Winter Model Discussion - The Hunt For Cold -18z 17/12/12

[TEITS]

Very impressive upper temp mean.

How ironic that we have been looking E and yet potentially could see colder weather arrive from the N between +144 to +168.

[Ice Day]

A very tasty ensemble mean for Boxing Day:

Looks very nice. The thing I'm struggling with as a novice, is that the volatility of the models at the moment makes picking any trends out difficult in the extreme. Tonight's output seems to show the middle of next week as potentially quite wintry, however this maybe gone tomorrow!

[Weathizard]

Very impressive upper temp mean.

How ironic that we have been looking E and yet potentially could see colder weather arrive from the N between +144 to +168.

Happy with that, -4 covering most of the country on the MEAN is very impressive? Must be some very decent runs in there?

[Panayiotis]

Judging by the short ensembles all members after the 27th have dipped below the 0C line, this IMO is evidence that we are getting more and more confident for a cold spell past Christmas (for my area) 45% chance of snow!!!

http://modeles.meteociel.fr/modeles/gens/graphe_ens3.php?x=&ext=1&y=&run=18&lat=51.53846153846153&lon=-3.30188679245283&runpara=0

[SE Blizzards]

Snow risk chart for next friday according to the GFS 18Z....

One too many Christmas sherry's down the local me thinks!!

Having said that, i am starting to feel though after viewing the charts today that some sort of Northerly may be on the cards shortly after Christmas day!?

Hopefully tomorrows runs will firm up on this idea!

Edited December 19, 2012 by SE Blizzards

[Don]

Could we possibly be heading towards a similar type of cold snap that occurred between Christmas and New Year 1985?

Edited December 19, 2012 by Don

[swilliam]

Happy with that, -4 covering most of the country on the MEAN is very impressive? Must be some very decent runs in there?

Lots of really good members but what is more impressive is the clustering.

Edited December 19, 2012 by swilliam

[peterf]

christ look at that precip rolling in from the SW.i fear for a lot of folk over the coming days.

sorry wrong thread mods feel free to remove.

[LomondSnowstorm]

Looks very nice. The thing I'm struggling with as a novice, is that the volatility of the models at the moment makes picking any trends out difficult in the extreme. Tonight's output seems to show the middle of next week as potentially quite wintry, however this maybe gone tomorrow!

The details are changing quite a bit but the basic theme has been the same in all the runs today: heights lowering to the northeast gradually sinking southwards introducing a northwesterly flow from Christmas Evening onwards. Just how cold and snowy this flow will be will be determined by how far southeast the low heights get, and the 18Z and its ensembles showed a particularly favorable setup where everything was a bit further southeast than on the 12Z. For it to really affect the south you have to have something like the 18Z but there is more margin for error further north. Some runs tomorrow will probably look a bit worse, others a bit better, but I think the general idea is likely to stay much the same.

[bradymk]

Very impressive upper temp mean.

How ironic that we have been looking E and yet potentially could see colder weather arrive from the N between +144 to +168.

yes very good teits, 18z ensembles impressive imo even the shorter ones show every run below 0c after day 7/8. The mean heights are pretty good aswell so far. I think its safe to say before any strat warming influence, we should be looking north for any cold and my confidence of a northerly/nwly have increased tonight.

[Zakos]

Very impressive upper temp mean.

http://modeles.meteo...21-0-192.png?18

How ironic that we have been looking E and yet potentially could see colder weather arrive from the N between +144 to +168.

Yes, things starting to get interesting now

I know, as soon the prospects an easterly are finally put in the grave, cold almost immediately attacks from the north.

Why? because of the distorted vortex and large Arctic high

This is exactly why I dont agree with the pessimism on here.

aslong as we have this amazing hemispheric pattern cold spells can appear out of nowhere with little warning.

[Ice Day]

The details are changing quite a bit but the basic theme has been the same in all the runs today: heights lowering to the northeast gradually sinking southwards introducing a northwesterly flow from Christmas Evening onwards. Just how cold and snowy this flow will be will be determined by how far southeast the low heights get, and the 18Z and its ensembles showed a particularly favorable setup where everything was a bit further southeast than on the 12Z. For it to really affect the south you have to have something like the 18Z but there is more margin for error further north. Some runs tomorrow will probably look a bit worse, others a bit better, but I think the general idea is likely to stay much the same.

Cheers LSS. Certainly the dip in the ensembles from around 26th is interesting; and as swiliiam says above, the clustering of them is impressive. Let's see what the 00z brings?

