Model Output Discussion - 28th November - 4th December

[Thundery wintry showers]

The other factor to take into account is that if we keep the NW-SE influence and bursts of cold zonality, and the polar vortex shifts west, then we inch closer to the scenario of the jet undercutting a developing Scandinavian High, which in the past has sometimes delivered battlegrounds between continental air and polar maritime air. I essentially see the UKMO/ECM vs GFS as a trade-off between cold but low chances of snowfall vs. generally milder but more chances of it turning snowy further down the line- though of course with it being the weather any "choice" is illusory as far as we're concerned!

I remember the blocking high of December 2006 and people wishing it further south, but this isn't a case of wishing the high further south, it's about whether we wish the high onto us in the first place (which most likely means a sustained spell of cold dry weather followed by the sinking of the high). However, it's a matter of personal preference- those who would prefer a cold dry spell are fully entitled to do so, I just don't want newbies to read the thread and confuse high cold potential with high snow potential when the two don't equate to each other.

[nick sussex]

I agree that GFS is the safest evolution into a cold spell, but I'm just mindful of the fact that a lot of people were disappointed during January 2011 because of the lack of snow in southern areas in spite of the month as a whole being near average to fairly cold. I'm also mindful of the fact that the winter of 1991/92 was arguably the most snowless of the twentieth century, despite not being particularly warm away from Scotland, primarily because of these mid-latitude highs.

Re. Jethro's questions, an interesting point- the UKMO/ECM are indeed suggesting some disruption to the polar vortex. On the UKMO the PV is becoming fragmented by T+144 while on the ECMWF it is heading west, which would again give potential for pressure rises over Scandinavia, even if they don't end up featuring on the operational run. Interestingly even the GFS with its mid-latitude high also has the PV fragmenting and the majority of it heading west. Thus, the strong polar vortex is showing consistent signs of fragmenting on the latest set of runs.

IMO TWS it's not really a proper split in the true sense, maybe someone could put up some archives showing a proper split PV.

[kold weather]

I essentially see the UKMO/ECM vs GFS as a trade-off between cold but low chances of snowfall vs. generally milder but more chances of it turning snowy further down the line- though of course with it being the weather any "choice" is illusory as far as we're concerned!

That is the problem though, I doubt the ECM/UKMO would turn snowy down the line, i suspect with the +AO regime we are in we are limited to two choices when it comes to cold, either WNW/NW airflows like currently which will be cold enough for snow in the north but only average down south and marginal snow away from the hills, or an upper high nearby that could either sink or eventually relocate further north into a better position. With the set-up aloft I'd not want to risk the jet getting going and like 06-07 get stuck in an endless jet beast...its a real risk if you get a +ve AO as strong as we have at the moment...

IMO the GFS is the way to go, take a look at Feb 91 for an examplwe of how you can evolve into a powerful easterly with some patience even in a powerful PV/+AO solution.

ECM slightly better for those in the north due to the marginal snow chances it has, GFS is much better for those in the south...because at least we'd actually get a decent air frost!!

[jethro]

Thanks for the answers folks. I just see big Purple blobs scattered around the NH and compare them to memories of previous years when there was only one big Purple blob, which apparently is not a good sign for snow here.

[Grimsby Snow Lover]

That is the problem though, I doubt the ECM/UKMO would turn snowy down the line, i suspect with the +AO regime we are in we are limited to two choices when it comes to cold, either WNW/NW airflows like currently which will be cold enough for snow in the north but only average down south and marginal snow away from the hills, or an upper high nearby that could either sink or eventually relocate further north into a better position. With the set-up aloft I'd not want to risk the jet getting going and like 06-07 get stuck in an endless jet beast...its a real risk if you get a +ve AO as strong as we have at the moment...

IMO the GFS is the way to go, take a look at Feb 91 for an examplwe of how you can evolve into a powerful easterly with some patience even in a powerful PV/+AO solution.

ECM slightly better for those in the north due to the marginal snow chances it has, GFS is much better for those in the south...because at least we'd actually get a decent air frost!!

At present the bbc seem to be following the gfs runs as they clearly expect to have a high pressure system in the Atlantic blocking weather patterns for next week end.

http://www.bbc.co.uk/weather/feeds/16025258

[Liam J]

ECM is rock solid with it's outputs, rather flat zonal setup with LP crossing west to east across the Atlantic and over the UK, UKMO also seems to want to back this evolution along with some of the lesser models. I would go with the majority on this one which seem to fit in better with the tele data at the current time.

