Model Output Discussion - 23rd Jan - 12z onwards

[Duane S.]

Good grief what is the 18z up to!? Running that not insignificant system SE to our W. Much interest

[shaky]

Forgive me for asking.. But is that good or bad?

its a good thing as blast from the past has just posted! ! Rather have that than the shortwave joining forces with the low coming out of the states!! Always a chance of too much energy going north east then!!

[Great Plum]

Is that a channel low that's just appeared?

[Snowman.]

18z gfs is approaching channel low territory.

Beaten by great plum from from sw bumping straight into some cold air.

Edited January 27, 2015 by SN0WM4N

[Captain Shortwave]

That shallow low misses the UK (precipitation goes into France, snow does graze the south coast just), but it may pull the trough back a little west as more Arctic air digs in behind the system.

This squeezes the Atlantic ridge, potentially increasing the chances of something being cut off in the north Atlantic

GFS still shows that disturbance running south too on Monday.

 

In the end it still ends up with the Atlantic high moving in, though the convective window has been extended a little longer, snow showers still going strong by next Wednesday.

Edited January 27, 2015 by Captain shortwave

[nick sussex]

More changes on the GFS 18hrs run, this time that shortwave separates cleanly ahead of the upstream low!

 

This was the original view of the outputs and especially of the GFS but this seemed to get dropped only to make a "special guest appearance" and a welcome one again now.

[Met4Cast]

Pub runs been on the booze.

 

Better amplification, better shortwave tracking and to top it all off, it almost gets a channel low going too

[Ice Day]

156 and the northerly still going strong, it's going to feel bitterly cold.  Superb run so far!

 

[Hotspur62]

Is that a channel low that's just appeared?

Yep it certainly looks like it but the way every run is changing goes to show that the models are really struggling to find consistency atm.At least the GFS is more positive like ECM and UKMET(mind you it is the pub run though lol!!)

[BLAST FROM THE PAST]

Many thanks BFTP and you're just down the road from me..

 

Indeed, also lok at the more vertical sheer of the trough W Atlantic, far more held back than the 12z, and also look at the disturbance coming down the UK on more N/S trajectory.....big improvement again...more like UKMO/ECM?....or maybe my drum beating?

 

 

BFTP

[John Cox]

Well well gfs reverts back to what it was showing 36 hours ago!! Shortwave cuts off in the atlantic and heads south east west of ireland!!

Well spotted and exactly my thoughts. I thought i was seeing things. Would be great for Ireland (MBY ) and NW parts of England (Snow wise)

[Mark Neal.]

156 and the northerly still going strong, it's going to feel bitterly cold.  Superb run so far!

 

Just wondering if that kink to the NE/E Anglia would bring precipitation? Looks interesting. Better 18z thus far! 

[Penrith Snow]

Yes, a great run and more in line with the latest MetO MRF on the Beeb that put any breakdown well into FI.

Don't forget early February is well know as a time that produces cold spells and HLB as the Atlantic weakens in response to lower SST so Stella charts are more likely to come off than any during December.

Andy

[chionomaniac]

So far we are seeing a 100hr toppler from T+72 programmed on the GFS 18Z. If that is dry for those 100hrs I'll be gobsmacked....

[damianslaw]

BBC a little perplexing, not sure how we can come under the influence of a high pressure with the Jetstream profile they were showing i.e. to our west with us on the cold side of the jet. The latest GFS combined with ECM are showing an increasingly likely scenario for early next week, i.e. shortwave activity to the NW prolonging the cold unsettled theme and sending the azores high northwards with no phasing of any low heights off the NE USA seaboard. A locked in cold unsettled theme.. with mix of polar and arctic maritime air lasting a number of days.

Edited January 27, 2015 by damianslaw

[Met4Cast]

So far we are seeing a 100hr toppler from T+72 programmed on the GFS 18Z. If that is dry for those 100hrs I'll be gobsmacked....

 

Is it still classified as a toppler if it lasts that long? I assumed topplers were 24 hour affairs.

 

GFS removes the shortwave that was over Iceland at 180hrs and begins building heights Northwards.. very, very similar to the later frames of the 12z ECM

[Banbury]

Yes, a great run and more in line with the latest MetO MRF on the Beeb that put any breakdown well into FI.

Don't forget early February is well know as a time that produces cold spells and HLB as the Atlantic weakens in response to lower SST so Stella charts are more likely to come off than any during December.

Andy

That was yesterdays

[chionomaniac]

Is it still classified as a toppler if it lasts that long? I assumed topplers were 24 hour affairs.

 

GFS removes the shortwave that was over Iceland at 180hrs and begins building heights Northwards.. very, very similar to the later frames of the 12z ECM

I consider it a toppler if we don't see undercutting, so in my eyes yes, because the jet is always riding on the top. But I'll take it!

[Ice Day]

-8's across the country at 186.  This could be one of  the best northerly's for many years and although the northern contingent should do best, this could deliver anywhere!

