Model output discussion - The run up to Christmas

[Daniel*]

Sensational EPS strengthening of blocking near Greenland being signalled very firmly established.

[Howie]

Just now, Daniel* said:

Sensational EPS strengthening of blocking near Greenland being signalled very firmly established.

I don't like that the low pressure is more south west than south east

[Frosty.]

The GEFS 12z mean shows an increasingly cold trend next week..more especially into early Janvier....temps struggling to reach freezing point in places...combined with plenty of snow potential too..hello winter indeed...very logical..apart from my charts being in an illogical order!

 

Edited December 23, 2020 by Jon Snow

[feb1991blizzard]

1 hour ago, chionomaniac said:

How ridiculous to post this. The stratospheric disturbance is still forecast. There is still a significant amount of wave 2 activity - and this is being trop driven. And, if the trop is the driver, then this is where we need to concentrate atm. The strat disruption will help later. So laugh away....

I do wished that big split right up top on the GFS was still showing though, that was just in the right place, if that would have been all the way to the mid strat with all this wave activity in the trop going on, would that have made it almost impossible to stop these Atlantic ridges gaining enough latitude? which up to this point has been the reason we have struggled to bring frigid air out of the Arctic.

Edited December 23, 2020 by feb1991blizzard

[Daniel*]

1 minute ago, Howie said:

I don't like that the low pressure is more south west than south east

Well perhaps but it could mean a tonne of snow for some...

[kold weather]

2 minutes ago, Howie said:

I was hoping the west based -NAO would do one, ugh 

We are probably early enough and the set-u good enough aloft that we can afford to roll the dice on this one. If it fails and we end up in a SW atlantic wave train I suspect we will get another bite of the cherry and things will rapidly switch back to cold. Just a gut feeling if it does end up too far west. We've come mighty close to that on both the 06z and the 12z deep in FI on the GFS.

[swfc]

4 minutes ago, northwestsnow said:

The plot thickens.....

Good grief!!!

[Daniel*]

1 minute ago, kold weather said:

We are probably early enough and the set-u good enough aloft that we can afford to roll the dice on this one. If it fails and we end up in a SW atlantic wave train I suspect we will get another bite of the cherry and things will rapidly switch back to cold. Just a gut feeling if it does end up too far west. We've come mighty close to that on both the 06z and the 12z deep in FI on the GFS.

More west based -NAO isn’t always a bad thing in fact in December 2010 it featured.

 

The GFS 12z perhaps has hints of extended EPS potential to be extremely snowy pattern.
 

[northwestsnow]

Mean looks more amplified in the Atlantic to me...

[stodge]

Evening all

Certainly some interesting output yesterday evening to get the discussion moving ranging from an underwhelming GEM to a much more "interesting" ECM and GFS. After a wet but mild day in downtown East London, we await the coming of the first cold snap for Christmas. The intriguing point was less whether we could get a second cold spell but for how long could it be sustained into 2021? 

12Z GEM: T+120 takes us to Monday next week and the LP is right over the British Isles, @snowkingput up a superb post on this yesterday which is recommended reading explaining all the factors at work when trying to assess potential for snowfall. The 850s are -4 but other factors look favourable especially for higher ground. By T+180 (New Year's Eve), the LP has filled significantly with residual centres to the north-east and south and the British Isles remains in the cold trough. The uppers remain reasonable even if the LP has filled.  The upstream profile looks messy but what we aren't seeing is a zonal Atlantic firing storms towards us. On to T+240 and a lot changes as little changes. AC strong HP over Greenland has forced the jet to the south but the next LP has sunk SE from west of Iceland to be just to the north-west of Scotland. The unsettled and cold pattern continues with a new push of cold air from the west and north-west. 

