'T+48' would refer to a model outcome 48 hours ahead.

 

 

U

 

UKMO - A weather model owned by the UK Met Office. It has comparable performance to ECMWF, although this model can only be viewed up to 144 hours ahead.

 

This model updates twice a day and is made up of the 00Z run and the 12Z run.

 

Undercutting - A situation where cold air undercuts warm air (thanks to 'forecaster' for the correction). Sometimes, on the charts, you may see approaching Low Pressure systems out West that try to track to the South under a High Pressure system to the North. This sort of scenario could be known as an 'undercutting low' with cold air from the North or North-East getting injected into the low and tries to mix out the warm air wrapped around it. Easterly winds can develop over the UK if the Low undercuts to the South of the island.

 

V

 

Vortex - Part of the Polar Vortex.

 

 

W

 

WAA - Warm Air Advection. This is where warm air from the South gets drawn up Northwards and moves into areas of cold air. In the Northern Hemisphere, the warm air would get drawn up on a Low Pressure system's Eastern side. When warm air gets 'advected' Northwards towards areas of cooler air, it can help strengthen High Pressure systems in that area, or even help new ones to form. This is why if you want a cold, blocked setup in Winter with High Pressure to our North or North-West, getting WAA heading up directly towards Greenland is seen as an encouraging sign.

 

Below is an example from January 1947 of WAA being pumped Northwards:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifWAA.jpg

 

The example above shows a long drawn Southerly airflow to the West of the UK bringing warm air up from the South (shown by the red arrows). Mild Upper Temperatures are also brought Northwards (see the example below):

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifWAA upper temperatures.gif

 

Prolonged Northern blocking soon followed after this event in 1947, and the rest of that Winter was very cold.

 

Weather Models - Charts, such as the ones you might be looking at right now, that display graphical weather forecasts and predictions, and use a large range of data. Their are a wide selection of weather models that all have varying purposes within our global Meteorological Weather operations. Satellites, radio equipment and weather balloons are used to detect the atmospheric and oceanic conditions on our planet, with special computers or super-computers working out all sorts of mathematical equations from the data that's gathered to then provide  forecasts on various models. There are many different types of models, and various different model viewers in which you can view the charts from. In the UK, you can view charts that cover a portion of the Northern hemisphere, or the whole of the Northern hemisphere. Viewing versions of models that show the whole of the Northern hemisphere is highly recommended if you want to get a broader picture of how the pressure conditions and atmospheric conditions are behaving. The way one pressure system behaves on one side of the world can have impacts as to what happens in other areas of the globe. This is why viewing versions of charts that cover a broad area can be useful.

 

Just one other thing to add is that you can also view historical charts dating to over 100 years back as well on sites such as Wetterzentrale (although I think Netweather has an archive somewhere, too)

 

WMA - Weather Model Addiction. Something some weather site member's are prone to catching. The addictions may include frequent model checking, frequent model analyzing and frequent viewing of the weather model threads across the World Wide Web.

 

 

X

 

Y

 

Z

 

Zonal Setup - The type of pattern that can bring about a North and South divide. On the whole, the Southern half of the UK remains settled with high pressure influential to the South, while the Northern part of the UK remains more cooler and unsettled with Low Pressure systems and Longwave troughs being more influential in those areas. Hence you begin to see why this is called a 'Zonal Setup'. Zonal patterns usually bring mild, or average conditions with a fairly long draw of winds from the West or South-West. However, not all Zonal setups bring mild or average temperatures. If winds come down from the North-West with High Pressure influential to the South-West of the UK and Low Pressure influential to the North/North-East, a cold Zonal setup can arise. Snow is also possible (if occurring in Winter), especially to the North and on high ground. 

 

 

(Updated) 850 hPa Temperatures - Some people refer to these as 'Upper Temperatures' or just 'Uppers' in the model output thread, although technically this is not correct. According to 'forecaster', "if you were to say "uppers" to a meteorologist (without any further clarification), he/she would automatically think you are talking about 500 hPa or 250 hPa, and most likely the geographical height, not the temperature." If you're think about using the term 'Uppers' in the model thread, don't do it no more.  http://f1.nwstatic.co.uk/forum//public/style_emoticons/default/blum.gif

 

The 850 hPa temperatures refer to the temperatures high up in the atmosphere (around 1,500 meters from the Earth's surface) and can be one of the factors that determine what the temperatures may be like at the surface. The warmer the 850 hPa temperatures are, the warmer the surface temperatures are likely to be. But this won't always necessarily be the case, as cloud, mist, or heavy rain can help to suppress the temperatures somewhat.

 

Likewise, the colder the 850 hPa/mb Temperatures are, the colder the surface temperatures are likely to be, although again, certain weather scenarios can have an impact on this. In a sunshine and shower setup, you really need the 850 hPa temperatures to be around -7*C or colder for snow to reach low levels. However, even 850 hPa temperatures around -5*C can suffice if the shower is very heavy (encouraging Evapourative Cooling), and if surface temperatures near the ground are particularly cold with no real layer of warm air above. The Dam-line (the 500 - 1000 hPa Thickness) needs to be around 527 or lower, as well, for showers to fall as sleet or snow. But again, like with the 850 hPa temperatures, you could get away with the 500 - 1000 hPa thickness being a little higher if the showers are extremely heavy.

 

For frontal bands of precipitation, you can get away with the 850 hPa temperatures being as warm as 0*C and snow can still fall to low levels. This, nonetheless, is mostly true in situations where an Atlantic front is forced to bang up against High Pressure to the North (with Low Pressure to our South). The front then slowly pivots and gets pulled Westwards/North-Westwards through the UK via cold Easterly or South-Easterly winds.

 

An example of an '850 hPa and Sea Level Pressure' chart below.

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifUpper Temperatures.png

 

The greens, blues and dark blues represent 850 hPa Temperatures around 0*C or colder, while the yellows, oranges and reds represent 850 hPa temperatures around 0*C or warmer. Helpfully, the 850 hPa Temperature gradients are marked by black lines with numbers over the top of the lines to show how warm or cold the 850 hPas are expected to be in that area. Over the far North of Scotland, for example, the 0*C 850 hPa Temperature line crosses a part of it, with the -5*C 850 hPa Temperature line crossing a part of Iceland further North-North-Westwards. You can also refer to the model's 850 hPa Temperature colour-coding line to the right of the chart to work out how warm or cold the 850 hPa temperatures are over your area. (However, if you have a Netweather Extra subscription, or are thinking about getting one, you can view the 850 hPa temperatures just for the UK only).

 

 

------------------------------------

 

(Updated) Weather FAQS: (Website recommendation by Johnholmes: http://weatherfaqs.org.uk/)
 
Edited by DiagonalRedLine

Share this post


Link to post
Share on other sites

Ramp: to greatley exaggerate the position of a scandi high at T+360 hrs

Share this post


Link to post
Share on other sites

One other thing perhaps worth pointing out is that "PV" is more frequently used to refer to "Potential Vorticity" (outside NW) whereas here it is of course used for the "Polar Vortex". Might prevent confusion for people hunting around the web. Especially given that PV is often used to look at the PV....

Share this post


Link to post
Share on other sites

'Polar Vortex' and 'Zonal Setup' terms: Updated

 

New terms added:

  • [*]Ramp [*]Bin / Binned

 

(all alterations included in the updated Model Dictionary underneath in the grey quote box). Posted Image

A

 

Amplification - When amplification occurs within the weather pattern. It is a bumpy looking pattern on the models' pressure charts where the general airflow becomes all wavy, usually thanks to pronounced ridges of High Pressure systems and notable *troughing of Low Pressure systems between the ridging High Pressure cells (*see 'Troughs' for more information). For example, you might get a long flat Westerly airflow over the UK that becomes more and more wavier as the pattern tries to amplify and High Pressure in the Atlantic tries to send some of its ridging Northwards to the West of us. Fairly potent periods of cool/cold North-Westerly winds can occur that spill down along the High Pressure system's Eastern side towards the UK. The amplification of the Jet Stream also helps along with a pressure pattern becoming all 'bumpy'. You can also get some portions of the pattern that's amplified and some portions that are flatter. Amplified patterns can, but not always, lead to Atlantic and/or Northern blocking further down a line, such as a High Pressure system trying to ridge in above us from the South.

 

An example of an amplified pattern:

Posted ImageECMWF amplified example.png

 

The black arrows shows the bumpiness of the pattern and the general airflow(s).

 

Atlantic Blocking - A High Pressure system that sets itself up either over us, to the West of us, or to East of us and stops the normal travel of the West to East progression of Atlantic Low Pressure systems/depressions. Effectively, the High Pressure system acts as a 'block' deflecting the Lows away from the UK.

 

Azores High - An area of High Pressure that hangs about to our South West in the Atlantic Ocean. It contributes to some of the weather in the UK where it sometimes extends some of its ridging towards us bringing warm or hot spells during the Summer.

 

 

B

 

Bartlett High - Cold and snow fans worst nightmare during the Winter. Mwha ha ha ha haa! It is an area of heights that sit stubbornly to our South-East. The High Pressure system can be very hard to budge. You can go weeks and weeks with winds coming from a mild South-Westerly direction with approaching Low Pressure systems from our West forced to track North-Eastwards. Spells of rain can occur fairly frequently as well, most especially to the North-West where the Low Pressure is more influential. Occasional wintry outbreaks of sleet and snow can happen (mostly for high-ground, but not necessarily so) via brief occurrences of winds swinging to a cool/cold Westerly or North-Westerly direction. The Bartlett High was named after the weather forecaster, Paul Bartlett.

 

The below chart shows a Bartlett High to our South and South-East, with a long draw of winds from the South-West. The Southern part of the UK would see the most mildest conditions in this setup, while much further North, it would be cooler:

Posted ImageBartlett High.gif

 

BI - Stands for British Isles. The shortened form of this is sometimes used in the model output thread.

 

(New!) Bin / Binned - If a particular model's outlook does not look right at all, and if it doesn't have support from any other model whatsoever, some people might 'bin' the outlook - essentially the person(s) will discard it. Most times 'binning' a chart's outlook is a good idea if it really is very unlikely to happen, although some people can have a habit of binning a models' outlook prematurely, and end up getting 'egg on their faces'.

 

post-10703-0-44154200-1381267657_thumb.p

 

Blocking - Where High Pressure systems set themselves up in places that interrupts the normal flow of weather patterns. Essentially, the High Pressure systems behave like a 'block'. (see 'Atlantic blocking' and 'Northern blocking' for examples).

 

 

C

 

Channel Low - Pretty much what the words refer to - A Low Pressure system over the Channel. In Winter, a channel Low often often brings spells of sleet and snow on its Northern side over the Southern UK as it engages with cold air to the East/North-East. A number of times, though, the very far South of the UK will just experience rain instead with milder air from the South being more influential.

 

Black circle showing a Channel Low.

Posted ImageChannel Low.png

 

CFS - Stands for 'Coupled Forecasting System'. It belongs to the same organization that produces the GFS model - NCEP. Generating charts up to 9 months ahead (yep, you heard that right), it is a long range model. But to use it effectively, the model should only really be used for trend spotting. It also produces anomaly charts up to several months ahead and this can also be useful for trend spotting. For example, if the anomaly charts have been continually producing a cold, blocked outlook for Christmas with higher than average heights to the North and lower than average heights to the South, then it could very well be the route we would head in. While charts for several months ahead will understandably change a huge amount, the CFS has been known to be close to perfect with some of its outlooks that have trended in the past.

 

However, if you were ever interested in long range weather forecasting, this site: http://www.weatherweb.net/wxwebchartscfshowtouse.php provides some useful tips on how to use charts like this effectively...

 

"1. Take a long view - step back from the charts and see how the weather is affecting large geographical areas, such as the whole of Europe.

2. Spot extremes - Look for any extremes of weather, heavy rain, high temperatures.

3. Visit daily - Take a look at the forecasts each day. This helps builds a picture of consistency; the more consistent the model from day to day, the more confidence you can have in it.

4. Don't take it literally - The CFS can only give guidance, not a forecast! Just because it says a particular day in 3 weeks may be warm, that doesn't mean that it will be, it should be taken that there is a chance of warmer weather during that period.