Unless of course the GFS is about to throw egg in the faces of quite a number of the models.

Edited December 4, 2011 by Liam J

[I remember Atlantic 252]

That is the problem though, I doubt the ECM/UKMO would turn snowy down the line, i suspect with the +AO regime we are in we are limited to two choices when it comes to cold, either WNW/NW airflows like currently which will be cold enough for snow in the north but only average down south and marginal snow away from the hills, or an upper high nearby that could either sink or eventually relocate further north into a better position. With the set-up aloft I'd not want to risk the jet getting going and like 06-07 get stuck in an endless jet beast...its a real risk if you get a +ve AO as strong as we have at the moment...

IMO the GFS is the way to go, take a look at Feb 91 for an examplwe of how you can evolve into a powerful easterly with some patience even in a powerful PV/+AO solution.

ECM slightly better for those in the north due to the marginal snow chances it has, GFS is much better for those in the south...because at least we'd actually get a decent air frost!!

agree with last bit, certainly prefer GFS would be very cold (inversion style) but feel 18Z will follow ECMWF 12Z, dont know any technical stuff, but ECMWF looks like we (south) are set for a mild Xmas, GFS who knows could well be very cold and frosty

[johnholmes]

http://www.emc.ncep..../html/acz6.html

will allow folk to see how the 'big' 3 models are at day 5 and day 6 for the whole noerthern hemisphere at 500mb.

Note how badly GFS was at the end of November and as it often is its behind both Met and ECMWF at the moment.

No bearing necessarily on how each performs at the far edges most of you seem to look at but its a fair insight into which to believe in my view.

Taking the 500mb anomaly charts with it and it gives a reasonable basis for the general trend of the upper air for 10-15 days ahead.

Edited December 4, 2011 by johnholmes

[kold weather]

The thing that I like about the ECM which hopefully comes off is its less positive with the AO...however I fear its rather under-estimating things on that run as nearly all the models have done in the last month or so.

At least none of the runs have anything above average right now...so count your blessings I suppose!!

[Thundery wintry showers]

IMO TWS it's not really a proper split in the true sense, maybe someone could put up some archives showing a proper split PV.

True, the PV is merely showing signs of starting to fragment in these runs, it certainly isn't splitting enough to allow any significant northern blocking to occur, only potential if the PV was to continue fragmenting in that way and eventually split altogether.

I see Kold Weather's point but I think we'll have to agree to disagree on it, as I think the disagreement owes more to personal preference than anything else. The high pressure scenario may be a low risk scenario for those whose main priority is to avoid a mild and/or completely "westerly" December, but it's a high risk scenario for those who, like myself, groan inwardly at the possibility of a prolonged spell of cold dry cloudy weather followed by a gradual sinking of the high towards Christmas. I don't think there is a right or wrong answer here.

A flatter pattern still looks odds-on to me judging by the teleconnection signals, rather than continued average to cool zonality, it all depends on where the high ends up, whether it ends up over us as per GFS or to the south as per ECM giving a milder westerly type.

[JP1972]

Slightly off topic so apologies in advance!

First post but been visiting for ooh about 4 years now, mainly in winter it has to be said!

Very green at all this and a lot of respect for the Murr's, Nick Sussex's, John Holmes's to name but a few. You all do a brilliant job at keeping novices like me abreast of what's going on and what we might expect.

Anyway back on topic now and I have to say a very interesting and exciting period ahead in terms of model watching. I have to laugh at how in the space of a week we have gone from a zonal express to something a little more interesting. If the situation can change so much in such a short space of time I think it's possibly best to keep an open mind as to what the models have in store for us in the days and weeks ahead.

[johnholmes]

At present the bbc seem to be following the gfs runs as they clearly expect to have a high pressure system in the Atlantic blocking weather patterns for next week end.

http://www.bbc.co.uk.../feeds/16025258

Having just watched that Peter Gibbs forecast I would say he followed, as far as it went to Thursday his own set of model and Fax charts. perhaps it might be fairer to say the two models are pretty similar to Thursday?

[nick sussex]

Slightly off topic so apologies in advance!

First post but been visiting for ooh about 4 years now, mainly in winter it has to be said!

Very green at all this and a lot of respect for the Murr's, Nick Sussex's, John Holmes's to name but a few. You all do a brilliant job at keeping novices like me abreast of what's going on and what we might expect.