12Z GFS OP - very solid output from GEM to start the evening. Now on to the American offering - will it be a fine turkey or a burnt offering? By T+120, GFS OP has the LP further south than GEM with some -4 850s but broadly zero to minus four but with some other factors possibly helping snowfall to higher ground. GFS OP then pulls the LP first to the south and then round to the east keeping the British Isles on the northern or western flank so it's a NE'ly backing to a N'ly by T+180. The airmass, as reflected by the 850s, only gets colder as we approach New Year. Elsewhere, there's a new LP forming around Iceland while pressure rises in mid-Atlantic. As 2021 arrives and get started, the new LP is phased into the trough which sinks south as pressure builds strongly over Greenland and the T+240 chart shows an E'ly for much of northern Britain. With 850s from -4 to -8, I suspect there's more snow on offer especially for higher ground. The problem with this evolution for cold fans is twofold - first, a strong Greenland HP often produces a west-based negative NAO whereby the ridge extends not south or south-east but south-west and LP comes around the southern end and can push NE towards western Europe and re-introduce milder air. The T+312 evolution is a knife-edge. The LP coming out of North America is initially steered SE  but phases with the residual trough over the British isles and deepens considerably near the Azores as the warmer ad colder airmasses interact and then swings towards the British Isles but the block to the north-west is strong enough to forces the LP to disrupt and elongate along the Channel. Milder air gets into southern Britain but the cold hangs on in the north.  FO ends with a complex trough o the north-west extending back into the Atlantic and heights over Greenland. The milder airmass is being chased away by a new PM airflow off the Atlantic. It looks very mild to the east over Europe.

12Z ECM - GFS OP is very interesting for cold weather fans tonight with plenty of synoptic interest throughout. Finally, the Europeans - will they complete a Deal for cold fans tonight or will we have to wait another day? The T+120 chart has the LP right over the British Isles with the mid-Atlantic HP perhaps more influential. 850s aren't exciting but as has been observed, perhaps not the only factor at work. By T+192, as we've seen elsewhere, the LP is just to the east and there's a weak N'ly flow over the British Isles. The 850s continue to struggle but it's not going to be mild. Upstream, a long ridge extends from Greenland with the next trough moving out from North America. On then to T+240 and something a bit different.  A mid-Atlantic LP is helping to prop up heights over southern Greenland. Further north, a new LP looks to be developing off the east Greenland coast. Closer to home, a shallow LP over Norfolk and a weak NE'ly flow.  The airmass remains cold over the British Isles.

Touching on other things, 12Z JMA brings the Atlantic HP closer and cuts off the bulk of the cold flow so a worrying sign from a model always to be considered. 12Z Control is cold for most of the run and would be well favoured by cold weather fans. Another worrying sign is 12Z GFS OP backing away sharply from splitting the PV at 10 HPA. The strat is extended and weakened but there's no split tonight. 

Conclusion - more exciting output for fans of cold tonight especially as we head toward the end of the year and the beginning of 2021. I'll leave others to comment on where and when snow will fall but I can't see why anywhere with higher ground won't do well in the next week to ten days. Beyond that, the main models seem keen on prolonging the cold spell but one or two warning signs tonight. JMA is less favourable and while other signals may show something different, the 10 HPA charts don't look as exciting as they did yesterday. We'll see where we are tomorrow but it's something to think about.

[Mike Poole]

2 minutes ago, kold weather said:

We are probably early enough and the set-u good enough aloft that we can afford to roll the dice on this one. If it fails and we end up in a SW atlantic wave train I suspect we will get another bite of the cherry and things will rapidly switch back to cold. Just a gut feeling if it does end up too far west. We've come mighty close to that on both the 06z and the 12z deep in FI on the GFS.

Yes, absolutely, bites at the cherry are coming along one after another re reloads of amplification.  And then the SSW while risking swallowing the whole cherry, if it doesn’t it gives us the big chance.  

Much closer to home, here for info is HARMONIE take on cumulative snow til lunchtime Christmas Day, some areas might get lucky:

[Geordiesnow]

1 hour ago, Tim Bland said:

Yes, now precip charts are out it’s clear it’s a big shift east. I’m going to say this is probably going to clip eastern areas at best the way it’s going! As for ‘surprise’ ‘unexpected’ snow ...ECM had been modeling this for 3 days now lol. It always makes me laugh how we talk about a snow event for days and then alL of a sudden there is a risk of a ‘surprise ’ event.  Even BBC have it on their weather bulletins today 00z vs 12z ECM below 

This is how I expect the PPN to go in relation to snowfall, any areas under that low with no wind could well recieve snow whilst other areas outside of it is likely to be rain due to mixing and the air not being overly cold.

[kold weather]

Looking at the ECM ensembles plenty of the suite have some sort of snow between 12z 28th and 12z 29th. Most aren't 'that' exciting but most also do have at least a dusting.

These types of events tend to get upgraded as we move closer to the time so its a positive sign to see reasonable agreement for there to be snow around. Whether it sticks all that well remains to be seen of course.