5. Look at each of the members and look for consistency between each.

One should exercise caution when using long range forecasting models such as the GFS. They should be used for guidance only, but the experience of using these is that they are better than nothing at all!"

 

 

D

 

E

 

ECMWF - Stands for 'European Centre for Medium-Range Weather Forecasts'. A European weather model. Some people use the term 'ECM' as its shortened version. It is regarded as one of the best performing models out their. As a general rule, the ECMWF is supposed to be better than the GFS at handling blocking patterns to the East, while the GFS is supposed to be better than the ECMWF (though not always) at handling developments to the North-West of us. And sometimes this can have a huge impact as to what happens over the UK. It is one reason why the GFS tends to pick out Northerly setups (with cool/cold air flowing down from the North) earlier than the European model.

 

This model updates twice a day and goes up to 240 hours (10 days) ahead. Made up of the 00Z and the 12Z runs.

 

Ensembles - What happens here is that an operational model run (main model run) is re-produced again at a lower resolution. This is then referred to as the 'control run'. Variations of this run are produced, each of which with different starting conditions. Along with the main operational model run, the results are then recorded as a line graph - each of these lines can be referred to as 'ensemble members'. They show various possibilities that could occur in regards to the weather. This is supposed to increase accuracy and help determine whether the particular operational model run is worth taking seriously or not. Most times you will find that the ensembles tell the bigger story of what will happen, than what the main model run shows. You can view various ensemble graphs, such as...

  • [*]
Air-Pressure [*]850 hPa Upper Temperatures [*]Precipitation amounts [*]Average Temperatures

You can also select what the ensembles show for your nearby region or town. How ever close the lines are on the graph reflects the confidence the model and its ensemble members has on a particular weather outlook. Underneath is an example of the Sea Level Pressure for London using GFS's ensembles:

 

Posted ImageLondon Ensemble memeber example.png

 

(GFS's ensemble set is also known as the 'GEFS ensembles')

 

The X axis shows the days ahead, while the Y axis shows the numbers relating to the type of weather, climate or atmospheric condition it is. The further along the graph you go, the more the ensemble members' confidence reduces, and thus the lines on the graph begin to scatter. The period at where the lines start to scatter depends on how well the operational model and its ensemble members are handling particular weather patterns. As you would likely expect, you can see that the lines for the first few days ahead are close together and reflects the confidence the GFS and its members has on the atmospheric conditions. The ensembles generally agree to a slight pressure drop around the 4th October (the dip between the 3rd October and the 5th October), with the pressure rising after that. Except for the odd ensemble member, the period at where the lines start to scatter becomes noticeable past the 7th October, with various solutions on offer. The red line (the ensemble mean) is the mean average of all of the ensemble members' outcomes. You can view the model's ensemble mean as an actual graphical model chart and see how it compares to the operational graphical model chart.

 

An example using the ECMWF's 00Z '500 hPa and Sea Level Pressure' ensemble mean chart (with the operational ECMWF 00Z '500 hPa and Sea Level Pressure' chart below it). Both of these showing the pressure conditions at 240 hours ahead:

Posted Imageecmwf ensemble mean.gif

Posted Imageecmwf operational run.gif

 

Since the ensemble mean is just an mean average of what the ensemble members show, and since the operational model run and its ensemble members' confidence drop the further ahead they predict, this will explain why the ensemble mean chart becomes less detailed the further ahead you're viewing it. It sort of works out a middle-ground-like solution of what its ensemble members show regarding what the weather conditions might be like several days ahead. The first few time-frames of the ensemble mean tends to be as detailed as the actual operational model run, and both will follow each other very closely, because of their high confidence in regards to the weather for the next few days. On the above example, however, the 00Z ECMWF ensemble mean expects that the High Pressure to the North-East of the UK on the operational model run will be more concentrated towards the East of the UK. Nor does the ensemble mean show the High Pressure to the East linking up with High Pressure over Greenland, like it does on the operational model run. This could mean that the operational ECMWF run has probably over-estimated the Northwards placement of the High Pressure to the North-East, and that it should be slightly further South/South-Eastwards. And if this was the case, then the operational run has probably also over-estimated how easily its High Pressure system to the North-East links up with the Greenland High Pressure system. For tend-spotting purposes, keeping an eye on future ensemble means and their ensembles from some of the models, can be handy to see where they keep heading in. Comparing a different operational model to an ensemble mean chart is probably a good idea as well, to see how close their outlook relates to each other. Should the operational outlook from a different model follow the ensemble mean closely all the way through, that could provide extra reassurance that the ensemble mean is heading in the right direction.

 

It can actually sometimes be much more reliable to follow the ensemble mean, rather than the operational model run, especially since the operational run only represents one line on an ensemble graph (the green one on the GFS's ensemble graph, for example), and can easily become an big 'outlier' in parts in relation to the ensemble mean, or even in relation to the other lines on the graph. In fact, you can see that the GFS operational run shows the biggest pressure drop around the 15th October on its ensemble graph, and doesn't follow the ensemble mean closely at all around that period, so its chances of coming off are almost virtually zero (unless I suppose another operational model run just happens to show very Low Pressure over the London area for 15th October, in which case its chances of coming off might be a little higher). This is why, however, if you're viewing the main model run, its worth checking the ensembles to see how well they follow the operational model run. If there is not much support for that run at all, then it's a good reason to be particularly cautious of what the operational model shows for a fair amount of days ahead. It's true that since a model's outlook several days ahead is liable to change a lot from one run to another that we shouldn't really trust what any run shows beyond a particular period. But it's still worth checking the main model run against the ensembles to see how confident the operational model run really could be and to see if any trends can be spotted. If the ensemble mean, let's say, continues to show a slight downward trend in pressure, with the general scatter of the other lines on the graph shaping out a slight downward trend, it could be a good sign to where we could head in the future in regards to the air-pressure.

 

For the sake of it, though, it is a good idea to also see what other main models runs are producing in relation to the one you're viewing, because even if a GFS operational model run may have strong ensemble support, it may not always mean too much if the other main operational models (such as the ECMWF, UKMO) completely disagree with GFS's weather scenarios. Either, it could mean the GFS and its ensembles are heading the wrong way, or that the other models are just not picking out the solutions the GFS and its ensembles show. What I will say, though, is even if their is notable large differences between the various models, which ever one has the strongest ensemble support is the one that is likely to lead the way in regards to its outlook. This could be particularly true if the GFS, on its last few runs, has continued to stick to a particular solution with its ensembles continuing to support it (John Holmes, though, does recommend comparing only the 00Z and 12Z GFS runs with each other, rather than all four of the GFS's daily runs when looking for trends). Also, if the other models have continued to offer solutions that vary widely from one run to another, then it's a good chance that they are not quite getting to grips with the weather outlook for the future. This doesn't always last, though, as models (and even the ensembles, especially if they're trending in the wrong way) do go through phrases where they won't perform as well as they could do.

 

 

F

 

(Updated) Fantasy Island - 'FI' for short. A term used to refer to a time-frame in the models where the accuracy in their predictions severely reduces. Usually around the 144 / 168 hour period and beyond in the models' outlooks - the sort of period where their outlooks become fantasy and dreamland-like, hence the term 'Fantasy Island'. However, if the models are having a tough time handling a particular pattern, people may use the 'FI' term for a time-frame in the models earlier than 144 hours. If, let's say, their are some fairly huge differences between the GFS, ECMWF and UKMO models as early as 96 hours, then this could count as Fantasy Island due to the uncertainty of the models' outcomes within that period. But even then, Fantasy Island could occur as early as 48 hours, if the models' ensembles show scatter on their graphs extremely early on.  It should be noted that some people use phrases such as "this model disagrees with another model" to refer to the differences in the evolutions the models show. Although a chart's FI outlook is unlikely to come off, occasionally it can happen. This is especially true if a model has picked up a trend regarding a particular pattern in its last few updates, and also if it has ensemble support.    

   

FAX Charts - White in colour, they are hand drawn by the Met Office, and are similar to the Pressure charts shown on the main models such as the GFS, ECMWF, UKMO and so on, but only without all the pretty colours in the background (although this is only true if you're viewing the '500 hPa Sea Level Pressure' charts from the GFS, UKMO etc, which have the rainbowy colours in it, rather than the just the 'Sea Level Pressure' charts - see example below).

 

('500 hPa and Sea Level Pressure' chart - has the colours of the rainbow in it)

Posted Imageh500 pressure chart.png

 

('Sea Level Pressure' chart without the 500 hPa's - doesn't have the colours of the rainbow in it)

Posted Imagenon-h500hpa slp chart.png

 

One advantage of FAX charts, over other kinds of charts, is because of the human input involved with this chart, they can be rather accurate - even up to 120 hours ahead. When the Met Office generate these charts, they look at the data from various models from the likes of their own UKMO models, the ECMWF, the JMA, the GFS and other data from the Met Office's super-computers. Due to some accuracy issues of the GFS model, the Met Office may not always include this model when basing their conclusions from the outlooks shown on various charts. As such, you will likely find that Met Office's FAX charts are very reminiscent of the pressure charts from the UKMO and/or ECMWF models.

 

FAX charts contain various symbols and features on them:

 

Posted Imagefax chart.png

  • [*]Low Pressure systems are marked by an '
L', while High Pressure systems are marked by an 'H'. [*]The bold black line with the semi-circles on it represents a 'warm front'. [*]The bold black line with the pointy triangles on it represents a 'cold front'. [*]The bold black line with both the semi-circles and the triangles on it is an 'occluded front'. [*]The bold black line with no triangles or semi-circles on it represents an 'upper trough'

 

Which ever way the semi-circles and triangles are pointing in shows the direction of travel of the weather fronts.

 

The FAX charts are viewable up to 120 hours ahead.

 

 

G

 

GFS - Stands for 'Global Forecasting System'. An American Weather Model. Created by NCEP. It contains billions of different types of charts, such as the standard pressure charts, convection potential, precipitation and its types, temperature charts and many more. This model, however, does come with a few issues: Whilst no model is entirely perfect, the GFS has the tendency to be over eager with the progression Low Pressure systems make from West to East, hence the term "the GFS is being too progressive" may be used in the model output thread. It can also have the habit of over-deepening Low Pressure systems (although it is not necessarily the only model that does this). NCEP often make adjustments to the model to further improve its performance.

 

This model updates four times a day and goes up to 384 hours (16 days) ahead. It is made up of the runs: 00Z, 06Z, 12Z and 18Z.

 

GEM - Stands for 'Global Enviromental Multiscale' model. It can be a suitable model for medium range forecasting and it has fairly good performance overall. The ECMWF and GFS are its only real competitors in that they are the only two other models that go up to ten days ahead and beyond (bar the ECM 32 day model and CFS long range model).

 

The GEM updates twice a day and is made up of the following runs: 00Z and 12Z.

 

 

H

 

High Pressure - 'HP' for short. Areas of stronger than average pressure - usually around 1016 hPa/millibars or higher - that fill up the gaps in the atmosphere between and around Low Pressure systems. Broadly speaking, they are not quite as circular as Low Pressure systems (though not all Lows are round). They tend to be oval or sausage-shaped and will often bring settled, calm conditions. How sunny it will be depends on the orientation and positioning of a High Pressure system. As an example for the UK, if you want warm, sunny weather, ideally the High Pressure needs to hang about just to our East with Southerly/South-Easterly winds drawing up warm, dry air from the continent. Wind tends not to be an issue with High Pressure systems - the isobars are usually well spaced out. In fact, if you're stuck under the centre of a High Pressure system, their may not be any wind at all. Winds, will, however, circulate clockwise around High Pressure cells. 'Anticyclone' is an alternative word people use to refer to areas of High Pressure.

