Model output discussion - proper cold spell inbound?

[tight isobar]

Definate trend in the correct direction.

6z ens....

[Chevron12345]

6 minutes ago, tight isobar said:

Definate trend in the correct direction.

6z ens....

Only 2 just reach -10 for a very short time. Surely there should be some better (colder) upper air showing on those. -5 upper just won't cut the mustard with regard to 2m temperatures. You can see everyone complaining about the meto showing 5-6 for towns and cities come the last week of Feb!

[Man With Beard]

Back to the nearer time ... must be a good chance of snow for some this coming Sunday night/Monday morning. Even on a westerly, uppers between -6C and -8C at this time of year can deliver snow. Just need a trough to form. Don't see why this shouldn't be possible on a straight westerly flow under lowish heights. 

      

[ICE COLD]

18 hours ago, Steve Murr said:

Early March are when the North Sea temps are at their lowest lol-

I will try & dunk a post in about 930 tonight

S

@Steve Murr what's happened to the 09:30 post . Been waiting for the past 15 hours . Getting tired now ?. 

[Frosty.]

Following the fabulous end to the Gfs 00z op which would have plunged the uk into the freezer soon after, the 6z ending is a variation on the same theme..something tells me the best of this winter is still to come as the upper level winds in the stratosphere are set to weaken considerably during the mid month period and we see strong height rises to the E / NE as the Gfs shows..fingers crossed we get the BEAST FROM THE EAST..nobody will be more delighted if we see a stonking  late Feb!!

[sebastiaan1973]

http://kachelmannwetter.com/de/modellkarten/euro/europa/m0_luftdruck/20180221-1200z.html

This would be terrible. An easterly but no beast! Nature is creative in disrupting our winter chances.

Edited February 7, 2018 by sebastiaan1973

[Catacol]

1 hour ago, Daniel Smith said:

6/6 GFS runs for blocking out in FI, ECM starting to sniff it at day 10 too, perhaps.

Yes - let's keep the score ticking. 06z however is the least "pretty" of the blocked evolutions for 19 Feb... but the block is clear along with the reverse undercut. Don't want that Euro high anomaly though.

Agree also on day 10. Slight sense of an earlier opportunity to get the trough disruption underway on the 06z and 00z and it isn't beyond the bounds of possibility that we could get an earlier setting up of the anticipated pattern. Right now, however, it feels a little too quick. Observation over many years suggests that trends tend to take longer to arrive than anticipated, and even though we are expecting a fast response to the split and reversal I suspect 72/96 hours might just be a bit too quick. However - what's in the process of happening now and early into next week is significantly unusual... so who knows: maybe it might all be brought forward.

Very exciting model watching. Real sense of anticipation.

[Kentish Snowman in Yorks]

14 minutes ago, Chevron12345 said:

Only 2 just reach -10 for a very short time. Surely there should be some better (colder) upper air showing on those. -5 upper just won't cut the mustard with regard to 2m temperatures. You can see everyone complaining about the meto showing 5-6 for towns and cities come the last week of Feb!

They will change in six hours anyway but if you have drier continental sourced air filtering across the UK the lower uppers required from a more maritime source are simply not required for snowfall.

I suspect that we will see some colder runs showing up in the near future if the SSW is to affect us as many are suggesting but even if we do not see this I would have thought uppers at circa -7 / -8 would be more than sufficient in terms of supporting snowfall / convection etc.

[Man With Beard]

6 minutes ago, Chevron12345 said:

Only 2 just reach -10 for a very short time. Surely there should be some better (colder) upper air showing on those. -5 upper just won't cut the mustard with regard to 2m temperatures. You can see everyone complaining about the meto showing 5-6 for towns and cities come the last week of Feb!

I see that we will start the 850s game should this easterly verify!!

My very oversimplified rule of thumb:

-10C 850s: should be snow for all

-8C 850s: snow for all on a SE/E/NEly except possibly extreme eastern coasts if there is a slack flow off the North Sea

-6C 850s from a E/NEly: mostly snow but possibly sleet for E coasts, or sleet more generally below 150m if the precipitation was light and the flow was very slack off the N Sea (this gives more time for sea air modification)

-6C 850s from a SEly: if the continent is under snow cover = snow for all. Otherwise, generally snow but sleety on the S coast.

