Model Output Discussion - Tuesday 20:25 -->

[cooling climate]

This could be a mega cold run with -15 to -20c 850's just off the Danish coast.

[snowking]

Whilst there is a warm sector associated with the LP out of scandinavia its still sub zero in terms of 850's...dependant on dew points that could well contain very little in the way of rain, particularly given the cold nature of the air either side of it, and bring more of a mix of sleet/snow....id certainly expect from something like that front and back edge snow

Shame its so far away still to prog with any certainty, but very encouraging stuff, and just look at the cold piling up behind the LP system ready to flood the UK at t+168

SK

[chionomaniac]

When viewing these runs it is important to check that the upper heights (the yellows, oranges, greens and blues) haven't changed too much. Surface events and 850 temps can be worried about nearer the time (within 72 hrs). On that basis this run is good again.

[Thundery wintry showers]

Yes, a real chance of some snow showers in many eastern parts of England as early as Sunday/Monday according to the GFS 12Z, which closely resembles this morning's ECMWF at that timeframe with a more pronounced easterly flow on the southern flank of the high to the north. It would still be marginal. The precipitation coming down from the north with warmer air would most likely be of rain but could turn to snow for a time in the south as it bumps into the cold continental air.

I don't think those shortwaves are a problem for cold/snow lovers on the 12Z run though I can see what Reef might be getting at- if the low around T+144 stalls in the North Sea it could keep the coldest air away to the east while allowing the Atlantic lows to close in. I don't see that happening on the GFS 12Z though which should be another snowy one out into FI and certainly a big upgrade on the snow chances all around relative to yesterday's GFS 12Z run. As long as high pressure holds to the N and the jet doesn't come powering in, the only way that shortwave can go is south, allowing the cold NE'lys in.

Edit: Check out the temperatures and, just as importantly, precipitation maps at T+180!

Edited December 9, 2009 by Thundery wintry showers

[kold weather]

Yep, its not a bad thing at all badboy...

PS, look at 192-216hrs, thats one of the best few charts I've EVER seen, strong cold pool with a rapidly developing low pressure sweeping down the north sea, no doubt that would give a big snow event.

[Iceberg]

The movement of the upper cold pool SW, so that some of it sits out of the north sea, provides a stonker of a snow fest(serious, serious snow) even within 200Z let alone after that, a real stunning run, cold and snow wise.

[MKN]

The low pressure over scandi is the one thing which stops the fun beginning earlier imo, if that wasnt as much of an influence the cold would come flooding our way earlier. As it is could end up a spoiler, or deliver better things just further down the line. we will see.

[phil nw.]

So far its not really an upgrade or a downgrade, its about the same as the 06z, maybe a tiny bit better to start with.

Anyway at 162hrs the upper high moving west, so we become at the mercy of the low pressure complexes that are circling each pther over E/S Europe. Upper jet really is very impressive indeed has to be said coming in from the east on this run, don't see that too often.

Yes Kold i don`t think the start of this cold spell could not be any better than this.Almost perfect alignment of the High to Greenland as the Low comes down through Scandi.towards us with that Cold air bringing up the rear.

Notice this push drives those lows across S.France.

Great to look at.

Edited December 9, 2009 by phil n.warks.

[Geordiesnow]

Decent enough run and most importantly it does not trend with the UKMO.

If i was to be selfish then i would say that shortwave in Scandi needs to come down quicker than the GFS is predicting as it would obviously bring the cold in much quicker.

[mountain shadow]

can you explain ?

The bitterly cold air is 'trapped' behind the low pressure system over Scandinavia, once it moves south the cold air will flood west towards the Uk. The Low Pressure is the dam.

[Tom D]

Some real eye-candy in this run, check these out!

[Snowaddict]

If this run were to verify then there would be widespread dirsuption to transportation across the majority of Europe, especially Scandi which would be shut off but also across the entire British Isles...

I would agree that after T+168 the evolution is a bit more dangerous so to speak, because if a low were to power up to the southwest then it could be troublesome..

However, the 'weight' of that much cold air in place should mean that things remain very cold..

An absolute stunner of a run..

It's just unbeliveable that we are seeing such synoptics, beginning firmly at T+96...

[phil nw.]

When viewing these runs it is important to check that the upper heights (the yellows, oranges, greens and blues) haven't changed too much. Surface events and 850 temps can be worried about nearer the time (within 72 hrs). On that basis this run is good again.

Yes quite right C.

The evolution to a cold setup is there.The details for next week still to be firmed up.

[Thundery wintry showers]

Can we reduce the number of one-liners please.