 

Black circle showing an area of High Pressure

Posted ImageHigh Pressure.jpg

 

 

I

 

J

 

Jet Stream - Fairly narrow passages of fast high-level winds - around 29,500 to 52,500 ft above the Earth's surface (thanks to 'The PIT' for the correction Posted Image). You could almost think of it as various pathways that direct Low Pressure systems along it, and can be one of the main controllers of our weather. Depending on the positioning of Jet Stream and where passages of high-levels winds flow along, it can have an affect on where High Pressure systems migrate to, and how pronounced their ridging is. You often get one of the main streams of the Jet flowing Eastwards through from America, all the way through to the UK, or just to the North of it. In Summer, the normal position of the main stream of the Jet is to the North of us, although in some of the recent Summers, with the exception of Summer 2013, it has been behaving very strangely - the main stream of the Jet going much further South than usual steering constant Lows over, or to the South of us. Where you find both the cold air to the North and the hot air to the South meet is where the main stream of the Jet tends to flow along (or at least close to this boundary). When ever heights build to our North, this can force the main area of the Jet Stream to go South, although there has also been debates about whether the arctic sea-ice melt has affected the normal flow and positioning of the Jet Stream. When the Jet Stream becomes weak, or fragmented, with streams of the Jet split into pieces, Atlantic or Northern blocking may occur, just like what a very amplified pattern could cause.

 

Some Jet Stream examples underneath:

 

Example 1:

Posted ImageJet Stream examples.jpg

 

In this situation, you have one of the main areas of the Jet Stream flowing to the South of us attempting to bring a Low Pressure system to the South-West of us. The cyclone collides against the High Pressure to the North-East of us thanks to the slight West-South-West alignment of the Jet stream to the South-West of us. And this is kind of helping to steer the Low Pressure up against the High to our North-East. The Low does not quite slide underneath the High, despite one of the main areas of the Jet Stream being to the South/South-West of the UK.

 

Example 2:

Posted ImageJet Stream examples 2.jpg

 

In this example, you have an area of a stream of the Jet flowing on a South-West to North-East direction to the North-West of the UK. The stream of the Jet then curves back down Southwards through the North-Sea to the East of the UK. But the curvy Jet to the North of the UK is still far enough North to allow a High from the South or South-West to build some of its heights North-Eastwards over the UK. A South-West to North-East alignment of the Jet with it staying out to the North-West of the UK often allows High Pressure systems from the South or South-West to ridge North-Eastwards towards us.

 

One other aspect about the Jet Stream is that it can sometimes split up into two main streams - one going to our South, and another going to our North. In this type of setup, the GFS apparently has a slight bias of overdoing the power of the main stream of the Jet going to the North of us.

 

JMA - Stands for 'Japan Meteorological Agency'. One of the slightly less well-known models, although it runs a MSM (Meso-Scale Model - these are short range models) to provide small forecasts in Japan. The JMA model can be a useful asset to weather operations across the globe. It is highly regarded by the UKMO and can occasionally pick out certain weather patterns earlier than some of the other models. 

 

This model updates twice a day and goes up to 192 hours ahead. It is made up of the 00Z and 12Z runs.

 

 

K

 

L

 

Low Pressure - 'LP' for short. Often circular or elongated in shape, they are areas of lower than average pressure - usually around 1012 hPa/millibars and below - that fill up the gaps between and around High Pressure systems in the atmosphere. They bring with them unsettled and changeable weather such as frontal bands of rain or snow, sunshine and showers... and even storms! The weather can be windy too, but the tighter the spaces between the isobars (white or black lines on the charts that mark out the Low and High Pressure cells) the windier the weather is likely to be, although the Jet Stream can have an impact on this as well. The winds spin anti-clockwise around Low Pressure systems. Low Pressure systems can also be referred to as 'Cyclones'. Another aspect about Low Pressure systems is that they come in different intensities: Deep Lows have a central pressure area lower than 980 hPa, while moderate Lows are those with a centre of around 980 to 1000 hPa. Shallow Lows are those that are around 1000 hPa, or higher. Furthermore, Lows can have a habit of 'filling out' if the centre of the cyclone becomes less and less deeper.

 

Black circle showing an area of Low Pressure:

Posted ImageLow Pressure.jpg

 

 

M

 

Model Posers - People, especially ladies, who pose as 'models' in magazines, newspapers, TV shows etc. Avoid talking about this type of model in the Model Output thread as it is off-topic and the Netweather moderators might banish you from the forum. Forever! (although if you're lucky, Chinomaniac and his moderating crew might give you a second chance). ;-)

Posted ImageModel Posers.png

 

 

N

 

NAVGEM - Stands for 'Navy Global Environmental Model'. There's quite an interesting tale behind this model. It never used to be the NAVGEM model. The former model, NOGAPS, used to run all the time. That model became discontinued and the NAVGEM replaced it. And it's very unlikely the NOGAPS model will be brought back to life. Knowing the NOGAPS was gone brought tears to the eyes of some of those who liked that model so much. Posted Image

 

But it's not all bad as the NAVGEM has supposedly had improvements done to it to increase its performance potential. Just like the expired NOGAPS, this model updates four times a day. It is made up of the following runs: 00Z, 06Z, 12Z and 18Z.

 

NMM - A Meso-scale Model which runs at a very high resolution of 12 kilometers. According to Paul, "it is high enough resolution to pick out small more localised weather features that lower resolution models will miss." (http://forum.netweather.tv/blog/189/entry-3452-new-nmm-hi-res-forecast-model-for-the-uk/). This means it can pick out even more details than the GFS model and even the ECMWF model - both of which are run at lower resolutions. (You can find more about the models' resolutions here: http://forum.netweather.tv/topic/72326-model-resolution/ - credit goes to lorenzo).

 

The NMM is a short range model and it updates twice a day, but you need a Netweather Extra subscription to view it.

 

Northern Blocking - Cold and snow fans number one setup in Winter. It is where blocking occurs to the North of us, and/or over the Artic region via High Pressure building to the North of us. Ideally, over the UK, you do not want High Pressure system(s) to the North of us to be too far North, otherwise Lows out West may not track far enough South to deliver cold South-Easterly, Easterly, or North-Easterly winds on their Northern side. When the Northern blocking is significant, such as a strong Greenland High Pressure system linking up with a Scandinavian High Pressure system (and with it being very close by to the North of the UK), this forces the Jet Stream to send a fair amount of its energy to the South of us and we can become locked into a prolonged cold spell with winds constantly coming in from an Easterly direction.

 

An example during late November 2010, where a High Pressure system to our North provided the UK with some chilly Easterly/North-Easterly winds:

Posted ImageNorthern Blocking.gif

 

At times, shortwaves, troughs or small Lows, usually over the Norway (or Scandinavia) area, will try to separate the link up of a High Pressure system to the North-West of us and a High Pressure system to the North-East of us into two, like so:

Posted ImageTrough trying to poke Northwards.jpg

 

But in the case of the 1947 example above, it might get itself trapped inside the High Pressure systems...

Posted ImageTrapped Low.jpg

 

An aspect you have to be careful of is that the Northern blocking isn't very weak as Low Pressure systems from below and above could easily attack the High Pressure system(s) destroying the blocking to the North. And, as such, we could easily revert back to a milder pattern with Westerly or South-Westerly winds. And this would be especially true if the Jet Steam tries to power back through over us from the West without it no longer staying to the South of us. Northern blocking can also become eradicated, if one of the streams of the Jet powers back up to the North and sends most of its energy and associated Low Pressure systems over the top of the blocking High(s) to our North and forces the High Pressure system(s) to sink Southwards over us.

 

Northerly Toppler - A short lasting period where cool or cold winds flow down from the North. High Pressure from the West is forced to 'spill' over us cutting off the Northerly flow, with Westerly or South-Westerly winds returning from the West. But sometimes, approaching Lows from the West might just flatten out an amplified High Pressure system to our West (but with the Northerly airflow still destroyed). Northerly topplers often last for around 2 days; perhaps longer, if a High Pressure system to our West is particularly strong and can withstand the power of the West to East track of the Lows for a few extra days. Despite this, one reason why Northerly airflows and Northerly topplers don't last too long is not only because they 'topple' or get broken down easily, but because of a strong stream of the Jet is always keen to try and blast through a block to the West of us. The Jet Stream is an enemy of the Northerly! Northerlies via a Greenland High, though, will tend to last longer and the chances of them toppling so easily is much less likely.

 

The charts below show the process for one of the types of 'Northerly Topplers' that happen. Example used from the period ' 26th January to 29th January 1935':

 

The Northerly Toppler part 1 - The Northerly airflow beings to get established with Low Pressure to our North-East, and an amplified High Pressure to our West. (dark blue arrows showing cold air draining down from the North)

Posted ImageNortherly toppler part 1.jpg

 

The Northerly Toppler part 2 - The whole of the UK gets affected by a direct Northerly, although warmer South-Westerly winds (indicated by the red arrows with Low Pressure to the North) try to blow over the top of the High Pressure, and it slowly begins to get knocked Eastwards/South-Eastwards towards us. (the black arrows show it slowly beginning to get knocked over towards our direction)

Posted ImageNortherly Toppler part 2.jpg

 

The Northerly Toppler part 3 - The Northerly has almost toppled completely, with only the South-Eastern UK holding onto cold air, which is now coming from the North-East, rather than directly from the North. This is thanks to Low Pressure to the North-West of the UK knocking a portion of the High Pressure system over us, and the far North of the UK begins to get affected by milder Westerly or South-Westerly winds (shown by the red arrows).

Posted ImageNortherly Toppler Part 3.jpg

 

The Northerly Toppler part 4 - It's game over for the Northerly! And the cold North-Easterly airflow now becomes concentrated over France and also areas further South-East, East, and North-East of it. The Low Pressure system which knocked a portion of the High Pressure over us, slowly makes a getaway North-Eastwards, and the orientation of the airflow over the UK is now a less-cold slack North-Westerly airflow.

Posted ImageNortherly Toppler part 4.jpg

 

 

O

 

Outlier - Where either a model's operational run, control run, or a variation of its control run is out of place amongst the other runs on the ensemble graph. The line representing the 'outlier' on a ensemble graph is very spaced out in relation to other lines on the graph, and is a good indication that the ensemble member's outlook is very unlikely to materialize.

 

Posted ImageOulier.png

 

On the London ensemble graph example, the black circle shows how one of the lines looks out of place amongst the other lines, although it does start meeting up with other lines towards the end of the graph. As such, an ensemble member won't be an outlier all the way through - especially at the beginning, where the ensemble members' confidence are high with the tight clustering of the lines. You can get the odd time, where one or two of the 'outliers' on a ensemble graph may have detected a new trend, and the other ensemble members (including the operational and control run), may follow suit on future runs.

 

 

P

 

Performance Statistics - Various checks are carried out on the models. And their performance in regards to how they've handled pressure patterns, and other atmospheric conditions, are recorded in numbers and graphs. You can view how accurate the models have been handling various atmospheric weather patterns here (although this is where it starts to get rather technical: http://www.emc.ncep.noaa.gov/gmb/STATS/STATS.html / http://www.emc.ncep.noaa.gov/gmb/STATS/html/new_acz6.html)

 

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifVerfification Stats (500mb charts NH).jpg

 

As an example, the Performance Statistics graph posted above shows the anomaly corrections of how well the various models have handled the 500 hPa and air pressure patterns in the Northern hemisphere in the last 31 days. The numbers on the Y axis on the graph relates to how well the models have handled the pressure patterns six days ahead. '1' means that a model handled the pressure patterns entirely perfectly with no errors whatsoever, while '0.2' means that a model was way off in the handling of their pressure patterns. Currently, the ECMWF model - the one in red - is performing the strongest with it receiving the highest average score of 0.808. The UKMO model, however - the one in orange - is close behind with an average score of 0.787. You can also see that the red line on the graph (representing the ECMWF) has generally been a touch higher than all the other lines on the graph (representing the other models) in the last 31 days. But their have been a few occasional brief periods within the last 31 days where the UKMO (representing the orange line) has been higher than the ECMWF's line.

 

One other aspect about this type of graph is if it has the 'NHX' part in its name, it's for the Northern hemisphere, and if it has the 'SHX' part in its name, it's for the Southern hemisphere.

 

(New!) Polar Front Jet - As its name implies, this Jet Stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 degree N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet.

