Model Output Discussion - 28th November - 4th December

[kold weather]

To be honest Nick I think the models are probably over-doing things, they've been too negative when it comes to the forecasted NAO for a long time now and I suspect this will be the same. ECM 00z op run IMO probably is close to the mark. Still probably an average set-up overall which given how shockingly bad other factors are, will do very nicely!

[nick sussex]

In answer to questions about the possible scenarios with that Canadian troughing, if we get more amplification we could see a decent northerly flow,

If we don't get sufficient amplification then the main cold thrust will probably just graze the far ne and head into Europe.

If the troughing flattens out too much then basically the ridge will just push into southern Europe and gloom will descend on this thread.

The absolute best I can see with my Santa glasses on is that we get a good northerly with a decent cut back which lowers heights over southern Europe, the ridge topples but favourably with a pull of east to ne winds before the high settles over the UK with surface cold.

This latter option is if everything goes right, at the moment I think its either option 1 or 2.

Looking at the ECM ensembles the operational run looks to be making too much of that ridge thrown up ahead of the low at 168hrs, the suggestion being the low is further south and the pattern a little more amplified than shown.

Edited November 30, 2011 by nick sussex

[Frosty.]

The models show a cool outlook but not a cold outlook as southern britain will frequently see any polar maritime air mixed out with the 528 dam line retreating up to northern scotland at times. At least winds look likely to be north of west for much of the time. There should be some thundery wintry showers at times in scotland and n.ireland during the cooler spell.

Edited November 30, 2011 by Polar_ Low

[Gavin Hannah]

Well, as one rather well known brand lives by, good things come to those that wait.

[Heisenberg]

In anwser to questions about the possible scenarios with that Canadian troughing, if we get more amplification we could see a decent northerly flow,

If we don't get sufficient amplification then the main cold thrust will probably just graze the far ne and head into Europe.

If the troughing flattens out too much then basically the ridge will just push into southern Europe and gloom will descend on this thread.

The absolute best I can see with my Santa glasses on is that we get a good northerly with a decent cut back which lowers heights over southern Europe, the ridge topples but favourably with a pull of east to ne winds before the high settles over the UK with surface cold.

This latter option is if everything goes right, at the moment I think its either option 1 or 2.

So nick, sorry to bother with another amateur question, but how far out is all of this? Is it all FI or something that can verify quite quickly?

ta

[Gavin Hannah]

So nick, sorry to bother with another amateur question, but how far out is all of this? Is it all FI or something that can verify quite quickly?

ta

The Low over canada is projected on the GFS next Wed. So, while it's not deep FI, it is still 7 days out.

Correction: From looking at the NH chart, the Low begins to take form on Monday over Alaska by the looks of it.

Edited November 30, 2011 by MadSnowboarder

[Heisenberg]

The Low over canada is projected on the GFS next Wed. So, while it's not deep FI, it is still 7 days out.

hmm still plenty of time for change then. Fingers crossed. Would be nice to see something more seasonal. Although im sure we are all happier with the current outlook than the last few weeks/months even.....Thanks MB for the answer.

[nick sussex]

So nick, sorry to bother with another amateur question, but how far out is all of this? Is it all FI or something that can verify quite quickly?

ta

The actual two key components are modelled within 168hrs so not really FI, the uncertainty is not whether these features will verify but what happens as they phase together in the eastern USA.

I'm not sure if you've read about WAA warm air advection but this is what will trigger any northerly for the UK, everytime you see a strong southerly flow heading in towards Greenland this helps to raise pressure there as this forces a displacement of colder air to the east of it, these pressure rises will likely only be temporary.

You will know if things are going well if you see the combined troughing beginning to become negatively tilted ie running nw/se not ne/sw, at worst north/south we can get away with.

Hope that helps.

[Gavin Hannah]

hmm still plenty of time for change then. Fingers crossed. Would be nice to see something more seasonal. Although im sure we are all happier with the current outlook than the last few weeks/months even.....Thanks MB for the answer.

Indeed, much happier today. Worst case scenario is that we end up with another Euro high in two weeks from now.. but over the last day or so, I have a had this feeling that the HP ridging into Greenland might just come off.