Anyway back on topic now and I have to say a very interesting and exciting period ahead in terms of model watching. I have to laugh at how in the space of a week we have gone from a zonal express to something a little more interesting. If the situation can change so much in such a short space of time I think it's possibly best to keep an open mind as to what the models have in store for us in the days and weeks ahead.

Welcome to the emotional rollercoaster known as the model discussion thread! and yes certainly your last sentence is very good advice.

[kold weather]

I see Kold Weather's point but I think we'll have to agree to disagree on it, as I think the disagreement owes more to personal preference than anything else. The high pressure scenario may be a low risk scenario for those whose main priority is to avoid a mild and/or completely "westerly" December, but it's a high risk scenario for those who, like myself, groan inwardly at the possibility of a prolonged spell of cold dry cloudy weather followed by a gradual sinking of the high towards Christmas. I don't think there is a right or wrong answer here.

Yeah, I'm not against a good westerly burst but knowing how the background set-up is at the moment and how super positive the AO is (record breaker by the way!) I'd fear we'd have a zonal and probably increasingly mild zonal set-up for a VERY long time. Also GFS would at least give E.Europe snowcover and cool the SST's, I suspect cool zonality won't really help us much in either of those aspects...

For now we are lucky that the pattern is so flat as its allowing a somewhat decent burst of cold zonality, but you've gotta wonder how sustainable a set-up like that really is...

Ian, I agree, the ECM is going to take a long time to evolve into anything more then cool zonality...however its certainly not a bad run, we are always flirting with colder air and I'm sure the ski resorts would be loving this pattern.

[JP1972]

Thanks Nick, I don't know why I haven't signed up sooner to be honest!

Funny how the teleconnections are almost saying one thing 'should' be happening but the models are hell bent on saying something completely different.

[Thundery wintry showers]

Probably not sustainable for too long I imagine, with the odds favouring a flattening of the pattern in about 7-10 days' time.

It will be interesting to see the ECMWF ensemble mean to see if there is growing support for the mid-latitude high scenario. There wasn't much support on the 00Z ensemble set, but with the GFS sticking to its guns it's a significant possibility.

[BARRY]

Thanks Nick, I don't know why I haven't signed up sooner to be honest!

Funny how the teleconnections are almost saying one thing 'should' be happening but the models are hell bent on saying something completely different.

Thing is after all the mild mush that we have been having the models always trend on the mild side, so the next 5 days will be intresting.

[bluearmy]

i thought that we would see a p/v that is tightening its grip on the NH, given the lag factor on a cold strat. instead, whilst not splitting by any stretch of the imagination, it is fragmenting somewhat on most models and the spread of cool uppers around the NH is not what i expect to see from a frigid strat profile. so i can only deduce that as so many have cautioned, the strat is only part of the puzzle and there are many other factors at play here. it would be foolish to presume that the whole of december cannot see a deep cold spell because the strat is currently too cold to allow it. it would also be foolish to predict that it will, based on what the models currently show. we are reasonably seasonal out to mid month and then - who knows !!

[kold weather]

BA, I think the AO has responded very well to the super cold start...we had the most positive AO EVER recorded today by quite some way and the 3rd most positive a few days ago as well...so in that respect its really shown itself. Rather than a central PV core that is tight and allowing some amplification of the jet we seem to have a large spread out feature that has dominated the whole of the arctic and kept things very flat. Of course its going to look weaker on the models because its going to be impossible to hold a record breaking feature for even a couple of days.

[Gibby]

Hi folks. Now the dusts settled on the 12z outputs, here's my interpretation of what to expect from GFS, UKMO and ECM.

The GFS operational tonight shows the weekdays to come as rather cold and unsettled with snow at times for Northern high ground and low ground too further North. For a time midweek temperatures will rise somewhat as winds back more Westerly and Atlantic fronts cross over bringing some rain for all. Late in the week cold air sweeps back SE over all areas with snow showers returning for many Northern and Western hills. Then from next weekend a trend develops which continues through FI. High pressure moves over and remains close to the UK in some shape or form from then until the end of the run. This would mean that areas of freezing fog and frost could form and become reluctant to clear through the days, making it locally very cold where this happens.

The GFS Ensembles show that after a cold week to come 850's become close to the long term average for London. In Aberdeen most members recover from members to a mild blip in mid term before becoming closer to the long term average. The operational was slightly above the long term mean here for the second half of the run.