 

(Suggested by: Johnholmes)

 

Polar Maritime - 'Pm' for short. An airflow that comes from a cool/cold region from the North-West, usually with a fairly amplified High Pressure system just to the West or South-West of us. It's a type of North-Westerly airflow that is fairly common over the UK and can usually bring a sunshine and shower setup if the air is unstable enough. Cool or cold 850 hPa upper temperatures usually help with the instability. In Winter, if the cold Upper Temperatures coming down from the North-West haven't been moderated enough by the warmish Atlantic seas, some of the showers can fall as snow, most especially in the North-West of the UK and over high ground. But in Summer, when daylight heating from the Sun plays a big role, the temperatures over the land become warmer than that of the ocean temperatures, and you will find that the heaviest showers become concentrated more towards the Eastern side of Britain.

 

Underneath is an example of a Polar Maritime airmass with Low Pressure to the North bringing cold air down from the North-West through the UK. (dark blue arrows showing direction of the airflow's travel).

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifPM airmass.png

 

(Updated) Polar Vortex / Potential Vorticity - 'Potential Vorticity' is technically meant to be the proper term for this, although you will find quite a few people will also use the term 'Polar Vortex'. It is blobs of dark blues and purples shown on the '500hPa Thickness and Sea Level Pressure' charts from ECMWF, GFS, UKMO, JMA, GEM, GME, NAVGEM etc. Some people will refer to the Potential Vorticity/Polar Vortex as the 'PV'. But it is essentially like an area of Low Pressure systems that, in our part of the world, dominate the Northern Hemisphere with High Pressure systems surrounding it. The Potential Vorticity occupies the Stratosphere and upper Troposphere levels and contains lots of very cold air. During the Winter where it gets very strong, Low Pressure systems become more powerful and increase in size as the Potential  Vorticity adds 'fuel' to them. This is thanks to lots of very cold air getting mixed in with warm air further South resulting in some very powerful Lows. But as the affects of the Potential Vorticity weakens significantly during Summer, Low Pressure systems tend to be much smaller in size and less powerful.

 

The '500 hPa and Sea Level Pressure' chart below [which I nicked from an old Model Output Discussion thread ;-)] shows the whole of the Northern Hemisphere and the areas of dark blues and purples, (the Polar Vortex/Potential Vorticity), that dominates some of it.

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifPolar Vortex.png

 

If wanting a prolonged cold setup, you never want the Polar Vortex to be too invasive to the North-West of us, and especially over Greenland and Iceland, as it can make it hard for Northern blocking to form. The exceptions would be if a 'SSW', or other form of Stratospheric Warming, event occurs (and even then, it would have to be in the right place), or if we get some notable 'WAA' going up into Greenland or Iceland. (see 'Sudden Stratospheric Warming' and 'WAA' for more details). (There's probably likely to be other methods that I've admittedly not thought about, or know about, that could put the Polar Vortex under threat).

 

 

Q

 

R

 

(New!) Ramp - Nope! Not this ramp...

 

post-10703-0-38545600-1381267681_thumb.p

 

It's the sort of ramp where someone greatly exaggerates what a chart beyond a certain outlook is showing. For example, if a model is showing an exceedingly warm deep-seated Southerly airflow in FI, some people might try and over-hype the outlook (usually for the fun of it), and to help raise excitement over the particular outlook.

 

Another example (from Richie V): exaggerating the position of a Scandi High at T+360 hrs.

 

Retrogressing High / Retrogression - When a pressure feature, such as a High Pressure cell, moves Westwards against the mean flow. For example, a High over the UK can sometimes retrogress towards Greenland. Whether a High retrogresses or not depends on various aspects such the dominance of the Polar Vortex (during Winter) and how obtrusive the Jet Stream is; if a powerful stream of the Jet is flowing over the top of the High, then it could have a hard time retrogressing (unless the main area of the Jet Stream suddenly heads South, or if the pattern significantly amplifies). It could also depend on other factors such as whether a Low Pressure system can pump up some 'Warm Air Advection' Northwards on the High Pressure system's Western side and help encourage heights to build further Northwards. Although if heights build Northwards only, rather than back Westwards or North-Westwards, then it doesn't really count as Retrogression.

 

Ridge / Ridging - A curvy or pointy area of a High Pressure system. It can be quite elongated. A High Pressure system will, at times, send some of its ridging from one area to another. This idea of 'ridging' means a High Pressure system is trying to extend its heights to other places.

 

An example of a ridge of High Pressure:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifRidge of High Pressure.png

 

 

S

 

Shortwave - It is essentially like a little Low Pressure system that hasn't quite become a proper closed-off circular Low (although they can sometimes develop into circular Low Pressure systems depending on the conditions within the atmosphere). Shortwaves are marked by a small(ish) curvy-shaped wave on the Pressure charts and are often a part of big Low Pressure systems and Longwave Troughs. Cold air above can also often be a part of Shortwave features.

 

Some examples of shortwaves on the chart below (circled in black):

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifShortwave examples.png

 

Slack airflow - Where the isobars on the pressure charts are very spaced out. A slack airflow is associated by gentle winds, or a gentle breeze. Virtually, the weather is non-windy. 

 

Sudden Stratospheric Warming - 'SSW' may also be used. A sudden warming event occurs within the Stratosphere and, if notable enough, can separate the Polar Vortex (and its blue/purple blobs) into separate segments. Or even just make it less stronger in general. Where ever an 'SSW' event occurs, heights will likely to keenly rise in that area. As such, if you wanted a cold setup with a strong, sturdy, Greenland High, a notable 'SSW' event would be ideal around that area to help split the Polar Vortex in that area into two, with a segment of it getting knocked further West and with a segment of it being knocked Eastwards towards Scandinavia. In fact, getting a segment of the Polar Vortex to drop down to the North East of us can help promote Scandinavian Troughs and, if close enough, the UK can become influenced by a very cold Northerly flow from a Trough/Low over Scandinavia.

 

 

T

 

Trough - An area of curvy Low heights away from a main Low Pressure system's centre. Weather fronts and convective weather can be a part of troughs. Troughs are also a part of stretchy Low Pressure systems, and they can by quite pointy shaped. In the United Kingdom, a trough will be marked by a solid line on the Fax charts. There are, nevertheless, different kind of troughs such as Upper-Air Troughs, Surface Troughs, Shortwave Troughs and Longwave Troughs.

 

The black circle on this chart shows an example of a trough tracking towards the UK:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifTrough Example.jpg

 

T+??? - To refer to a time-frame within the models. For example, 'T+48' would refer to a model outcome 48 hours ahead.

 

 

U

 

UKMO - A weather model owned by the UK Met Office. It has comparable performance to ECMWF, although this model can only be viewed up to 144 hours ahead.

 

This model updates twice a day and is made up of the 00Z run and the 12Z run.

 

Undercutting - A situation where cold air undercuts warm air (thanks to 'forecaster' for the correction). Sometimes, on the charts, you may see approaching Low Pressure systems out West that try to track to the South under a High Pressure system to the North. This sort of scenario could be known as an 'undercutting low' with cold air from the North or North-East getting injected into the low and tries to mix out the warm air wrapped around it. Easterly winds can develop over the UK if the Low undercuts to the South of the island.

 

 

V

Vortex - Part of the Polar Vortex/Potential Vorticity.

 

 

W

 

WAA - Warm Air Advection. This is where warm air from the South gets drawn up Northwards and moves into areas of cold air. In the Northern Hemisphere, the warm air would get drawn up on a Low Pressure system's Eastern side. When warm air gets 'advected' Northwards towards areas of cooler air, it can help strengthen High Pressure systems in that area, or even help new ones to form. This is why if you want a cold, blocked setup in Winter with High Pressure to our North or North-West, getting WAA heading up directly towards Greenland is seen as an encouraging sign.

 

Below is an example from January 1947 of WAA being pumped Northwards:

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifWAA.jpg

 

The example above shows a long drawn Southerly airflow to the West of the UK bringing warm air up from the South (shown by the red arrows). Mild Upper Temperatures are also brought Northwards (see the example below):

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gifWAA upper temperatures.gif

 

Prolonged Northern blocking soon followed after this event in 1947, and the rest of that Winter was very cold.

 

Weather Models - Charts, such as the ones you might be looking at right now, that display graphical weather forecasts and predictions, and use a large range of data. Their are a wide selection of weather models that all have varying purposes within our global Meteorological Weather operations. Satellites, radio equipment and weather balloons are used to detect the atmospheric and oceanic conditions on our planet, with special computers or super-computers working out all sorts of mathematical equations from the data that's gathered to then provide  forecasts on various models. There are many different types of models, and various different model viewers in which you can view the charts from. In the UK, you can view charts that cover a portion of the Northern hemisphere, or the whole of the Northern hemisphere. Viewing versions of models that show the whole of the Northern hemisphere is highly recommended if you want to get a broader picture of how the pressure conditions and atmospheric conditions are behaving. The way one pressure system behaves on one side of the world can have impacts as to what happens in other areas of the globe. This is why viewing versions of charts that cover a broad area can be useful.

 

Just one other thing to add is that you can also view historical charts dating to over 100 years back as well.

 

WMA - Weather Model Addiction. Something some weather site member's are prone to catching. The addictions may include frequent model checking, frequent model analyzing and frequent viewing of the weather model threads across the World Wide Web.

 

 

X

 

Y

 

Z

 

(Updated) Zonal Setup - The type of pattern that can bring about a North and South divide. On the whole, the Southern half of the UK remains settled with high pressure influential to the South, while the Northern part of the UK remains more cooler and unsettled with Low Pressure systems and Longwave troughs being more influential in those areas. Hence you begin to see why this is called a 'Zonal Setup'. Zonal patterns usually bring mild, or average conditions with a fairly long draw of winds from the West or South-West. It is a flat pattern overall.

 

However, not all Zonal setups bring mild or average temperatures. If winds come down from the North-West with High Pressure influential to the South-West of the UK and Low Pressure influential to the North/North-East, a cold Zonal setup can arise. Snow is also possible (if occurring in Winter), especially to the North and on high ground. People will also refer to a 'Zonal Setup' as 'Zonality'.

 

 

123456789...

 

850 hPa Temperatures - Some people refer to these as 'Upper Temperatures' or just 'Uppers' in the model output thread, although technically this is not correct. According to 'forecaster', "if you were to say "uppers" to a meteorologist (without any further clarification), he/she would automatically think you are talking about 500 hPa or 250 hPa, and most likely the geographical height, not the temperature." If you're think about using the term 'Uppers' in the model thread, don't do it no more.  http://f1.nwstatic.co.uk/forum//public/style_emoticons/default/blum.gif.pagespeed.ce.xv8tK9qKzu.gif

 

The 850 hPa temperatures refer to the temperatures high up in the atmosphere (around 1,500 meters from the Earth's surface) and can be one of the factors that determine what the temperatures may be like at the surface. The warmer the 850 hPa temperatures are, the warmer the surface temperatures are likely to be. But this won't always necessarily be the case, as cloud, mist, or heavy rain can help to suppress the temperatures somewhat.

 

Likewise, the colder the 850 hPa/mb Temperatures are, the colder the surface temperatures are likely to be, although again, certain weather scenarios can have an impact on this. In a sunshine and shower setup, you really need the 850 hPa temperatures to be around -7*C or colder for snow to reach low levels. However, even 850 hPa temperatures around -5*C can suffice if the shower is very heavy (encouraging Evapourative Cooling), and if surface temperatures near the ground are particularly cold with no real layer of warm air above. The Dam-line (the 500 - 1000 hPa Thickness) needs to be around 527 or lower, as well, for showers to fall as sleet or snow. But again, like with the 850 hPa temperatures, you could get away with the 500 - 1000 hPa thickness being a little higher if the showers are extremely heavy.

 

For frontal bands of precipitation, you can get away with the 850 hPa temperatures being as warm as 0*C and snow can still fall to low levels. This, nonetheless, is mostly true in situations where an Atlantic front is forced to bang up against High Pressure to the North (with Low Pressure to our South). The front then slowly pivots and gets pulled Westwards/North-Westwards through the UK via cold Easterly or South-Easterly winds.