-4C 850s from a E/NEly: rain on E coast, marginal below 150m, still snow on 150m-300m unless light precip and slack flow, snow above that.

-4C 850s from a SEly: rain/sleet on S coast unless the continent is very cold at surface level (more likely to be snow the more SE rather than SW). Inland, probably still snow but could be marginal if other factors not in favour.

-2C 850s from a E/NEly: generally rain unless a front cutting across cold air already in situ

-2C 850s from a SEly: could still be snow if continent is freezing and the flow is strong - otherwise rain unless a front cutting across cold air already in situ

I'm fully aware that other factors such as dew points are important but this kind of guide could be useful when the 850s are all we have to go by

[stodge]

Morning all

Onwards and sideways into February we go. A very cold morning in lowland East London but is this winter's last hurrah or a sign of things to come ?

This morning's medium term output takes us to Saturday February 17th:

Starting as always with ECM 00Z at T+240:

A typical winter chart (or indeed for almost any other time of the year). The LP just to the west of the Hebrides is trundling eastwards but filling rapidly having been 955 MB just 48 hours earlier. Frontal systems crossing the British Isles west to east with the mild sector over the SW at this time so cloud and rain for many with perhaps snow to Scottish mountains. Some signs of a slackening Atlantic and hints of pressure rising further west.

GEM 00Z at the same time:

A very different profile this morning and more akin to the pattern we've seen for much of the winter. The LP has filled and is disrupting SE into Europe between a much more prominent Azores HP and heights to the east and NE. The Atlantic is still strong with new LP coming and a very vigorous feature over the far NE of Canada but the vortex looks under huge pressure so the evolution isn't clear.

GFS 00Z OP at the same time:

A mild or very mild airflow over the British Isles between a deep LP just off Greenland and HP over Europe so a classic SW'ly airflow. Some signs of trough development to the SW which could presage rain for western areas but more than a hint of spring from that especially for the south and east. Further into FI and pressure builds strongly through the British Isles and to the north opening the door to a very cold E'ly airflow. If there was ever an example of very cold weather starting from very mild this is it.

GFS 06Z OP at T+234:

As you'd expect, not too different but the Azores profile is different with a larger HP area albeit a bit flabby in places (aren't we all ?). We get to a nice cold evolution for those who like that sort of thing but more slowly oddly enough.

GFS 06Z Control at T+240:

Subtle differences and more defined heights already in place to the NE with the SE already getting close to a continental feed but nothing spectacular. Further into FI and the Atlantic remains in charge albeit with the vortex looking severely weakened.

Looking at the GEFS at T+240:

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=1&ech=240

It's a mess - plenty of options as you'd expect but many trying to handle the breakdown of the Atlantic flow - do we see an undercut here, a slider there ? Well, maybe but far too early to tell. Certainly at the far edge of FI there continue to be a lot of cold or very cold options.

In summary, then, it remains a case of "as you were". Next week remains unresolved with some output suggesting an evolution to almost spring-like conditions while other suggesting more LP dominance and colder air over the British Isles. After mid month it's all to play for - GFS 00Z OP was close to nirvana for many cold fans and it's far from unsupported and perhaps more likely than a Greenland based HLB at this time but other evolutions are possible and in much of the output the Atlantic and the PV remnant are fighting hard to stay in control.

As ever, more runs are needed.

 

[tight isobar]

31 minutes ago, Chevron12345 said:

Only 2 just reach -10 for a very short time. Surely there should be some better (colder) upper air showing on those. -5 upper just won't cut the mustard with regard to 2m temperatures. You can see everyone complaining about the meto showing 5-6 for towns and cities come the last week of Feb!

No it won't cut it yet!!..

Due to the upper atmospheric changes.

I highlighted a 'trend' in direction-and thats what begining to note...mean and others showing a decline.....although im fully expecting that trend to coninue over coming days.

With more member adjusting on the downwards track.

[Chesil View]

Just now, Kentish Snowman said:

They will change in six hours anyway but if you have drier continental sourced air filtering across the UK the lower uppers required from a more maritime source are simply not required for snowfall.

I suspect that we will see some colder runs showing up in the near future if the SSW is to affect us as many are suggesting but even if we do not see this I would have thought uppers at circa -7 / -8 would be more than sufficient in terms of supporting snowfall / convection etc.