[bluearmy]

2 minutes ago, Daniel* said:

More west based -NAO isn’t always a bad thing in fact in December 2010 it featured.

 

The GFS 12z perhaps has hints of extended EPS potential to be extremely snowy pattern.
 

Dec 2010 saw the chunk of vortex tumble just west of the U.K. with a west based neg nao but this was on the back of a a few weeks of sub zero temps  and snow covered ground - once you have that level of deeply cold embedded in you can afford to sail much closer to the wind. 

[Scott Ingham]

1 hour ago, chionomaniac said:

This is the kind of pattern that has #surprises written all over it.  One area has sleety rain, and down the road has a snow fest 

Couldnt agree more its a classic set up! Some will be disappointed some will be making snowmen!

[I remember Atlantic 252]

2 minutes ago, Scott Ingham said:

Couldnt agree more its a classic set up! Some will be disappointed some will be making snowmen!

Stafford rain, Walsall snaw

[MATTWOLVES 3]

Its a cracking mean and it stays good out to day 14...on a funny note and this is the truth...I tried doing a Snowking earlier,and spent quite a bit of time trying to explain those 850 temps to my gran...Needless to say it didn't end well.

[Duane S.]

Stunning charts, just stunning. No point fixating on the details, it will of course change.

What a thing of beauty this is...

[Allseasons-Si]

ECM/Gefs mean at 240+ pressure and ens

day ten EPS and clusters and extended

^all looking pretty good to me

sorry for short text as i am trying to eat my tea at the same time lol,it's now a microwave jobby

 

 

 

[feb1991blizzard]

4 minutes ago, bluearmy said:

Dec 2010 saw the chunk of vortex tumble just west of the U.K. with a west based neg nao but this was on the back of a a few weeks of sub zero temps  and snow covered ground - once you have that level of deeply cold embedded in you can afford to sail much closer to the wind. 

It did go back to average beforehand and it wasn't a lucky one just the right side of Marginal that Liverpool and Preston got a right dumping off, it was a WNW flow, but embedded within -10c uppers.

[Captain Shortwave]

9 minutes ago, Daniel* said:

More west based -NAO isn’t always a bad thing in fact in December 2010 it featured.

 

The GFS 12z perhaps has hints of extended EPS potential to be extremely snowy pattern.
 

The important part is to have those low heights already established in Europe to jet the jetstream pushed as far south as possible, hence the lows track in a way to allow for heavy snowfall on the north flank of any frontal systems that move through.

Really impressed by the ECM in terms of the outcome once that wedge forms, the reamplification and subsequent attempt at forming a fully fledged block close to Greenland. Not worried to much about the lack of deep cold as a set up like that should allow the trough to our east to reinvigorate as Arctic air moves south into Europe and the UK.

ECM/GEFs at day 9

   

I am pretty sure that last time we saw a set up like this well advertised the majority on here were writing off several weeks of summer.... 

[bluearmy]

Ecm extended clusters show  50% option  headed west based and getting across pretty much the whole country by day 15. Other two options are much better. 
 

the eps meteogram for me shows I have a 90% chance of seeing some snow over the next fortnight...

Edited December 23, 2020 by bluearmy

[Singularity]

I recall Tamara saying in 2018-19 that a Greenland block that’s partially over Canada is more likely to reach far east  enough to put the UK in a cold airmass during Dec-Jan than in Feb-Mar. To do with seasonal wavelengths... though I think a fully west-based NAO could still be trouble unless it was VERY negative (does happen!).

[snowlessayr]

Absolutely fascinating model watching at present and synoptic charts that haven't been seen for quite a while. 

This is before what could happen with the stratosphere, massive thank you for the posters explaining what stratosphere charts are showing as it is a massive help at times. 

With the fascinating period ahead us it is a shame to see some very negative posts, don't understand the reasoning behind it if I am honest, most winters you would bite your hand off for what we have at the moment. 

The longer the colder spell goes on the less marginal some setups are as the ground gets colder and colder. Hopefully that will finally lay the uppers debate to rest for  a while, not holding much hope mind

Merry Christmas to you all and let's hope we all get some snow.

[Mike Poole]

ECM clusters good to excellent. 

T192-T240 and T264-T360:

I’m surprised that there is only 1 cluster in the T192-T240 period, the mean and spreads on Meteociel didn’t suggest that, maybe ‘other’ reasons.  

But the later clusters, well high lat blocking party time, any which road.  

Edited December 23, 2020 by Mike Poole