[Jason M]

Big upgrade on ensembles out to 192. Could still do we a slight tweak east but pretty good agreement. Only a quick look but the opp looks a mild outlier to me.

Anyways, lets see what the morrow brings.

Jason

[nick sussex]

The GFS 18hrs run looks similar to the JMA with a crucial split in the energy train out of the eastern USA.

Without this you can't get enough forcing on troughing near the UK, the ECM keeps an elongated troughing into the UK which tilts sw/ne.

So I think we must see that split upstream, another thing we have to factor in is shortwave development around Iceland which might still appear given the timeframes involved.

Because the pattern flattens out again across the USA your start point re location of the jet will impact the track of low pressure into the UK after this point.

Its very difficult however to maintain a southerly tracking jet into the UK with a western based negative NAO as the weak point near the UK will see the jet curve ne at this point.

So although the GFS 18hrs run was a small step in the right direction, we still need to see the pattern edged further east and south.

[karlos1983]

The GFS 18hrs run looks similar to the JMA with a crucial split in the energy train out of the eastern USA.

Without this you can't get enough forcing on troughing near the UK, the ECM keeps an elongated troughing into the UK which tilts sw/ne.

So I think we must see that split upstream, another thing we have to factor in is shortwave development around Iceland which might still appear given the timeframes involved.

Because the pattern flattens out again across the USA your start point re location of the jet will impact the track of low pressure into the UK after this point.

Its very difficult however to maintain a southerly tracking jet into the UK with a western based negative NAO as the weak point near the UK will see the jet curve ne at this point.

So although the GFS 18hrs run was a small step in the right direction, we still need to see the pattern edged further east and south.

So proceed with caution is the advice at the moment I guess. . . Perhaps I'll give the 00z a miss, or look with one eye open through the gaps of my fingers!

[mountain shadow]

The possible synoptic pattern for Scandi trough and Greenland heights has been in the model output for a good while, the delaying factor was the persistant russian high pressure.

If we were facing a northerly toppler we would normally expect it to shunt eastwards as we approached t+0. Perhaps with the possible west based -NAO we will see it move more neutral and deliver a bullseye.

Answers in the morning?

[Panayiotis]

Shows how people can jump on to conclusions to quickly by just viewing one run eg On Sunday many people were going for a mild Christmas, but for me (ive always said this) that this Christmas would be cool and wet and certainly not mild. Ensembles show this quite clearly for the 18z but many runs today have been cooler for Christmas..

Edited December 20, 2012 by Panayiotis

[Dunstable Snow]

Not posted for a while but tried to keep up with this thread in between very busy work commitments. Fully agree with Nick S assesment and those of a couple of other learned members earlier. Perhaps our start point given the experience of the last few weeks should be to assess what can and is likely to go wrong when we see a set of encouraging charts ( as per 18Z tonight) before we assess the positives . At least then if things stay the same or upgrade the mood can only lighten as we go forward ! I have to say that I am very nervous about proceeding down the west based NAO route. If this is the road we start to journey down as we proceed into the new year I fear for the mood on this thread.

My teenage sons keep telling me what a miserable old man I am ( I'm not that old by the way !) and the above comments suggest they may be right. I am going to end on a positive however :- The North West will I feel do quite well out of this set up next week and developments within the stratosphere and the continued pressure placed on the polar vortex will, I believe, utlimately lead to a cold and wintry outbreak. I suspect that this may not arrive until last week of January however, but I believe will be long lasting- up to 3 weeks - .We are, I suspect, going to endure some pain before hand.

Cheers.

[Dunstable Snow]

Sorry just re-read my post and should have said west based negative NAO route - apologies - too much red wine tonight !

[nick sussex]

So proceed with caution is the advice at the moment I guess. . . Perhaps I'll give the 00z a miss, or look with one eye open through the gaps of my fingers!

It's best to factor in noted biases of the GFS at this range in terms of the southwards extent of cold uppers. This often occurs because it fails to pick up shortwaves at longer range, especially in this instance where you have that deep cold hitting the Iceland area.

Also the ECM keeps a train of energy running east, this wouldn't be such a problem if you had the jet south of the UK but as it is that has a western neg NAO with a trend to angle the jet more sw/ne later.

I think to get something better we're going to need something to help pull the Azores high further west and north to slow the next low and add a bit of oomph to the push south of colder air south into the UK, so we need to see more amplification off the eastern USA.

We'll see if that happens over the next few runs, overall to be honest the models look rather underwhelming and the GFS 18hrs run is a bit better but we do need to see some biggish changes to develop anything resembling true winter weather.