 

[Geordiesnow]

BBC a little perplexing, not sure how we can come under the influence of a high pressure with the Jetstream profile they were showing i.e. to our west with us on the cold side of the jet. The latest GFS combined with ECM are showing an increasingly likely scenario for early next week, i.e. shortwave activity to the NW prolonging the cold unsettled theme and sending the azores high northwards with no phasing of any low heights off the NE USA seaboard. A locked in cold unsettled theme.. with mix of polar and arctic maritime air lasting a number of days.

 

I wonder if the BBC uses the 00Z ukmo run on the longer range forecasts as the chart Nina Ridge shows did not look like the UKMO 12Z run too me although that said, they would have more data so maybe that is the way it would of headed?

 

If tomorrow UKMO runs show something you described then expect the BBC's longer range forecast to change also. 

[Severe Siberian icy blast]

A fantastic pub run upto t180 , absolutely no point in taking it serious from this point in . Just for fun and trends remember . A very unstable northerly that's for sure . The coldest spell of the year . Great .

[Mucka]

More changes on the GFS 18hrs run, this time that shortwave separates cleanly ahead of the upstream low!

 

This was the original view of the outputs and especially of the GFS but this seemed to get dropped only to make a "special guest appearance" and a welcome one again now.

 

Yes quite a surprise that Nick, looked like that option had been dropped and we had to worry about it phasing with the next low.

A bit like a celeb who has fallen from grace and suddenly turns up on a panel show - it looks a bit desperate but reassuringly familiar

Edited January 27, 2015 by Mucka

[J10]

 

-8's across the country at 186.  This could be one of  the best northerly's for many years and although the northern contingent should do best, this could deliver anywhere!

 

 

 

OF course the added advantage of a sustained "northerly" is that any mild weather from this week or even the less cold spells on Friday will have long since left the building.

 

How often in recent times have we had cold air up in the atmosphere but milder air at the surface "spoiling" things.

[Vorticity0123]

What is it about this particular northerly airstream that makes it so unstable? Is it the proximity of the Low Pressure?

 

Short answer

 

The short answer to your question is yes, the presence of low pressure (at higher altitudes, being coincident with very cold upper air), makes the air more unstable, as the gradient in temperature between the surface and higher altitudes becomes larger.

 

Long answer

 

The long answer requires some explanation on stability. To avoid things from getting too complex, I will not go into detail about Skew-T diagrams. (If one wishes to have an explanation via Skew-T charts, just ask )

 

Stability of the atmosphere

 

(Un)stability has to do with the 'tendency' of a parcel of air to rise from a certain position (in altitude) or to stay at the same position. This tendency is related to the temperature a parcel has compared to its environment.

 

Imagine a parcel starts to rise from a certain altitude (say, 1000 meters). The parcel then cools adiabatically (meaning it does not 'mix' with its environment) up to a certain height. If a parcel then finds itself being cooler than its environment (stable conditions), it will drop back to its original position (remember that a certain volume of cold air is in general heavier than an equal volume of warm air). However, if the parcel is warmer than its surroundings (unstable conditions), it will continue to lift to even higher altitudes until it reaches a height when the parcel becomes saturated. This height is the height where clouds start to form. Thereafter, the parcel will still continue to rise up to where it finds itself in an environment that is warmer than the parcel itself. (Note that the cooling process during ascent of a parcel is different when the parcel is saturated, but goes too far to treat this in detail). The parcel then stabilizes, and this can (under great simplifications) indicate the height of a cloud.

What this comes down to is that when the air is unstable, showers are easier to form based on the parcel analogy described above.

 

A good measure of stability is the change of temperature with height. If the temperature drops sharply with height, the atmosphere can be considered unstable (referring back to the parcel analogy). When the temperatures decreases only weakly with height or even increases with height, the atmosphere is stable (from the parcel analogy: a parcel will find itself colder than its environment after ascent, meaning it will drop back to its original position).

 

To illustrate this, below is a series of images showing the parcel analogy:

 

Stable situation

 

Unstable situation

 

In the images above, the x-axis indicates the temperature, while the vertical axis (y-axis) denotes height.

For both graphs, the red line indicates the change in temperature over height of the environment of a certain parcel (technically spoken: lapse rate). Note that the environmental temperature drops much more with height in the unstable situation than in the stable situation.

The black dot indicates a parcel on a random level. The arrow pointing to the upper-left stands for the adiabatic rising (and the accompanied cooling) of this parcel. For both images, this parcel cools at a same rate (so the black arrow has the same slope to the left on both images).

 

As can be seen in the stable situation, the parcel becomes colder than its environment after rising. Therefore, it is being forced downward again. On the other hand, in the unstable situation, the parcel becomes warmer (and thus lighter) than its environment, indicating the parcel will continue to rise.