Regarding the latter stages of the 12Z they have a strong "February 1979" look about them with the Scandinavian Low feeding some less cold air across from the NW following what looks like it would be a colossal snow event for >80% of the country, but the southerly tracking jet remaining ever-present. Chances are other runs will be completely different, but it is certainly interesting to see these old-fashioned synoptics being played out.

Interesting times, but one important thing to look for now is whether or not the UKMO backs down from its 00Z scenario and whether the ECMWF continues to throw up a similar scenario to the GFS and the Met Office's 6-15 day forecasts.

[Neilsouth]

I don't think ive ever seen a run with cold in the close range well beyond in FI!!

http://www.wzkarten.de/pics/Rtavn2162.png

[Nick L]

Looks like a "battleground" to me at the end of the run, potentially a lot of snow from this for those north enough, but those too far south it will be a washout!

http://charts.netweather.tv/gfsimages/gfs.20091209/12/312/h500slp.png

http://charts.netweather.tv/gfsimages/gfs.20091209/12/312/h850t850eu.png

All in all though, a belter of a run.

[NorthantsSnow]

Look at the upper 850hpa temps. As low as -12 across most of England and Eastern Scotland:

http://212.100.247.145/viewimage.pbx?type=gfs;date=20091209;time=12;ext=228;file=tmp850;sess=13cd7970749c428bf2c0daaaf68f1abd;

[kold weather]

well the 12z ends up in something of a poor end synoptically with the -ve aO just imploding, wouldn't surprise me if its way too progressive on that front but we shall see.

[NorthernRab]

Lovely run for the Highlands of Scotland there, we'd be having a good traditional run up to Christmas with snow like that

Anyway, that's just eye candy. The synoptics are almost set in stone, we can comfortably say that we are about to head into a nice cold spell. I wouldn't be disappointed about the apparent breakdown in FI, it's the reliable time frame that we are concerned with and it's behaving very, very well

[Thundery wintry showers]

well the 12z ends up in something of a poor end synoptically with the -ve aO just imploding, wouldn't surprise me if its way too progressive on that front but we shall see.

I think most cold/snow lovers wouldn't mind the setup imploding at the end of the run if we were to get that kind of ultra-snowy setup for a week or so beforehand- the only unfortunate bit being that the mild air would arrive in time for Christmas. But there's no point in looking at the detail that far out. One thing that can be said of the models is that while, prior to projected wintry spells, they tend to bring the cold in too readily (which is why we need to be sceptical regarding possible downgrades), once such a wintry spell gets underway they often try to break it down too quickly. For example many runs at the end of February 2006 showed southerly tracking lows cutting off the northerly flow and giving mild south-westerlies for most of the first half of March.

[kold weather]

A very cold run at the surface from the 12z GFS though it does get rid of the upper high very quickly indeed, IMO probably too rapidly but becuase we keep a generally ESE airflow for much of the run we still end up having a stonkingly cold set-up, its very similar to the set-up from the 60s in many ways that didn't look that impressive but because of the snowfields, it'd be surprisingly cold at the surface...mind you wouldn't take much change to keep the surface conditions cold for the xmas period, though this run just fails to do it.

[londonsnow]

great 12z, although i havent, on purpose, perused deep FI. 510's flirting with the east coast in early FI. very interesting stuff.

[SP1986]

I think the run to 180 hours is starting to look a little more assured, there has been a consistent and unbroken emphasis from a large number of models towards an easterly of some description, the long it goes on the more likely it becomes. FI can be ignored or treated only as eye candy but the building blocks are there for a least a short potent easterly.

If we look at the models they are quite borderline in terms of dew point in the realistic timeframe so I imagine that if snow is to occur, we'll have to wait for it and be patient.

[NorthantsSnow]

great 12z, although i havent, on purpose, perused deep FI. 510's flirting with the east coast in early FI. very interesting stuff.

The 510 air does just about make it, with sub 522 DAM air over the entire British Isles:

http://212.100.247.145/viewimage.pbx?type=gfs;date=20091209;time=12;ext=228;file=hgt500-1000;sess=e4a0c739801d9d7e4debfc8171cb461d;

[Iceberg]

I am not saying that UKMO was right at 00Z, it was almost certainty reacting to a gradual shift of the southerly undercut northwards.

This pattern has continued with the 12Z GFS and alternative IMO.

Meto 12Z looks better anyway, but it does look more and more like a massive snow event of some description and a pretty abrupt removal of the northly high. It should be a gradualish progression of the Atlantic back in though.