Indeed so Kentish. this must be minus ten uppers for snow is very misleading as it depends on the source of the air and its moisture content. 

The deepest snow I have ever seen here in Dorset (12-18 inches on the level and 20 foot drifts)came from Atlantic fronts stalling against a dry southeasterly feed off the continent with uppers at just minus 2!!!

[Frosty.]

Well really there is nothing for those who prefer mild weather, just slightly milder / less cold at times associated with atlantic frontal systems but predominantly cold zonal with wintry showers and overnight frosts / ice..it's then very much game on for a change to more settled and even colder weather during mid feb or soon after..confidence is growing and is reflected in the mid / longer range GEFS  / GFS so far today..very exciting times ahead i feel..cautiously of course!! 

[Timmytour]

2 hours ago, warrenb said:

Models will always try to revert to Zonal as that climatic norm. When a signal is so strong for blocking it often takes the model time to get there.

I'm not sure the models don't underplay zonal.   We seem to get a far bigger proportion of easterlies and northerlies modelled than we ever actually end up experiencing

[MR EXTREMES]

Ok all I can say is the GFS brings in some exciting cold from the west northwest.

And shallow heights to our northeast building with each run especially on the GFS.

Also looking at the stratosphere event on going you can see clearly a huge split with European section of the vortex spinning from the east to west over the south of the uk.

This paves the way for lower heights to our south and by God opens the door to some pretty impressive northern blocking.

As the vortex takes one last gasp of energy and then explodes spewing out cold air already bottled up in the northern hemisphere.

In fact I'm actually blown away by the intensity of cold around.

Most certainly some snow events showing up.

Of coarse still us roasting south coast crew still awaiting a event.

but seen some snowflakes so not all bad.

And from past events I really believe this will be a quick response to the stratosphere this time.

Vortex absolutely ripped apart.

[mb018538]

25 minutes ago, Timmytour said:

I'm not sure the models don't underplay zonal.   We seem to get a far bigger proportion of easterlies and northerlies modelled than we ever actually end up experiencing

Maybe, but as they go against the climatic norm....they are harder to predict with accuracy. When you chuck a major SSW into the mix too....it's going to be even harder for the models to predict what will happen, especially at distance. Add to that no two SSWs are the same, or result in exactly the same outcomes....you can see where the problems lie! We are still going to have to wait for this to draw closer before a real handle is shown.

[Chevron12345]

1 hour ago, Man With Beard said:

I see that we will start the 850s game should this easterly verify!!

My very oversimplified rule of thumb:

-10C 850s: should be snow for all

-8C 850s: snow for all on a SE/E/NEly except possibly extreme eastern coasts if there is a slack flow off the North Sea

-6C 850s from a E/NEly: mostly snow but possibly sleet for E coasts, or sleet more generally below 150m if the precipitation was light and the flow was very slack off the N Sea (this gives more time for sea air modification)

-6C 850s from a SEly: if the continent is under snow cover = snow for all. Otherwise, generally snow but sleety on the S coast.

-4C 850s from a E/NEly: rain on E coast, marginal below 150m, still snow on 150m-300m unless light precip and slack flow, snow above that.

-4C 850s from a SEly: rain/sleet on S coast unless the continent is very cold at surface level (more likely to be snow the more SE rather than SW). Inland, probably still snow but could be marginal if other factors not in favour.

-2C 850s from a E/NEly: generally rain unless a front cutting across cold air already in situ

-2C 850s from a SEly: could still be snow if continent is freezing and the flow is strong - otherwise rain unless a front cutting across cold air already in situ

I'm fully aware that other factors such as dew points are important but this kind of guide could be useful when the 850s are all we have to go by

Thats great...for snow falling. But 2m temps at around -5 wont mean freezing so any snow falling will just melt. Anyway not the place for that discussion. 

[Daniel*]

Just now, Chevron12345 said:

Thats great...for snow falling. But 2m temps at around -5 wont mean freezing so any snow falling will just melt. Anyway not the place for that discussion. 

Not always in UK we have had ice days with positive 850s in fact last winter in London managed an ice day from a southerly flow off France / near continent, was cold! There’s always exceptions to the rule. 