[johnholmes]

Its a quickfire ridge BA, when it comes to those sorts of ridges the strat doesn't really come into it, I've seen many times us have super cold uppers in the bad old days and still get some fairly potent, if short lived topplers. Plus if you look at the flow which is broadly W-E in the central Atlantic anyway at the moment its not all that difficult to amplify that a little more in that broadly zonal set-up to get some colder air down.

I'd be a little more surprised if we don't have a high trying to ridge into Europe by the 15th, but we'll see!!

I'd also be a little surprised if the models were not overdoing any mid-atlantic ridging...

very true K re the Stratosphere and cold weather over the UK.

There does NOT have to be any signs in the Stratosphere for a cold spell. Its perhaps another example of folk reading something and taking too much from it. The right events in the Stratosphere do help to give prolonged deep cold but cold spells of a few days are likely and have occurred previously without its support and will occur again.

[nick sussex]

As was the case for much of December last year I will be on eastern USA trough watch this evening!

It's a simple scenario here, you really don't have to worry about whats happening in Europe, just look at whats happening upstream, if you get the right amplitude of that troughing you should see a nw/n flow developing over the UK.

The one thing I can't see though is an extended northerly, any ridging over Greenland will likely get flattened but thats for another day, lets just see if we can at least get a polar flow out of this.

[Rocheydub]

Is it too early to conclude that the first benefit of the cooling trend portrayed by the models is the absence of GavinD's high pressure charts?

That and the emoticon!

Looking more likely as everyone has said for cold next week from Sunday on. Still some great concern about Sunday night and the gales... One to watch as these can shift even (though rare) in the +36 hour to +12 hour timeframe!

[wimblettben]

That and the emoticon!

Looking more likely as everyone has said for cold next week from Sunday on. Still some great concern about Sunday night and the gales... One to watch as these can shift even (though rare) in the +36 hour to +12 hour timeframe!

Looks like GFS has downgraded the winds for Sunday and Monday now though!

[Grimsby Snow Lover]

Looks like GFS has downgraded the winds for Sunday and Monday now though!

For now it has,although the GFS did the same during yesterdays 12z only for the 18z and 00z to upgrade it again. Nothing is set in stone by any means.

[Gavin Hannah]

It looks like the Highlands will def see a regular accumulation of snow over the next 7 days. Sunday onwards will introduce the chance of low level snow to most of the uk. Higher chance of low level snow the further north you go.

[Uncle_Barty]

A lot of it will most likely be marginal though away from high ground, and the continuing strat pattern/PV makes it likely IMO that the cold will get downgraded as the pattern eventually flattens out.

Would put FI at 120 hours for that reason.

I do not think there will be any snow south of the M4 except on the highest ground, personally.

No real sign of a reversion to the very mild conditions of most of this month at the moment however.

Edited November 30, 2011 by Jim_AFCB

[TEITS]

very true K re the Stratosphere and cold weather over the UK.

There does NOT have to be any signs in the Stratosphere for a cold spell. Its perhaps another example of folk reading something and taking too much from it. The right events in the Stratosphere do help to give prolonged deep cold but cold spells of a few days are likely and have occurred previously without its support and will occur again.

That isn't technically correct. A cold stratosphere makes Greenland blocking unlikely but doesn't rule out a cold spell from the E via blocking over Scandi. The only thing I will add is ideally you would then need the PV to be centred just to the W of Greenland to allow the E,lys to arrive.

Just wanted to add this because our weather isn't as simplistic as you suggest.

[Frosty.]

It looks like the Highlands will def see a regular accumulation of snow over the next 7 days. Sunday onwards will introduce the chance of low level snow to most of the uk. Higher chance of low level snow the further north you go.

I doubt very much whether there will be any low ground snow for the southern half of the uk, the models show cool and unsettled, not cold except for the scottish mountains.

Edited November 30, 2011 by Polar_ Low

[johnholmes]

That isn't technically correct. A cold stratosphere makes Greenland blocking unlikely but doesn't rule out a cold spell from the E via blocking over Scandi. The only thing I will add is ideally you would then need the PV to be centred just to the W of Greenland to allow the E,lys to arrive.

Just wanted to add this because our weather isn't as simplistic as you suggest.

I disagree TEITS it does not have to be supportive for cold spells but does for prolonged spells, but let's just agree to differ on that.