 

An example of an '850 hPa and Sea Level Pressure' chart below.

http://f1.nwstatic.co.uk/forum//public/style_images/Simple/attachicon.gif.pagespeed.ce.hPGJFKgqtP.gifUpper Temperatures.png

 

The greens, blues and dark blues represent 850 hPa Temperatures around 0*C or colder, while the yellows, oranges and reds represent 850 hPa temperatures around 0*C or warmer. Helpfully, the 850 hPa Temperature gradients are marked by black lines with numbers over the top of the lines to show how warm or cold the 850 hPas are expected to be in that area. Over the far North of Scotland, for example, the 0*C 850 hPa Temperature line crosses a part of it, with the -5*C 850 hPa Temperature line crossing a part of Iceland further North-North-Westwards. You can also refer to the model's 850 hPa Temperature colour-coding line to the right of the chart to work out how warm or cold the 850 hPa temperatures are over your area. (However, if you have a Netweather Extra subscription, or are thinking about getting one, you can view the 850 hPa temperatures just for the UK only).

 

 

------------------------------------

 

Weather FAQS: (Website recommendation by Johnholmes: http://weatherfaqs.org.uk/

 

 

(Image attachments for the 'Ramp' and 'Bin / Binned' terms underneath just in case they don't show up in the grey quote box above)

post-10703-0-38545600-1381267681_thumb.ppost-10703-0-44154200-1381267657_thumb.p

Edited by DiagonalRedLine

Share this post


Link to post
Share on other sites

Hold on, I should have clarified better. 

 

Polar Vortex: as you were...

 

Potential Vorticity (PV): It's a mathematical construct, which combines vorticity with stability. This makes it difficult to imagine, because it combines a thermodynamic contribution (stability) with a dynamical element (vorticity). However, it's useful because it is conserved in the absence of friction and diabatic (meaning energy is transferred in and out of the boundary of the air parcel) effects. So people use it as a "tracer". Displayed as a field, it has many applications, and one of them is to identify the polar vortex. There will be a gradient of potential vorticity in the north/south direction.

 

So Potential Vorticity is not the correct term for the Polar Vortex; they are two completely different things. It's just that conventionally, PV is used to mean Potential Vorticity, whereas on NW is used to mean Polar Vortex. This is a potential source of confusion. 

Share this post


Link to post
Share on other sites

Thanks. My fault really for not checking to see what 'Potential Vorticity' meant. Will get that corrected at some point today. Must it admit, it does seem kinda confusing having two terms where their shortened form is 'PV'. :)

Share this post


Link to post
Share on other sites

Potential Vorticity is what meteorologists use (PV) and I suspect that the other PV is something that has come along since the advent of the web, although it does appear to have more general use with the meteorological fraternity than it did?

Share this post


Link to post
Share on other sites

Corrections made to 'Potential Vorticity' and 'Polar Vortex'. Both of which are different terms as pointed out by 'forecaster.' Potential Vorticity is also a new term I've added to the updated Dictionary underneath.

 

Also added a little new section to the 'Ensembles' section, and made some slight tweaks to the 'Polar Maritime' description. Wherever else you see the word (Udpated) next to a term, it means I've replaced the words 'Uppers' or 'Upper Temperatures' with 850 hPa temperatures, when ever it was used (with the exception of the '850 hPa temperatures' term which was already corrected a week or so ago).

 

New term added: '500 hPa'

 

Ideas for terms to add in future: CAPE, Spanish Plume, Meridional flow, the North Atlantic Oscillation (and what it looks like on the charts), and others. :) 

 

The Model Dictionary up-to-date as of 09/10/3013 @ 22:23pm (see underneath):

 

------------------------

 

A

 

Amplification - When amplification occurs within the weather pattern. It is a bumpy looking pattern on the models' pressure charts where the general airflow becomes all wavy, usually thanks to pronounced ridges of High Pressure systems and notable *troughing of Low Pressure systems between the ridging High Pressure cells (*see 'Troughs' for more information). For example, you might get a long flat Westerly airflow over the UK that becomes more and more wavier as the pattern tries to amplify and High Pressure in the Atlantic tries to send some of its ridging Northwards to the West of us. Fairly potent periods of cool/cold North-Westerly winds can occur that spill down along the High Pressure system's Eastern side towards the UK. The amplification of the Jet Stream also helps along with a pressure pattern becoming all 'bumpy'. You can also get some portions of the pattern that's amplified and some portions that are flatter. Amplified patterns can, but not always, lead to Atlantic and/or Northern blocking further down a line, such as a High Pressure system trying to ridge in above us from the South.

 

An example of an amplified pattern:

post-10703-0-04569900-1381351592_thumb.p

 

The black arrows shows the bumpiness of the pattern and the general airflow(s).

 

Atlantic Blocking - A High Pressure system that sets itself up either over us, to the West of us, or to East of us and stops the normal travel of the West to East progression of Atlantic Low Pressure systems/depressions. Effectively, the High Pressure system acts as a 'block' deflecting the Lows away from the UK.

 

Azores High - An area of High Pressure that hangs about to our South West in the Atlantic Ocean. It contributes to some of the weather in the UK where it sometimes extends some of its ridging towards us bringing warm or hot spells during the Summer.

 

 

B

 

Bartlett High - Cold and snow fans worst nightmare during the Winter. Mwha ha ha ha haa! It is an area of heights that sit stubbornly to our South-East. The High Pressure system can be very hard to budge. You can go weeks and weeks with winds coming from a mild South-Westerly direction with approaching Low Pressure systems from our West forced to track North-Eastwards. Spells of rain can occur fairly frequently as well, most especially to the North-West where the Low Pressure is more influential. Occasional wintry outbreaks of sleet and snow can happen (mostly for high-ground, but not necessarily so) via brief occurrences of winds swinging to a cool/cold Westerly or North-Westerly direction. The Bartlett High was named after the weather forecaster, Paul Bartlett.

 

The below chart shows a Bartlett High to our South and South-East, with a long draw of winds from the South-West. The Southern part of the UK would see the most mildest conditions in this setup, while much further North, it would be cooler:

post-10703-0-86056700-1381351583_thumb.g

 

BI - Stands for British Isles. The shortened form of this is sometimes used in the model output thread.

 

(New!) Bin / Binned - If a particular model's outlook does not look right at all, and if it doesn't have support from any other model whatsoever, some people might 'bin' the outlook - essentially the person(s) will discard it. Most times 'binning' a chart's outlook is a good idea if it really is very unlikely to happen, although some people can have a habit of binning a models' outlook prematurely, and end up getting 'egg on their faces'.

 

post-10703-0-69434200-1381351586_thumb.p

 

Blocking - Where High Pressure systems set themselves up in places that interrupts the normal flow of weather patterns. Essentially, the High Pressure systems behave like a 'block'. (see 'Atlantic blocking' and 'Northern blocking' for examples).

 

 

C

 

Channel Low - Pretty much what the words refer to - A Low Pressure system over the Channel. In Winter, a channel Low often often brings spells of sleet and snow on its Northern side over the Southern UK as it engages with cold air to the East/North-East. A number of times, though, the very far South of the UK will just experience rain instead with milder air from the South being more influential.

 

Black circle showing a Channel Low.

post-10703-0-21997900-1381351589_thumb.p

 

CFS - Stands for 'Coupled Forecasting System'. It belongs to the same organization that produces the GFS model - NCEP. Generating charts up to 9 months ahead (yep, you heard that right), it is a long range model. But to use it effectively, the model should only really be used for trend spotting. It also produces anomaly charts up to several months ahead and this can also be useful for trend spotting. For example, if the anomaly charts have been continually producing a cold, blocked outlook for Christmas with higher than average heights to the North and lower than average heights to the South, then it could very well be the route we would head in. While charts for several months ahead will understandably change a huge amount, the CFS has been known to be close to perfect with some of its outlooks that have trended in the past.

 

However, if you were ever interested in long range weather forecasting, this site: http://www.weatherwe...cfshowtouse.php provides some useful tips on how to use charts like this effectively...

 

"1. Take a long view - step back from the charts and see how the weather is affecting large geographical areas, such as the whole of Europe.

2. Spot extremes - Look for any extremes of weather, heavy rain, high temperatures.

3. Visit daily - Take a look at the forecasts each day. This helps builds a picture of consistency; the more consistent the model from day to day, the more confidence you can have in it.

4. Don't take it literally - The CFS can only give guidance, not a forecast! Just because it says a particular day in 3 weeks may be warm, that doesn't mean that it will be, it should be taken that there is a chance of warmer weather during that period.

5. Look at each of the members and look for consistency between each.

One should exercise caution when using long range forecasting models such as the GFS. They should be used for guidance only, but the experience of using these is that they are better than nothing at all!"

 

 

D

 

E

 

ECMWF - Stands for 'European Centre for Medium-Range Weather Forecasts'. A European weather model. Some people use the term 'ECM' as its shortened version. It is regarded as one of the best performing models out their. As a general rule, the ECMWF is supposed to be better than the GFS at handling blocking patterns to the East, while the GFS is supposed to be better than the ECMWF (though not always) at handling developments to the North-West of us. And sometimes this can have a huge impact as to what happens over the UK. It is one reason why the GFS tends to pick out Northerly setups (with cool/cold air flowing down from the North) earlier than the European model.

 

This model updates twice a day and goes up to 240 hours (10 days) ahead. Made up of the 00Z and the 12Z runs.

 

(Updated) Ensembles - What happens here is that an operational model run (main model run) is re-produced again at a lower resolution. This is then referred to as the 'control run'. Variations of this run are produced, each of which with different starting conditions. Along with the main operational model run, the results are then recorded as a line graph - each of these lines can be referred to as 'ensemble members'. They show various possibilities that could occur in regards to the weather. This is supposed to increase accuracy and help determine whether the particular operational model run is worth taking seriously or not. Most times you will find that the ensembles tell the bigger story of what will happen, than what the main model run shows. You can view various ensemble graphs, such as...

  • [*]
Air-Pressure [*]850 hPa Temperatures [*]Precipitation amounts [*]Average Temperatures

You can also select what the ensembles show for your nearby region or town. How ever close the lines are on the graph reflects the confidence the model and its ensemble members has on a particular weather outlook. Underneath is an example of the Sea Level Pressure for London using GFS's ensembles:

 

post-10703-0-91641000-1381351646_thumb.p

 

(GFS's ensemble set is also known as the 'GEFS ensembles')

 

The X axis shows the days ahead, while the Y axis shows the numbers relating to the type of weather, climate or atmospheric condition it is. The further along the graph you go, the more the ensemble members' confidence reduces, and thus the lines on the graph begin to scatter. The period at where the lines start to scatter depends on how well the operational model and its ensemble members are handling particular weather patterns. As you would likely expect, you can see that the lines for the first few days ahead are close together and reflects the confidence the GFS and its members has on the atmospheric conditions. The ensembles generally agree to a slight pressure drop around the 4th October (the dip between the 3rd October and the 5th October), with the pressure rising after that. Except for the odd ensemble member, the period at where the lines start to scatter becomes noticeable past the 7th October, with various solutions on offer. The red line (the ensemble mean) is the mean average of all of the ensemble members' outcomes. You can view the model's ensemble mean as an actual graphical model chart and see how it compares to the operational graphical model chart.

 

An example using the ECMWF's 00Z '500 hPa and Sea Level Pressure' ensemble mean chart (with the operational ECMWF 00Z '500 hPa and Sea Level Pressure' chart below it). Both of these showing the pressure conditions at 240 hours ahead:

post-10703-0-94997700-1381351596_thumb.g

post-10703-0-73042700-1381351598_thumb.g

 

Since the ensemble mean is just an mean average of what the ensemble members show, and since the operational model run and its ensemble members' confidence drop the further ahead they predict, this will explain why the ensemble mean chart becomes less detailed the further ahead you're viewing it. It sort of works out a middle-ground-like solution of what its ensemble members show regarding what the weather conditions might be like several days ahead. The first few time-frames of the ensemble mean tends to be as detailed as the actual operational model run, and both will follow each other very closely, because of their high confidence in regards to the weather for the next few days. On the above example, however, the 00Z ECMWF ensemble mean expects that the High Pressure to the North-East of the UK on the operational model run will be more concentrated towards the East of the UK. Nor does the ensemble mean show the High Pressure to the East linking up with High Pressure over Greenland, like it does on the operational model run. This could mean that the operational ECMWF run has probably over-estimated the Northwards placement of the High Pressure to the North-East, and that it should be slightly further South/South-Eastwards. And if this was the case, then the operational run has probably also over-estimated how easily its High Pressure system to the North-East links up with the Greenland High Pressure system. For tend-spotting purposes, keeping an eye on future ensemble means and their ensembles from some of the models, can be handy to see where they keep heading in. Comparing a different operational model to an ensemble mean chart is probably a good idea as well, to see how close their outlook relates to each other. Should the operational outlook from a different model follow the ensemble mean closely all the way through, that could provide extra reassurance that the ensemble mean is heading in the right direction.