First step would be a big push east in any positive heights modelled to the nw.

Edited December 20, 2012 by nick sussex

[Isolated Frost]

Really interesting synoptics atm

Displaced bartlett hp split over Greenland and Siberia!

Displaced Icelandic low in Scandinavia?

Anyway...

Bye bye vortex- the canadian heights (the ones that wouldn't been conductive to any colder weather), long stretch northerlies with an elongated lp section to the NE of us- this isn't going to be your typical mild pattern, definitely cold-orientated, especially for those further north

[Captain Shortwave]

Yeah you are probably right Nick, but it's nice to have some signs of hope though. I guess it's best to keep your estimations low. Just got to look for further amplification off the Eastern seaboard to pull the Azores ridge away from it's Spanish holiday and do us some real favours.

If all go to our hopes, we might not get a white Christmas, but by then we might be able to Nail on some real prospects for a cold spell. Wouldn't be Christmas without a bit of ramping

[Zakos]

Really interesting synoptics atm

Displaced bartlett hp split over Greenland and Siberia!

Displaced Icelandic low in Scandinavia?

Come to think of it, there seem to be displaced Bartlett systems all over the hemisphere at present? a worrying trend.

To the newer members who are no doubt wondering what on earth we are on about :

http://forum.netweat...artlett-set-up/

"A 'Bartlett' High (also referred to as a 'Eurotrash' High) is a very persistent area of high pressure situated over Europe during the winter months - for Britain this means weeks or months of very mild though often wet and windy weather under a predominantly southwesterly airflow. The worst possible conditions for anyone wanting cold and snow. A typical 'Bartlett Winter' was '88/'89 - one of the mildest winters on record."

There is no such thing as a "displaced Bartlett" because by definition it has to meet these requirements.

There is nothing that resembles a Bartlett on show in any of the models at present.

Even in the depths of FI on the 12z, there is nothing that resembles a Bartlett high.

There was a trend to build HP to the south of the UK. However the models are now trending away from this idea, with the 18z the latest to continue this trend

See my post here as to how I interpret this trend - http://forum.netweat...60#entry2452063

Edited December 20, 2012 by Zakos

[gaia rules]

It's best to factor in noted biases of the GFS at this range in terms of the southwards extent of cold uppers. This often occurs because it fails to pick up shortwaves at longer range, especially in this instance where you have that deep cold hitting the Iceland area.

Also the ECM keeps a train of energy running east, this wouldn't be such a problem if you had the jet south of the UK but as it is that has a western neg NAO with a trend to angle the jet more sw/ne later.

I think to get something better we're going to need something to help pull the Azores high further west and north to slow the next low and add a bit of oomph to the push south of colder air south into the UK, so we need to see more amplification off the eastern USA.

We'll see if that happens over the next few runs, overall to be honest the models look rather underwhelming and the GFS 18hrs run is a bit better but we do need to see some biggish changes to develop anything resembling true winter weather.

First step would be a big push east in any positive heights modelled to the nw.

Am I right in thinking that the UKMO and NOGAPS go for the development of the central atlantic high which develops from the Azores and Greenland highs at about T144 whereas the ECM and GFS go for continued jet streaming across pushing back the Atlantic. The decision point appears around Christmas Day (assuming the world has not ended by then but none of the ensemble members have picked that yet....who knows though that even at T24 they are struggling!)

[Bonjangle]

There’s a chance of colder air pushing south between Christmas and the New Year. That still seems possible this evening. The cold block to the east of us will continue to be eroded in the days ahead, which will allow weather systems across the North Atlantic and western Europe to become more mobile. So that means low pressure systems will be able track further east, leaving the UK on their western side for a time, with the winds coming from a more northerly or north westerly direction. Without a build of pressure over Greenland this type of pattern doesn’t usually last long, with the next low pressure zipping in from the west to to cut off the cold flow over the UK. Nonetheless, we can get snow from this set-up, and I think January 2003 or 2004 (I can’t remember which year, they all blur together after a while!) provided a couple of decent examples. The London 12z GEFS 850hPA chart below has a number of runs which do bring colder air a long way south between Christmas and the New Year, but as you’ll see there is a lot of scatter between the runs after Boxing Day. Remember scatter is bad from a weather forecasting viewpoint because it means a range of different solutions are being generated by the computer model, so confidence in any particular one being correct is reduced. At the moment it remains a case of watching and waiting, but don’t think it’s yet worth doing a blue v green analysis like I did a few weeks ago. The main weather event in the short term is the heavy rain and flood risk, so do keep checking your local forecast. This is from TwoBuzz!