Certainly I am very well aware of how complex weather patterns and forecasting are. As always I try to help the newer members by making things sound not too complex to try and prevent being put off if the explanations are too complex. The excellent technical thread and Stratospheric thread are the places to do that.

just a few ideas using the 500mb anomaly charts, its just rough work so not all that tidy in essay format. It gives my ideas on how the weather may turn out in a general fashion into the 2nd week of December.

500mb anomaly ideas-30 nov 2011.pdf

Edited November 30, 2011 by johnholmes

[snow drift]

I disagree TEITS it does not have to be supportive for cold spells but does for prolonged spells, but let's just agree to differ on that.

Certainly I am very well aware of how complex weather patterns and forecasting are. As always I try to help the newer members by making things sound not too complex to try and prevent being put off if the explanations are too complex. The excellent technical thread and Stratospheric thread are the places to do that.

just a few ideas using the 500mb anomaly charts, its just rough work so not all that tidy in essay format. It gives my ideas on how the weather may turn out in a general fashion into the 2nd week of December.

500mb anomaly ideas-30 nov 2011.pdf

Thanks John. Informative and easy to understand.

The models show something similar. Down here there will be little of interest but further north it looks quiet exciting.

[jonnybradley]

I disagree TEITS it does not have to be supportive for cold spells but does for prolonged spells, but let's just agree to differ on that.

Certainly I am very well aware of how complex weather patterns and forecasting are. As always I try to help the newer members by making things sound not too complex to try and prevent being put off if the explanations are too complex. The excellent technical thread and Stratospheric thread are the places to do that.

just a few ideas using the 500mb anomaly charts, its just rough work so not all that tidy in essay format. It gives my ideas on how the weather may turn out in a general fashion into the 2nd week of December.

Great read John. I for one really appreciate the efforts of yourself and others on here that do try to explain things to newbies like myself who really don't have a clue

Interesting that the Azeros high still seems to be having such an effect on where these systems track. At least this is one feature I can easily pick up on, even if I don't full understand its impact.

Thanks

Edited November 30, 2011 by jonnybradley

[nick sussex]

Here comes the Canadian low, we'll know within a few minutes whether we'll see a northerly for the UK.

Oh the tension......

Earlier the GFS doesn't develop that shortwave which is good news for those that don't like gales, also less of a warm sector, some snow on its northern flank especially for higher ground.

[Snowstorm1]

Quite like this run so far less mildish air getting in for as long which means were in the colder upper air longer which could mean better chance of snowfall at lower levels at the moment Id say to be in with a shout of snow at low levels you have to be midlands Northwards

Edited November 30, 2011 by Snowstorm1

[SuttonStorm]

Here comes the Canadian low, we'll know within a few minutes whether we'll see a northerly for the UK.

Oh the tension......

Earlier the GFS doesn't develop that shortwave which is good news for those that don't like gales, also less of a warm sector, some snow on its northern flank especially for higher ground.

Nick

which frame does it appear...make sure I am looking at the right one?

SS

[Thundery wintry showers]

On the "cold stratosphere" issue, I don't think a cold stratosphere prevents cold snowy weather from arriving from the east, but it does stack the odds far more heavily against, because the tendency for a strong polar vortex means that any high pressure to the NE will struggle to extend its influences to the west of central Scandinavia, giving us a lot of southerly and south-easterly winds.

It is still possible to get frontal battleground snowfalls if we get a NW-SE tilted jet with depressions undercutting a Scandinavian high, giving battlegrounds between cold continental air and fairly cold polar maritime air (this scenario can also produce a fair amount of cold zonality in the absence of heights over Scandinavia) but for the most part a Scandinavian high and cold stratosphere brings a dry cloudy stalemate over the British Isles with cold continental air trapped on the other side of the North Sea.

The current GFS runs are suggesting an extended spell spent in polar maritime air, meaning a good 4-6 days or so of "sunshine and showers"- I always have doubts when I see this showing in the charts, going by numerous experiences of secondary lows turning up at short notice. There are already hints of the airmass having to "return" a longer distance over the Atlantic meaning a watered-down version of what was previously shown, though the GFS 12Z still supports snowfalls in Scotland and northern England around Monday/Tuesday, and maybe over high ground across the Midlands with Sunday's frontal system.