Model Output Discussion - 23rd Jan - 12z onwards

[nick sussex]

The updated fax charts look interesting:

 

T96hrs occlusion heading south, the 528 dam is well clear of the UK:

 

 

The T120hrs looks very wintry:

 

 

 

[pages]

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....

Still hasn't toppled at 204hr  . think this very unlikely thou i expect to tomorrow runs to show that low of the south west of greenland at 132hr disrupting with better height towards Greenland-Iceland. I bet at least one of the models shows a proper easterly around 240hr tomorrow.

[Ice Day]

At 222 we have cold now encroaching from the east, is the beast starting to stir????

 

[SMU]

Nice post Vorticity - so good ive actually liked it - which I never bother to do!

 

GFS at 234 sees the -8c line heading back towards the E & SE of the UK

 

http://modeles.meteociel.fr/modeles/gfs/runs/2015012718/gfsnh-0-234.png?18

http://modeles.meteociel.fr/modeles/gfs/runs/2015012718/gfsnh-1-234.png?18

 

great-  possible ice days!

 

S

[weather eater]

Wow lots of changes really quite early on that demonstrate what absolute forecasters nightmare this could be. One thing for certain this is not looking like a standard northerly tippler and therefore it seems to me all bets are on the table including heights to our NW, however,  a MLB over or close to the UK still looks the form horse for the end of next week.

[Mark Neal.]

Nice post Vorticity - so good ive actually liked it - which I never bother to do!

 

GFS at 234 sees the -8c line heading back towards the E & SE of the UK

 

http://modeles.meteociel.fr/modeles/gfs/runs/2015012718/gfsnh-0-234.png?18

http://modeles.meteociel.fr/modeles/gfs/runs/2015012718/gfsnh-1-234.png?18

 

great-  possible ice days!

 

S

Lovely stuff showing, what you think of the fax charts, Steve as Nick S has posted, look wintry to say the least! Interesting.

[Banbury]

The updated fax charts look interesting:

 

T96hrs occlusion heading south, the 528 dam is well clear of the UK:

 

fax96s.gif

 

The T120hrs looks very wintry:

 

fax120s.gif

The 120 looks excellent but N Ridge saying dry inland over the weekend ?? Puzzled a bit

[Met4Cast]

The 120 looks excellent but N Ridge saying dry inland over the weekend ?? Puzzled a bit

 

That's just an outlook, a possible outcome, forecasts are likely to change between now and then. 

[Weather Wizard]

I think the next week will be a lot more noteworthy for its temperature (specifically wind chill) than its lack of precipitation

Back to trend for the 12z. A week of northerlies then high pressure. Could well be a very dry period for some places.

 

[Duane S.]

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 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 more easy to form based on the parcel analog 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 analog). When the temperatures decreases only weakly with height or even increases with height, the atmosphere is stable (from the parcel analog: a parcel will find itself colder than its environment after ascent, meaning it will drop back to its original position).

 

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 unstability than during summer.

 

Combining 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 presece 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 neighborhood 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 analog 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:

 

 

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/

 

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=

 

What a great post. Any newbies out there would do well to bookmark that and use it for future reference while learning.

 

Put my earlier short response to the same question to shame!

[TEITS]

 

I will add much of the above is aimed at S parts of the UK. Some parts of NW England/Scotland/Ireland could see heavy snowfalls.

. Beyond and although the evidence points towards this cold spell ending around next Wednesday, I am not yet convinced it will. What could happen is instead of a return to milder W,lys we see high pressure become centred across the UK. Obviously the snow risk fades but the cold temps remain. Another option is an NE/E,ly which cannot be ruled out.

Come to daddy!

 

http://www.wetterzentrale.de/pics/Rtavn2401.gif

http://www.wetterzentrale.de/pics/Recm2401.gif

 

So potentially snow tomorrow/thurs. More snow at the weekend into next week via a N,ly. Finally potentially an E,ly end of next week. Have to laugh though as i didn't rate this cold spell a few days ago. Glad I have been wrong yet again this winter.

[bluearmy]

The 120 looks excellent but N Ridge saying dry inland over the weekend ?? Puzzled a bit

I've never seen a day 5 fax verify!

It will look different by tomorrow's T96

[Eugene]

I think the charts we are are seeing are coming at the right time, next week is February and February traditionally is when the jet quietens down and anticyclones become more dominant, if it was December/most of January its more likely the jet would smash through, it definately feels to me we are entering a quieter drier phase, February on average is one of the driest months of the year.

[SMU]

Lovely stuff showing, what you think of the fax charts, Steve as Nick S has posted, look wintry to say the least! Interesting.

 

Yes very wintry & with each passing system the air gets colder...

 

If you can build up a bit of snow cover then ice days become a factor.

 

Its certainly not going to be dry- most places will get in on the action-  I suspect past  day 5 all troughs will be all snow....

 

S

Edited January 27, 2015 by Steve Murr

[pages]

Come to daddy!