 

Temperature difference representation between surface and aloft

 

Coupling the part given above back to the presence of low pressure at higher heights and stability, one can realize that the difference in temperature between the surface and aloft (I'll be using the 500 hPa level, being about 6 km, as a reference for now) must be very large in order to have an unstable atmosphere. If the atmosphere can be more or less unstable when the temperature at the surface stays the same, the temperature at 500 hPa has to vary accordingly. In other words, changes in stability can be explained by variations in temperature at 500 hPa level.

 

Simplifying a bit, one can assume as a general rule that low pressure activity at higher altitudes is accompanied by lower temperatures at that same level. (more in-depth explanation can be found here). This means that, in general, low pressure at higher altitudes indicates the atmosphere is more unstable than when high pressure is present at higher altitudes (and thus showers are by approximation more likely to form when low pressure is present at higher altitudes)

 

Seasonality in stability

 

An important difference between summer and winter regarding stability is that the surface is usually colder during winter than summer. This means that the upper air has to be colder in winter to acquire instability than during summer.

 

Combing to current weather

 

The weather that we are about to observe this Thursday up to the weekend is a very nice example to illustrate the relation between stability and the presence of low pressure at higher altitudes. Therefore, given below is the pressure forecast of the GFS for next Thursday:

 

GFS surface level pressure and 500 hPa heights (colours), 18Z T+48

 

It is important to focus solely on the 500 hPa heights, indicated by colours. As a rough guide, purple/blue colours indicate low heights (lower pressure activity at 500 hPa height) while yellow/red colours indicate high heights (high pressure presence at 500 hPa height).

 

Note that there is a very deep trough (low pressure area) present at 500 hPa height over Western Scandinavia and Northeastern UK. Referring to the explanations, low pressure at 500 hPa should coincide with lower 500 hPa temperatures. Much higher heights (relatively higher pressure) are present to the southwest, west and north of the UK. Therefore, the 500 hPa temperatures for the same timeframe (from the GFS forecast) are given below:

 

GFS 500 hPa temperatures, 12Z T+54

 

The runs of the GFS are two different ones (18Z above, 12Z below), but they are valid for the same timeframe. Since big changes between runs for 2 days out are not likely, I'll therefore assume that both runs show the same situation.

 

Note that there is a large swathe of very cold 500 hPa temperatures present to the east of the UK (down to -38*C). This is associated with the very deep trough present to the east and over the UK. Much warmer 500 hPa temperatures can be found to the south and west of the UK, while the 500 hPa temps are also slightly warmer to the north of the UK.

 

The surface temperatures do not vary much in the neighbourhood of the UK at this timeframe (except for land/sea effects). The surface temperature chart for this Thursday can be found here.

 

Thinking of the parcel analogy given in the beginning of this post, it becomes evident that showers are more likely to develop over or to the east of the UK than to the north, west or south (assuming equal surface temperatures).

 

Northerlies and stability

 

Regarding wind, there are northerlies present over and to the north of the UK, while to the east of the UK there is barely any wind. (you can find the wind forecast from the GFS here). However, as we can see above, the air to the north of the UK is less cold than over the UK itself. This means that despite the fact that the northerlies are stronger to the north of the UK are stronger than the ones over the UK, the air over the UK is more unstable (due to the lower upper temperatures).

 

Exceptions

 

One possible exception is the presence of a polar low. Such systems may pop up out of nowhere and yield a lot of snow, being completely overlooked by global models. Quoting from s4lancia:

 

Worth remembering that the much overused polar low can only truly form in a true AM airmass and one of a decent duration. Will be worth keeping an eye nearer the time for the formation of these rare little beasts from next weekend when a northerly kicks in. No point trying to look outside 12 hours or so as they normally pop out out of nowhere and disappear as quickly as they arrived. Great fun to be underneath one at the time though!

 

Summary

 

To summarize the relationship: low pressure at high heights is coincident with cold upper air, yielding a bigger temperature difference between the surface and aloft. This yields a more unstable atmosphere.

 

It has to be kept in mind, though, that this relationship is simplified, so it does not have to match the actual conditions in any case.

 

Conclusion

 

Even a very short question can have a very long answer, and in fact there was much more that possibly could have been told about this. I hope this answers you question sufficiently . If something is not clear, do not hesitate to ask! Furthermore, I am by no means an expert on this subject, so any additions/corrections are also very welcome!

 

Finally, if one would like some explanation about this via Skew-T diagrams, that's possible (probably with some delay ). A good read about Skew-T diagrams, which could also serve to visualize stability, is given below:

 

https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

 

EDIT: Added graphical representation of stability.

 

Sources:

http://www.wetterzentrale.de/topkarten/fsavneur.html

http://www.keesfloor.nl/weerkunde/10neerslag/10neerslag.htm

https://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/

https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=ukuk&MODELL=gfs&MODELLTYP=1&BASE=-&VAR=z500&HH=48&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

Edited January 27, 2015 by Vorticity0123

[A.J]

a few posts moved over to the ramps thread (in case your wondering where they are)