UKMO tonight shows this week to be dominated cold Westerly winds with wintry showers in Northern and Western regions but a lot of drier weather for the east and southeast. Through midweek a milder interlude seems likely in the South as frontal troughs push wind and rain across many areas quickly followed by a return to cold NW winds and wintry showers by Friday. An interesting situation develops on Saturday as Low pressure develops over the English Channel and the North Sea keeping cold and unsettled conditions going with snow on high ground widely and even the chance of a flake or two in the South.

ECM too keeps a cold and showery West flow interrupted by less cold and cloudier periods as rain belts spread East through this week. Unlike GFS it prefers to keep an unsettled Westerly flow going until term of the run with occasional snow to higher ground, especially in the North.

In Summary GFS is keen to bring a change in weather type by next weekend as it replaces the cold West or NW flow with high pressure moving in from the SW. UKMO and ECM both show alternative outcomes with both showing what looks to be a continuation of rather cold conditions with the risk of snow at times in the North and rain or showers elsewhere. Temperatures look like staying on the lower side of normal for the majority of the time on the surface throughout.

[Glacier Point]

BA, I think the AO has responded very well to the super cold start...we had the most positive AO EVER recorded today by quite some way and the 3rd most positive a few days ago as well...so in that respect its really shown itself. Rather than a central PV core that is tight and allowing some amplification of the jet we seem to have a large spread out feature that has dominated the whole of the arctic and kept things very flat. Of course its going to look weaker on the models because its going to be impossible to hold a record breaking feature for even a couple of days.

Monitoring tells us that the cooling is most pronounced around 60N, which is further south than under most strong vortex events. What is really striking at the moment is the modelling of the tropospheric vortex; the primary centre west of Greenland; the second forming in the Bering Straights - totally out of kilter with Ninas and likely to cause no end of problems with NWP as they will be preconditioned to programme a ridge where troughing is developing. Slowly, slowly, starting to develop as a hemispheric pattern but going to take a long while to blossom.

[chionomaniac]

BA, I think the AO has responded very well to the super cold start...we had the most positive AO EVER recorded today by quite some way and the 3rd most positive a few days ago as well...so in that respect its really shown itself. Rather than a central PV core that is tight and allowing some amplification of the jet we seem to have a large spread out feature that has dominated the whole of the arctic and kept things very flat. Of course its going to look weaker on the models because its going to be impossible to hold a record breaking feature for even a couple of days.

I think KW that you are right to point out that the polar vortex is spread out rather than splitting as such. This can be demonstrated in the potential vorticity charts at 380K. Last year we saw a proper split of the vortex led by local wave breaking from the troposphere into the stratosphere right up to the 30 hPa level. The diffluent but strong polar vortex seen in the tropsophere this year has no support from the stratosphere and is therefore extremely unlikely to lead to anything other than displacement distortions by Rossby waves at the vortex's edge.

[Partholon]

Monitoring tells us that the cooling is most pronounced around 60N, which is further south than under most strong vortex events. What is really striking at the moment is the modelling of the tropospheric vortex; the primary centre west of Greenland; the second forming in the Bering Straights - totally out of kilter with Ninas and likely to cause no end of problems with NWP as they will be preconditioned to programme a ridge where troughing is developing. Slowly, slowly, starting to develop as a hemispheric pattern but going to take a long while to blossom.

Monitoring tells us that the cooling is most pronounced around 60N, which is further south than under most strong vortex events. What is really striking at the moment is the modelling of the tropospheric vortex; the primary centre west of Greenland; the second forming in the Bering Straights - totally out of kilter with Ninas and likely to cause no end of problems with NWP as they will be preconditioned to programme a ridge where troughing is developing. Slowly, slowly, starting to develop as a hemispheric pattern but going to take a long while to blossom.

Are you saying that signals are emerging to support what you expect to unfold in 6 or 7 weeks time?

[Glacier Point]

Tentatively yes, particularly in the Pacific which is now starting to develop an upper low which will keep the main core of the Pacific ridge in the NE off Alaska setting the downstream wave length. I've flagged the possible role of the Madden Julian Oscillation wave in the Indian Ocean in October as a possible route map for pattern change in January, and one of the key December developments was an upper low starting to form in this area. Hopefully in the next 7-10 days I can post the pressure anomaly for this month and the comparison with those MJO analogue years which should begin to give extra confidence for change in January.

[Matty M]

Well the 18z is nicer at this point.