 

http://www.wetterzentrale.de/pics/Rtavn2401.gif

http://www.wetterzentrale.de/pics/Recm2401.gif

 

So potentially snow tomorrow/thurs. More snow at the weekend into next week via a N,ly. Finally potentially an E,ly end of next week. Have to laugh though as i didn't rate this cold spell a few days ago. Glad I have been wrong yet again this winter.

The Beast is coming, not from the ridge toppling and linking with the russian high thou. It will come from disruption around 120-140hr, creating a wedge between Greenland and scandie. the candian vortex is still being modeled to strong, watch subsequent runs slowly increase the amount of energy going under. In my opinion they won't pick this up until within 120hrs. Think it will be tomorrow afternoons runs that will suddenly show this. 

[nick sussex]

The 120 looks excellent but N Ridge saying dry inland over the weekend ?? Puzzled a bit

You're not the only one! lol Generally with northerly topplers the period of instability weakens quite quickly as ridging in the Atlantic moves in . But the Euro troughing is parked for quite a few days, and the fax charts show two occlusions moving s/se in unstable air so I'm somewhat bemused by the BBC forecasts.

 

Of course the fax charts are subject to change and occlusions and troughs can come and go but I can't see this set up being your bog standard showers just on coastal fringes scenario.

[Mucka]

Come to daddy!

 

http://www.wetterzentrale.de/pics/Rtavn2401.gif

http://www.wetterzentrale.de/pics/Recm2401.gif

 

So potentially snow tomorrow/thurs. More snow at the weekend into next week via a N,ly. Finally potentially an E,ly end of next week. Have to laugh though as i didn't rate this cold spell a few days ago. Glad I have been wrong yet again this winter.

 

 

If you have an opinion on what the future weather will bring then you will be wrong sooner or later. 

I thought this would be a 3 or 4 day toppler too at first but more than happy to be wrong.

The only fly in the ointment for me is the lack of cold 850's to our N and NE which I think we will pay for on Friday where many places (certainly low lying) will probably see rain and a slushy thaw of any lying snow. That is very frustrating because if the cold air had been there to tap into this would of been a very cold and snowy spell.

 

I can get over that though as we are quickly back in the colder air and if we can get some blocking and deeper cold later it could still be a magical spell of winter weather.

 

 

 

The control run backs the Op and has the shortwave dives SE with no phasing with the low behind.

 

Edited January 27, 2015 by Mucka

[Geordiesnow]

You're not the only one! lol Generally with northerly topplers the period of instability weakens quite quickly as ridging in the Atlantic moves in . But the Euro troughing is parked for quite a few days, and the fax charts show two occlusions moving s/se in unstable air so I'm somewhat bemused by the BBC forecasts.

 

Of course the fax charts are subject to change and occlusions and troughs can come and go but I can't see this set up being your bog standard showers just on coastal fringes scenario.

 

Even if the winds goes too NNW'ly which imo is the perfect set up for the shower wishbone effect? If the winds are more sue North/NNE'ly then Eastern areas get more in the way of showers converserly, if the winds are NW'ly then it be Western areas aslong as the instability and pressure is low enough.

 

Still time for details like to change mind which will make all the difference regarding how widespread any shower activity will be, UKMO is probably the best for this potential really. 

[Duane S.]

What the 18z shows is how important that early seperation and driving SE of energy is to sustaining the Northerly and would also help open the door later on to a Continental flow. A great run to conclude a very good day of model output I would say!

Edited January 27, 2015 by s4lancia

[Mucka]

I really hope this 18z suite isn't rogue or lacking data and is the start of a trend because the ensembles are a decent upgrade too, with this sort of thing appearing in the mid term.

 

 

Edited January 27, 2015 by Mucka

[pages]

This is what i think will be the closed to verifying at t126. Just fits the trend and the background signals.

 

whatever is goes on to show, i think more energy will go under in reality.

[Pembroke Dangler]

Even if the winds goes too NNW'ly which imo is the perfect set up for the shower wishbone effect? If the winds are more sue North/NNE'ly then Eastern areas get more in the way of showers converserly, if the winds are NW'ly then it be Western areas aslong as the instability and pressure is low enough.

 

Still time for details like to change mind which will make all the difference regarding how widespread any shower activity will be, UKMO is probably the best for this potential really.

In this set up the isobars show direct north to south alignment come Sunday, but am I right in saying that the wind direction will be nnw because in a low pressure system the wind is drawn towards the centre? I may be completely wrong but sure iv seen this explained before in here

[Ice Day]

I really hope this 18z suite isn't rogue or lacking data and is the start of a trend because the ensembles are a decent upgrade too, with this sort of thing appearing in the mid term.

 

 

 

Mucka, is your comment re 'lacking data' based on something concrete or is it just a 'too good to be true' feeling?  Fantastic chart by the way!

[The Enforcer]

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

Thank you so much for all that detail. Is it worth copying to the detailed thread, so it doesn't get lost in here?

[East_England_Stormchaser91]

I really hope this 18z suite isn't rogue or lacking data and is the start of a trend because the ensembles are a decent upgrade too, with this sort of thing appearing in the mid term.

 

 

Oh my word, now that would bring chaos to the Southern Coastal areas big time! What an extreme historical winter storm that would be if it came off!