 

It can actually sometimes be much more reliable to follow the ensemble mean, rather than the operational model run, especially since the operational run only represents one line on an ensemble graph (the green one on the GFS's ensemble graph, for example), and can easily become an big 'outlier' in parts in relation to the ensemble mean, or even in relation to the other lines on the graph. In fact, you can see that the GFS operational run shows the biggest pressure drop around the 15th October on its ensemble graph, and doesn't follow the ensemble mean closely at all around that period, so its chances of coming off are almost virtually zero (unless I suppose another operational model run just happens to show very Low Pressure over the London area for 15th October, in which case its chances of coming off might be a little higher). This is why, however, if you're viewing the main model run, its worth checking the ensembles to see how well they follow the operational model run. If there is not much support for that run at all, then it's a good reason to be particularly cautious of what the operational model shows for a fair amount of days ahead. It's true that since a model's outlook several days ahead is liable to change a lot from one run to another that we shouldn't really trust what any run shows beyond a particular period. But it's still worth checking the main model run against the ensembles to see how confident the operational model run really could be and to see if any trends can be spotted. If the ensemble mean, let's say, continues to show a slight downward trend in pressure, with the general scatter of the other lines on the graph shaping out a slight downward trend, it could be a good sign to where we could head in the future in regards to the air-pressure.

 

For the sake of it, though, it is a good idea to also see what other main models runs are producing in relation to the one you're viewing, because even if a GFS operational model run may have strong ensemble support, it may not always mean too much if the other main operational models (such as the ECMWF, UKMO) completely disagree with GFS's weather scenarios. Either, it could mean the GFS and its ensembles are heading the wrong way, or that the other models are just not picking out the solutions the GFS and its ensembles show. What I will say, though, is even if their is notable large differences between the various models, which ever one has the strongest ensemble support is the one that is likely to lead the way in regards to its outlook. This could be particularly true if the GFS, on its last few runs, has continued to stick to a particular solution with its ensembles continuing to support it (John Holmes, though, does recommend comparing only the 00Z and 12Z GFS runs with each other, rather than all four of the GFS's daily runs when looking for trends). Also, if the other models have continued to offer solutions that vary widely from one run to another, then it's a good chance that they are not quite getting to grips with the weather outlook for the future. This doesn't always last, though, as models (and even the ensembles, especially if they're trending in the wrong way) do go through phrases where they won't perform as well as they could do.

 

But... should viewing the ensembles in graph form not take your fancy, you can view each of the ensemble members from a model's run as mini graphical chart images, just like in this example:

 

post-10703-0-81790600-1381351645_thumb.p

 

...And this can be a great way to see how each of the ensemble members handle the atmospheric conditions in a visual style!

 

 

F

 

(Updated) Fantasy Island - 'FI' for short. A term used to refer to a time-frame in the models where the accuracy in their predictions severely reduces. Usually around the 144 / 168 hour period and beyond in the models' outlooks - the sort of period where their outlooks become fantasy and dreamland-like, hence the term 'Fantasy Island'. However, if the models are having a tough time handling a particular pattern, people may use the 'FI' term for a time-frame in the models earlier than 144 hours. If, let's say, their are some fairly huge differences between the GFS, ECMWF and UKMO models as early as 96 hours, then this could count as Fantasy Island due to the uncertainty of the models' outcomes within that period. But even then, Fantasy Island could occur as early as 48 hours, if the models' ensembles show scatter on their graphs extremely early on.  It should be noted that some people use phrases such as "this model disagrees with another model" to refer to the differences in the evolutions the models show. Although a chart's FI outlook is unlikely to come off, occasionally it can happen. This is especially true if a model has picked up a trend regarding a particular pattern in its last few updates, and also if it has ensemble support.    

   

FAX Charts - White in colour, they are hand drawn by the Met Office, and are similar to the Pressure charts shown on the main models such as the GFS, ECMWF, UKMO and so on, but only without all the pretty colours in the background (although this is only true if you're viewing the '500 hPa Sea Level Pressure' charts from the GFS, UKMO etc, which have the rainbowy colours in it, rather than the just the 'Sea Level Pressure' charts - see example below).

 

('500 hPa and Sea Level Pressure' chart - has the colours of the rainbow in it)

post-10703-0-96171800-1381351604_thumb.p

 

('Sea Level Pressure' chart without the 500 hPa's - doesn't have the colours of the rainbow in it)

post-10703-0-11595800-1381351659_thumb.p

 

One advantage of FAX charts, over other kinds of charts, is because of the human input involved with this chart, they can be rather accurate - even up to 120 hours ahead. When the Met Office generate these charts, they look at the data from various models from the likes of their own UKMO models, the ECMWF, the JMA, the GFS and other data from the Met Office's super-computers. Due to some accuracy issues of the GFS model, the Met Office may not always include this model when basing their conclusions from the outlooks shown on various charts. As such, you will likely find that Met Office's FAX charts are very reminiscent of the pressure charts from the UKMO and/or ECMWF models.

 

FAX charts contain various symbols and features on them:

 

post-10703-0-07536700-1381351603_thumb.p

  • [*]
Low Pressure systems are marked by an 'L', while High Pressure systems are marked by an 'H'. [*]The bold black line with the semi-circles on it represents a 'warm front'. [*]The bold black line with the pointy triangles on it represents a 'cold front'. [*]The bold black line with both the semi-circles and the triangles on it is an 'occluded front'. [*]The bold black line with no triangles or semi-circles on it represents an 'upper trough'

 

Which ever way the semi-circles and triangles are pointing in shows the direction of travel of the weather fronts.

 

The FAX charts are viewable up to 120 hours ahead.

 

 

G

 

GFS - Stands for 'Global Forecasting System'. An American Weather Model. Created by NCEP. It contains billions of different types of charts, such as the standard pressure charts, convection potential, precipitation and its types, temperature charts and many more. This model, however, does come with a few issues: Whilst no model is entirely perfect, the GFS has the tendency to be over eager with the progression Low Pressure systems make from West to East, hence the term "the GFS is being too progressive" may be used in the model output thread. It can also have the habit of over-deepening Low Pressure systems (although it is not necessarily the only model that does this). NCEP often make adjustments to the model to further improve its performance.

 

This model updates four times a day and goes up to 384 hours (16 days) ahead. It is made up of the runs: 00Z, 06Z, 12Z and 18Z.

 

GEM - Stands for 'Global Enviromental Multiscale' model. It can be a suitable model for medium range forecasting and it has fairly good performance overall. The ECMWF and GFS are its only real competitors in that they are the only two other models that go up to ten days ahead and beyond (bar the ECM 32 day model and CFS long range model).

 

The GEM updates twice a day and is made up of the following runs: 00Z and 12Z.

 

 

H

 

High Pressure - 'HP' for short. Areas of stronger than average pressure - usually around 1016 hPa/millibars or higher - that fill up the gaps in the atmosphere between and around Low Pressure systems. Broadly speaking, they are not quite as circular as Low Pressure systems (though not all Lows are round). They tend to be oval or sausage-shaped and will often bring settled, calm conditions. How sunny it will be depends on the orientation and positioning of a High Pressure system. As an example for the UK, if you want warm, sunny weather, ideally the High Pressure needs to hang about just to our East with Southerly/South-Easterly winds drawing up warm, dry air from the continent. Wind tends not to be an issue with High Pressure systems - the isobars are usually well spaced out. In fact, if you're stuck under the centre of a High Pressure system, their may not be any wind at all. Winds, will, however, circulate clockwise around High Pressure cells. 'Anticyclone' is an alternative word people use to refer to areas of High Pressure.

 

Black circle showing an area of High Pressure

post-10703-0-76125600-1381351609_thumb.j

 

 

I

 

J

 

Jet Stream - Fairly narrow passages of fast high-level winds - around 29,500 to 52,500 ft above the Earth's surface (thanks to 'The PIT' for the correction Posted Image). You could almost think of it as various pathways that direct Low Pressure systems along it, and can be one of the main controllers of our weather. Depending on the positioning of Jet Stream and where passages of high-levels winds flow along, it can have an affect on where High Pressure systems migrate to, and how pronounced their ridging is. You often get one of the main streams of the Jet flowing Eastwards through from America, all the way through to the UK, or just to the North of it. In Summer, the normal position of the main stream of the Jet is to the North of us, although in some of the recent Summers, with the exception of Summer 2013, it has been behaving very strangely - the main stream of the Jet going much further South than usual steering constant Lows over, or to the South of us. Where you find both the cold air to the North and the hot air to the South meet is where the main stream of the Jet tends to flow along (or at least close to this boundary). When ever heights build to our North, this can force the main area of the Jet Stream to go South, although there has also been debates about whether the arctic sea-ice melt has affected the normal flow and positioning of the Jet Stream. When the Jet Stream becomes weak, or fragmented, with streams of the Jet split into pieces, Atlantic or Northern blocking may occur, just like what a very amplified pattern could cause.

 

Some Jet Stream examples underneath:

 

Example 1:

post-10703-0-87274000-1381351642_thumb.j

 

In this situation, you have one of the main areas of the Jet Stream flowing to the South of us attempting to bring a Low Pressure system to the South-West of us. The cyclone collides against the High Pressure to the North-East of us thanks to the slight West-South-West alignment of the Jet stream to the South-West of us. And this is kind of helping to steer the Low Pressure up against the High to our North-East. The Low does not quite slide underneath the High, despite one of the main areas of the Jet Stream being to the South/South-West of the UK.

 

Example 2:

post-10703-0-39799900-1381351633_thumb.j

 

In this example, you have an area of a stream of the Jet flowing on a South-West to North-East direction to the North-West of the UK. The stream of the Jet then curves back down Southwards through the North-Sea to the East of the UK. But the curvy Jet to the North of the UK is still far enough North to allow a High from the South or South-West to build some of its heights North-Eastwards over the UK. A South-West to North-East alignment of the Jet with it staying out to the North-West of the UK often allows High Pressure systems from the South or South-West to ridge North-Eastwards towards us.

 

One other aspect about the Jet Stream is that it can sometimes split up into two main streams - one going to our South, and another going to our North. In this type of setup, the GFS apparently has a slight bias of overdoing the power of the main stream of the Jet going to the North of us.

 

JMA - Stands for 'Japan Meteorological Agency'. One of the slightly less well-known models, although it runs a MSM (Meso-Scale Model - these are short range models) to provide small forecasts in Japan. The JMA model can be a useful asset to weather operations across the globe. It is highly regarded by the UKMO and can occasionally pick out certain weather patterns earlier than some of the other models. 

 

This model updates twice a day and goes up to 192 hours ahead. It is made up of the 00Z and 12Z runs.

 

 

K

 

L

 

Low Pressure - 'LP' for short. Often circular or elongated in shape, they are areas of lower than average pressure - usually around 1012 hPa/millibars and below - that fill up the gaps between and around High Pressure systems in the atmosphere. They bring with them unsettled and changeable weather such as frontal bands of rain or snow, sunshine and showers... and even storms! The weather can be windy too, but the tighter the spaces between the isobars (white or black lines on the charts that mark out the Low and High Pressure cells) the windier the weather is likely to be, although the Jet Stream can have an impact on this as well. The winds spin anti-clockwise around Low Pressure systems. Low Pressure systems can also be referred to as 'Cyclones'. Another aspect about Low Pressure systems is that they come in different intensities: Deep Lows have a central pressure area lower than 980 hPa, while moderate Lows are those with a centre of around 980 to 1000 hPa. Shallow Lows are those that are around 1000 hPa, or higher. Furthermore, Lows can have a habit of 'filling out' if the centre of the cyclone becomes less and less deeper.