[stodge]

2 minutes ago, Daniel* said:

Not always in UK we have had ice days with positive 850s in fact last winter in London managed an ice day from a southerly flow off France / near continent, was cold! There’s always exceptions to the rule. 

Indeed and under midwinter anticyclonic conditions this has happened on numerous occasions but not recently.

I can't remember the last truly anticyclonic winter spell with 4-5 days of HP over the British isles and a lovely inversion bringing fog, frost and ice days. It now seems we have a more mobile and turbulent atmosphere and a more powerful jet.

I wonder why..

[Seasonality]

1 hour ago, sebastiaan1973 said:

http://kachelmannwetter.com/de/modellkarten/euro/europa/m0_luftdruck/20180221-1200z.html

This would be terrible. An easterly but no beast! Nature is creative in disrupting our winter chances.

Nature....

 

[Singularity]

21 minutes ago, Chevron12345 said:

Thats great...for snow falling. But 2m temps at around -5 wont mean freezing so any snow falling will just melt. Anyway not the place for that discussion. 

It's actually a serious balancing act because with -5 uppers you've usually got a bit more moisture to work with and heavier snow is more likely to overwhelm any positive ground temperatures and accumulate - and then do so rapidly. In Nov 2010 I saw +7*C and rain transition to 0*C and heavy settling snow in the space of 10 minutes... on the south coast of all places!

At the other end of the spectrum with -10 or below there is a risk of the air being so dry that snow is very fine and has a tendency to sublime both on the way down (making the snow more scattered) and after settling. Such behaviour appears to have occurred in many places yesterday and overnight into today. LP system forcing uplift of less cold, less dry air from lower levels is usually required to overcome that and produce the fabled 'drifting powder snow'.

Hmm I need a model chart... how about this from the NASA model, which is worth a gander as it shows a scenario I've been looking out for as a possible alternative to the next low after the weekend blowing up into a major system of its own;

From a runner into the base of the previous trough I can see a faster route to getting low heights back into Europe. Shame this run cuts out at this point but GEM sort of shows what I imagine could follow;

[Frosty.]

39 minutes ago, MR EXTREMES said:

 

Vortex absolutely ripped apart.

This is looking increasingly likely, been a while since I posted this..lets hope the expected much anticipated HLB favours the uk and brings a memorable last third to February and continues into march...cold zonality on the latest models followed by very cold from the east (per MO and various output) would be the perfect happy ending.

[Chesil View]

Just now, Singularity said:

It's actually a serious balancing act because with -5 uppers you've usually got a bit more moisture to work with and heavier snow is more likely to overwhelm any positive ground temperatures and accumulate - and then do so rapidly. In Nov 2010 I saw +7*C and rain transition to 0*C and heavy settling snow in the space of 10 minutes... on the south coast of all places!

At the other end of the spectrum with -10 or below there is a risk of the air being so dry that snow is very fine and has a tendency to sublime both on the way down (making the snow more scattered) and after settling. Such behaviour appears to have occurred in many places yesterday and overnight into today. LP system forcing uplift of less cold, less dry air from lower levels is usually required to overcome that and produce the fabled 'drifting powder snow'.

Hmm I need a model chart... how about this from the NASA model, which is worth a gander as it shows a scenario I've been looking out for as a possible alternative to the next low after the weekend blowing up into a major system of its own;

From a runner into the base of the previous trough I can see a faster route to getting low heights back into Europe. Shame this run cuts out at this point but GEM sort of shows what I imagine could follow;

Quite so James. There are many more parameters involved in snow falling and settling including rate of fall and rate of accumulation. A heavy fall of accumulated snow is not easy to shift even at temps above freezing. 

The argument about longer daylight hours and stronger sun in mid late feb increasing melt is of course based on the assumption that the sun is actually shining. Worth peeps looking at feb 47 in that regard where daytime temps and snowmelt was held back due to almost permanently cloudy skies.  The record for consecutive sunless days in London was set in the winter of 1947. 

 

[winterof79]

Svalbard region  is looking like the Bookies favourite for an initial pressure rise 

From there on

it just gets tastier

 

[winterof79]

GEFS mean today

240Z

384Z

And the jet mean today

End of the run bye bye 

 

Edited February 7, 2018 by winterof79