 

Black circle showing an area of Low Pressure:

post-10703-0-15740900-1381351652_thumb.j

 

 

M

 

Model Posers - People, especially ladies, who pose as 'models' in magazines, newspapers, TV shows etc. Avoid talking about this type of model in the Model Output thread as it is off-topic and the Netweather moderators might banish you from the forum. Forever! (although if you're lucky, Chinomaniac and his moderating crew might give you a second chance). ;-)

post-10703-0-97417700-1381351654_thumb.p

 

 

N

 

NAVGEM - Stands for 'Navy Global Environmental Model'. There's quite an interesting tale behind this model. It never used to be the NAVGEM model. The former model, NOGAPS, used to run all the time. That model became discontinued and the NAVGEM replaced it. And it's very unlikely the NOGAPS model will be brought back to life. Knowing the NOGAPS was gone brought tears to the eyes of some of those who liked that model so much. Posted Image

 

But it's not all bad as the NAVGEM has supposedly had improvements done to it to increase its performance potential. Just like the expired NOGAPS, this model updates four times a day. It is made up of the following runs: 00Z, 06Z, 12Z and 18Z.

 

NMM - A Meso-scale Model which runs at a very high resolution of 12 kilometers. According to Paul, "it is high enough resolution to pick out small more localised weather features that lower resolution models will miss." (http://forum.netweat...del-for-the-uk/). This means it can pick out even more details than the GFS model and even the ECMWF model - both of which are run at lower resolutions. (You can find more about the models' resolutions here: http://forum.netweat...del-resolution/ - credit goes to lorenzo).

 

The NMM is a short range model and it updates twice a day, but you need a Netweather Extra subscription to view it.

 

post-10703-0-32841300-1381351656_thumb.p

 

Northern Blocking - Cold and snow fans number one setup in Winter. It is where blocking occurs to the North of us, and/or over the Artic region via High Pressure building to the North of us. Ideally, over the UK, you do not want High Pressure system(s) to the North of us to be too far North, otherwise Lows out West may not track far enough South to deliver cold South-Easterly, Easterly, or North-Easterly winds on their Northern side. When the Northern blocking is significant, such as a strong Greenland High Pressure system linking up with a Scandinavian High Pressure system (and with it being very close by to the North of the UK), this forces the Jet Stream to send a fair amount of its energy to the South of us and we can become locked into a prolonged cold spell with winds constantly coming in from an Easterly direction.

 

An example during late November 2010, where a High Pressure system to our North provided the UK with some chilly Easterly/North-Easterly winds:

post-10703-0-10840400-1381351743_thumb.g

 

At times, shortwaves, troughs or small Lows, usually over the Norway (or Scandinavia) area, will try to separate the link up of a High Pressure system to the North-West of us and a High Pressure system to the North-East of us into two, like so:

post-10703-0-39452600-1381351908_thumb.j

 

But in the case of the 1947 example above, it might get itself trapped inside the High Pressure systems...

post-10703-0-36277200-1381351894_thumb.j

 

An aspect you have to be careful of is that the Northern blocking isn't very weak as Low Pressure systems from below and above could easily attack the High Pressure system(s) destroying the blocking to the North. And, as such, we could easily revert back to a milder pattern with Westerly or South-Westerly winds. And this would be especially true if the Jet Steam tries to power back through over us from the West without it no longer staying to the South of us. Northern blocking can also become eradicated, if one of the streams of the Jet powers back up to the North and sends most of its energy and associated Low Pressure systems over the top of the blocking High(s) to our North and forces the High Pressure system(s) to sink Southwards over us.

 

Northerly Toppler - A short lasting period where cool or cold winds flow down from the North. High Pressure from the West is forced to 'spill' over us cutting off the Northerly flow, with Westerly or South-Westerly winds returning from the West. But sometimes, approaching Lows from the West might just flatten out an amplified High Pressure system to our West (but with the Northerly airflow still destroyed). Northerly topplers often last for around 2 days; perhaps longer, if a High Pressure system to our West is particularly strong and can withstand the power of the West to East track of the Lows for a few extra days. Despite this, one reason why Northerly airflows and Northerly topplers don't last too long is not only because they 'topple' or get broken down easily, but because of a strong stream of the Jet is always keen to try and blast through a block to the West of us. The Jet Stream is an enemy of the Northerly! Northerlies via a Greenland High, though, will tend to last longer and the chances of them toppling so easily is much less likely.

 

The charts below show the process for one of the types of 'Northerly Topplers' that happen. Example used from the period ' 26th January to 29th January 1935':

 

The Northerly Toppler part 1 - The Northerly airflow beings to get established with Low Pressure to our North-East, and an amplified High Pressure to our West. (dark blue arrows showing cold air draining down from the North)

post-10703-0-78407300-1381351666_thumb.j

 

The Northerly Toppler part 2 - The whole of the UK gets affected by a direct Northerly, although warmer South-Westerly winds (indicated by the red arrows with Low Pressure to the North) try to blow over the top of the High Pressure, and it slowly begins to get knocked Eastwards/South-Eastwards towards us. (the black arrows show it slowly beginning to get knocked over towards our direction)

post-10703-0-97567600-1381351673_thumb.j

 

The Northerly Toppler part 3 - The Northerly has almost toppled completely, with only the South-Eastern UK holding onto cold air, which is now coming from the North-East, rather than directly from the North. This is thanks to Low Pressure to the North-West of the UK knocking a portion of the High Pressure system over us, and the far North of the UK begins to get affected by milder Westerly or South-Westerly winds (shown by the red arrows).

post-10703-0-79639300-1381351733_thumb.j

 

The Northerly Toppler part 4 - It's game over for the Northerly! And the cold North-Easterly airflow now becomes concentrated over France and also areas further South-East, East, and North-East of it. The Low Pressure system which knocked a portion of the High Pressure over us, slowly makes a getaway North-Eastwards, and the orientation of the airflow over the UK is now a less-cold slack North-Westerly airflow.

post-10703-0-10772500-1381351741_thumb.j

 

 

O

 

Outlier - Where either a model's operational run, control run, or a variation of its control run is out of place amongst the other runs on the ensemble graph. The line representing the 'outlier' on a ensemble graph is very spaced out in relation to other lines on the graph, and is a good indication that the ensemble member's outlook is very unlikely to materialize.

 

post-10703-0-72846300-1381351744_thumb.p

 

On the London ensemble graph example, the black circle shows how one of the lines looks out of place amongst the other lines, although it does start meeting up with other lines towards the end of the graph. As such, an ensemble member won't be an outlier all the way through - especially at the beginning, where the ensemble members' confidence are high with the tight clustering of the lines. You can get the odd time, where one or two of the 'outliers' on a ensemble graph may have detected a new trend, and the other ensemble members (including the operational and control run), may follow suit on future runs.

 

 

P

 

Performance Statistics - Various checks are carried out on the models. And their performance in regards to how they've handled pressure patterns, and other atmospheric conditions, are recorded in numbers and graphs. You can view how accurate the models have been handling various atmospheric weather patterns here (although this is where it starts to get rather technical: http://www.emc.ncep....TATS/STATS.html / http://www.emc.ncep....l/new_acz6.html)

 

post-10703-0-57341000-1381351913_thumb.j

 

As an example, the Performance Statistics graph posted above shows the anomaly corrections of how well the various models have handled the 500 hPa and air pressure patterns in the Northern hemisphere in the last 31 days. The numbers on the Y axis on the graph relates to how well the models have handled the pressure patterns six days ahead. '1' means that a model handled the pressure patterns entirely perfectly with no errors whatsoever, while '0.2' means that a model was way off in the handling of their pressure patterns. Currently, the ECMWF model - the one in red - is performing the strongest with it receiving the highest average score of 0.808. The UKMO model, however - the one in orange - is close behind with an average score of 0.787. You can also see that the red line on the graph (representing the ECMWF) has generally been a touch higher than all the other lines on the graph (representing the other models) in the last 31 days. But their have been a few occasional brief periods within the last 31 days where the UKMO (representing the orange line) has been higher than the ECMWF's line.

 

One other aspect about this type of graph is if it has the 'NHX' part in its name, it's for the Northern hemisphere, and if it has the 'SHX' part in its name, it's for the Southern hemisphere.

 

Polar Front Jet - As its name implies, this Jet Stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 degree N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet.

 

(Suggested by: Johnholmes)

 

(Updated) Polar Maritime - 'Pm' for short. An airflow that comes from a cool/cold region from the North-West, usually with a fairly amplified High Pressure system just to the West or South-West of us. It's a type of North-Westerly airflow that is fairly common over the UK and can usually bring a sunshine and shower setup if the air is unstable enough. Cool or cold 850 hPa temperatures combined with low 500 hPa thickness (roughly around 528 or lower) usually help with the instability. In Winter, if the cold 850 hPa temperatures coming down from the North-West haven't been moderated enough by the warmish Atlantic seas, some of the showers can fall as snow, most especially in the North-West of the UK and over high ground. But in Summer, when daylight heating from the Sun plays a big role, the temperatures over the land become warmer than that of the ocean temperatures, and you will find that the heaviest showers become concentrated more towards the Eastern side of Britain.

 

Underneath is an example of a Polar Maritime airmass with Low Pressure to the North bringing cold air down from the North-West through the UK. (dark blue arrows showing direction of the airflow's travel).

post-10703-0-52211200-1381351747_thumb.p

 

(Updated) Polar Vortex - It is blobs of dark blues and purples shown on the '500hPa Thickness and Sea Level Pressure' charts from ECMWF, GFS, UKMO, JMA, GEM, GME, NAVGEM etc. Some people will refer to the Polar Vortex as the 'PV' in the model output thread (although this is where it starts to get confusing as the shortened form of 'PV' is really meant to stand for 'Potential Vorticity'). But it is essentially like an area of Low Pressure systems that, in our part of the world, dominate the Northern Hemisphere with High Pressure systems surrounding it. The Polar Vortex occupies the Stratosphere and upper Troposphere levels and contains lots of very cold air. During the Winter where it gets very strong, Low Pressure systems become more powerful and increase in size as the Polar Vortex adds 'fuel' to them. This is thanks to lots of very cold air getting mixed in with warm air further South resulting in some very powerful Lows. But as the affects of the Polar Vortex weakens significantly during Summer, Low Pressure systems tend to be much smaller in size and less powerful.

 

The '500 hPa and Sea Level Pressure' chart below [which I nicked from an old Model Output Discussion thread ;-)] shows the whole of the Northern Hemisphere and the areas of dark blues and purples, (the Polar Vortex), that dominates some of it.

post-10703-0-67633100-1381351749_thumb.p

 

If wanting a prolonged cold setup, you never want the Polar Vortex to be too invasive to the North-West of us, and especially over Greenland and Iceland, as it can make it hard for Northern blocking to form. The exceptions would be if a 'SSW', or other form of Stratospheric Warming, event occurs (and even then, it would have to be in the right place), or if we get some notable 'WAA' going up into Greenland or Iceland. (see 'Sudden Stratospheric Warming' and 'WAA' for more details). (There's probably likely to be other methods that I've admittedly not thought about, or know about, that could put the Polar Vortex under threat).

 

(New!) Potential Vorticity (PV) - It's a mathematical construct, which combines vorticity with stability. This makes it difficult to imagine, because it combines a thermodynamic contribution (stability) with a dynamical element (vorticity). However, it's useful because it is conserved in the absence of friction and diabatic (meaning energy is transferred in and out of the boundary of the air parcel) effects. So people use it as a "tracer". Displayed as a field, it has many applications, and one of them is to identify the polar vortex. There will be a gradient of potential vorticity in the north/south direction.

 

post-10703-0-51380500-1381351978_thumb.j

(More Info: http://iopscience.iop.org/1751-8121/43/17/172001/fulltext/)

 

(Suggested by 'forecaster')

 

 

Q

 

R

 

Ramp - Nope! Not this ramp...

 

post-10703-0-24168600-1381351755_thumb.p

 

It's the sort of ramp where someone greatly exaggerates what a chart beyond a certain outlook is showing. For example, if a model is showing an exceedingly warm deep-seated Southerly airflow in FI, some people might try and over-hype the outlook (usually for the fun of it), and to help raise excitement over the particular outlook.

 

Another example (from Richie V): exaggerating the position of a Scandi High at T+360 hrs.

 

Retrogressing High / Retrogression - When a pressure feature, such as a High Pressure cell, moves Westwards against the mean flow. For example, a High over the UK can sometimes retrogress towards Greenland. Whether a High retrogresses or not depends on various aspects such the dominance of the Polar Vortex (during Winter) and how obtrusive the Jet Stream is; if a powerful stream of the Jet is flowing over the top of the High, then it could have a hard time retrogressing (unless the main area of the Jet Stream suddenly heads South, or if the pattern significantly amplifies). It could also depend on other factors such as whether a Low Pressure system can pump up some 'Warm Air Advection' Northwards on the High Pressure system's Western side and help encourage heights to build further Northwards. Although if heights build Northwards only, rather than back Westwards or North-Westwards, then it doesn't really count as Retrogression.

 

Ridge / Ridging - A curvy or pointy area of a High Pressure system. It can be quite elongated. A High Pressure system will, at times, send some of its ridging from one area to another. This idea of 'ridging' means a High Pressure system is trying to extend its heights to other places.

 

An example of a ridge of High Pressure:

post-10703-0-53935700-1381351768_thumb.p

 

 

S

 

Shortwave - It is essentially like a little Low Pressure system that hasn't quite become a proper closed-off circular Low (although they can sometimes develop into circular Low Pressure systems depending on the conditions within the atmosphere). Shortwaves are marked by a small(ish) curvy-shaped wave on the Pressure charts and are often a part of big Low Pressure systems and Longwave Troughs. Cold air above can also often be a part of Shortwave features.

 

Some examples of shortwaves on the chart below (circled in black):

post-10703-0-11111700-1381351771_thumb.p

 

Slack airflow - Where the isobars on the pressure charts are very spaced out. A slack airflow is associated by gentle winds, or a gentle breeze. Virtually, the weather is non-windy. 

 

Sudden Stratospheric Warming - 'SSW' may also be used. A sudden warming event occurs within the Stratosphere and, if notable enough, can separate the Polar Vortex (and its blue/purple blobs) into separate segments. Or even just make it less stronger in general. Where ever an 'SSW' event occurs, heights will likely to keenly rise in that area. As such, if you wanted a cold setup with a strong, sturdy, Greenland High, a notable 'SSW' event would be ideal around that area to help split the Polar Vortex in that area into two, with a segment of it getting knocked further West and with a segment of it being knocked Eastwards towards Scandinavia. In fact, getting a segment of the Polar Vortex to drop down to the North East of us can help promote Scandinavian Troughs and, if close enough, the UK can become influenced by a very cold Northerly flow from a Trough/Low over Scandinavia.

 

 

T

 

Trough - An area of curvy Low heights away from a main Low Pressure system's centre. Weather fronts and convective weather can be a part of troughs. Troughs are also a part of stretchy Low Pressure systems, and they can by quite pointy shaped. In the United Kingdom, a trough will be marked by a solid line on the Fax charts. There are, nevertheless, different kind of troughs such as Upper-Air Troughs, Surface Troughs, Shortwave Troughs and Longwave Troughs.

 

The black circle on this chart shows an example of a trough tracking towards the UK:

post-10703-0-84581100-1381351901_thumb.j

 

T+??? - To refer to a time-frame within the models. For example, 'T+48' would refer to a model outcome 48 hours ahead.

 

 

U

 

UKMO - A weather model owned by the UK Met Office. It has comparable performance to ECMWF, although this model can only be viewed up to 144 hours ahead.

 

This model updates twice a day and is made up of the 00Z run and the 12Z run.

 

Undercutting - A situation where cold air undercuts warm air (thanks to 'forecaster' for the correction). Sometimes, on the charts, you may see approaching Low Pressure systems out West that try to track to the South under a High Pressure system to the North. This sort of scenario could be known as an 'undercutting low' with cold air from the North or North-East getting injected into the low and tries to mix out the warm air wrapped around it. Easterly winds can develop over the UK if the Low undercuts to the South of the island.

 

 

V

Vortex - Part of the Polar Vortex.

 

 

W

 

WAA - Warm Air Advection. This is where warm air from the South gets drawn up Northwards and moves into areas of cold air. In the Northern Hemisphere, the warm air would get drawn up on a Low Pressure system's Eastern side. When warm air gets 'advected' Northwards towards areas of cooler air, it can help strengthen High Pressure systems in that area, or even help new ones to form. This is why if you want a cold, blocked setup in Winter with High Pressure to our North or North-West, getting WAA heading up directly towards Greenland is seen as an encouraging sign.

 

Below is an example from January 1947 of WAA being pumped Northwards:

post-10703-0-84245700-1381351922_thumb.j

 

The example above shows a long drawn Southerly airflow to the West of the UK bringing warm air up from the South (shown by the red arrows). Mild 850 hPa temperatures are also brought Northwards (see the example below):

post-10703-0-22832500-1381351915_thumb.g

 

Prolonged Northern blocking soon followed after this event in 1947, and the rest of that Winter was very cold.

 

Weather Models - Charts, such as the ones you might be looking at right now, that display graphical weather forecasts and predictions, and use a large range of data. Their are a wide selection of weather models that all have varying purposes within our global Meteorological Weather operations. Satellites, radio equipment and weather balloons are used to detect the atmospheric and oceanic conditions on our planet, with special computers or super-computers working out all sorts of mathematical equations from the data that's gathered to then provide  forecasts on various models. There are many different types of models, and various different model viewers in which you can view the charts from. In the UK, you can view charts that cover a portion of the Northern hemisphere, or the whole of the Northern hemisphere. Viewing versions of models that show the whole of the Northern hemisphere is highly recommended if you want to get a broader picture of how the pressure conditions and atmospheric conditions are behaving. The way one pressure system behaves on one side of the world can have impacts as to what happens in other areas of the globe. This is why viewing versions of charts that cover a broad area can be useful.

 

Just one other thing to add is that you can also view historical charts dating to over 100 years back as well.

 

WMA - Weather Model Addiction. Something some weather site member's are prone to catching. The addictions may include frequent model checking, frequent model analyzing and frequent viewing of the weather model threads across the World Wide Web.

 

 

X

 

Y

 

Z

 

Zonal Setup - The type of pattern that can bring about a North and South divide. On the whole, the Southern half of the UK remains settled with high pressure influential to the South, while the Northern part of the UK remains more cooler and unsettled with Low Pressure systems and Longwave troughs being more influential in those areas. Hence you begin to see why this is called a 'Zonal Setup'. Zonal patterns usually bring mild, or average conditions with a fairly long draw of winds from the West or South-West. It is a flat pattern overall.

 

However, not all Zonal setups bring mild or average temperatures. If winds come down from the North-West with High Pressure influential to the South-West of the UK and Low Pressure influential to the North/North-East, a cold Zonal setup can arise. Snow is also possible (if occurring in Winter), especially to the North and on high ground. People will also refer to a 'Zonal Setup' as 'Zonality'.

 

 

123456789...

 

(New!) 500 hPa - Occupying the atmosphere at around 5,500 meters above the Earth's Surface, these can be used to determine how warm or cold the weather may be. Most people on here use the combined '500 hPa and Sea Level Pressure' charts, not because of all their pretty colours, but because of the fact they can be used to determine various weather conditions, as well as what the flow will be like, at around the 500 hPa/mb level. Not only is 500 hPa around 5,500 meters above the surface of the Earth, but it is also around 5,500 meters below the Tropopause level (where our weather is formed). 500 hPa is essentially the pressure area mid-way up the atmosphere.

 

There are also the '500 hPa to 1000 hPa Thickness' charts you can use. The difference between the pressure at around 1000 hPa and 500 hPa is the thickness level, and determines the amount of volume of air their is between those two levels. A thickness of around 570 decameters (dm) or higher is often associated with warm or hot weather (they occupy the oranges and reds on the charts), while a thickness of around 527 decameters (dm) or lower usually guarantees any precipitation that falls from the sky will be wintry (they occupy the pale and dark blues on the charts).

 

Moreover, the 500 hPa charts can be used to work out what the instability in the atmosphere will be like. In Winter, the turquoise greeny colours, pale blues and dark blues, can indicate great instability.

 

An example of a 500 hPa (combined with 'Sea Level Pressure') chart below. The colour code line to the left represents the 'thickness' amount.  

post-10703-0-96171800-1381351604_thumb.p 

 

 

850 hPa Temperatures - Some people refer to these as 'Upper Temperatures' or just 'Uppers' in the model output thread, although technically this is not correct. According to 'forecaster', "if you were to say "uppers" to a meteorologist (without any further clarification), he/she would automatically think you are talking about 500 hPa or 250 hPa, and most likely the geographical height, not the temperature." If you're thinking about using the term 'Uppers' in the model thread, don't do it no more.  Posted Image

 

The 850 hPa temperatures refer to the temperatures high up in the atmosphere (around 1,500 meters from the Earth's surface) and can be one of the factors that determine what the temperatures may be like at the surface. The warmer the 850 hPa temperatures are, the warmer the surface temperatures are likely to be. But this won't always necessarily be the case, as cloud, mist, or heavy rain can help to suppress the temperatures somewhat.

 

Likewise, the colder the 850 hPa/mb Temperatures are, the colder the surface temperatures are likely to be, although again, certain weather scenarios can have an impact on this. In a sunshine and shower setup, you really need the 850 hPa temperatures to be around -7*C or colder for snow to reach low levels. However, even 850 hPa temperatures around -5*C can suffice if the shower is very heavy (encouraging Evapourative Cooling), and if surface temperatures near the ground are particularly cold with no real layer of warm air above. As mentioned in the '500 hPa' term above, the 500 - 1000 hPa Thickness needs to be around 527 dm/dam-line or lower, as well, for showers to fall as sleet or snow. But again, like with the 850 hPa temperatures, you could get away with the 500 - 1000 hPa thickness being a little higher if the showers are extremely heavy.

 

For frontal bands of precipitation, you can get away with the 850 hPa temperatures being as warm as 0*C and snow can still fall to low levels. This, nonetheless, is mostly true in situations where an Atlantic front is forced to bang up against High Pressure to the North (with Low Pressure to our South). The front then slowly pivots and gets pulled Westwards/North-Westwards through the UK via cold Easterly or South-Easterly winds.

 

An example of an '850 hPa and Sea Level Pressure' chart below.

post-10703-0-40733300-1381351910_thumb.p

 

The greens, blues and dark blues represent 850 hPa Temperatures around 0*C or colder, while the yellows, oranges and reds represent 850 hPa temperatures around 0*C or warmer. Helpfully, the 850 hPa Temperature gradients are marked by black lines with numbers over the top of the lines to show how warm or cold the 850 hPas are expected to be in that area. Over the far North of Scotland, for example, the 0*C 850 hPa Temperature line crosses a part of it, with the -5*C 850 hPa Temperature line crossing a part of Iceland further North-North-Westwards. You can also refer to the model's 850 hPa Temperature colour-coding line to the right of the chart to work out how warm or cold the 850 hPa temperatures are over your area. (However, if you have a Netweather Extra subscription, or are thinking about getting one, you can view the 850 hPa temperatures just for the UK only).

 

 

------------------------------------

 

Weather FAQS: (Website recommendation by Johnholmes: http://weatherfaqs.org.uk/

 

Share this post


Link to post
Share on other sites

Surely it's time for Shannon entropy to be defined.

 

Personally I hate this term when it is a measure of uncertainty or volatility.  On the model threads it's used to indicate high uncertainty but really it's just a measure of uncertainty and can be high or low.

 

Entropy just means informational content - and following on from this the likelihood of a predicted event occurring beyond that you get into logarithms pretty quickly.  

 

A shannon is just the unit measure of an event occurring 1 shannon = 50% given the forecast of future events. 

 

The only important thing to take away is high Shannon entropy = high uncertainty (volatility of potential future outcomes).

Edited by Trom

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...