Unless you're a gardener/agricultural worker in the north

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It is classified as Nightmare on Bart Street. 

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True meaning of it - had to google it: http://weatherfaqs.org.uk/node/117

 

 

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12:00 temps IMBY over the next few days

5c 10c 7c 4c 6c 7c 4c 6c 9c 7c 11c 8c 5c 5c 3c 3c

Not that many days one can nip to the beach in Budgie Smugglers

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Following James's excellent post earlier on the strat temps I confess I've just had a look across three models at 10hPa temps, and concede that on GFS at least the temp at 10hPa above Greenland rises from -80 to -10 in the space of 7 days beginning on Thursday.

Now please do not misinterpret this observation as anything other than that - I have minimal understanding of why it does that and what effects it might have, but taken in isolation it is an interesting phenomenon nevertheless.

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Well, this is relevant to models because as a longstanding colleague said to me just a few days ago: "It was easier in the old days with just one model to scrutinise. Now, we are victims of our own success, with each forecast failure made into a big deal by snipers. We are often guilty of examining too many models, too many competing solutions, and then terrifying ourselves over the validity of a forecast 14 days away when we haven't even yet nailed what a severe thunderstorm will do 14 minutes away."

 

From IF on the other side

 

How true is that

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I've always said this. It's why I find the whole notion of LRFs so utterly utterly pointless as things stand. 

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There is nothing new to report here this morning - the huge sea of deeply low 500mb uppers across Canada continues well into the New Year. There will be no HLB around our longitudes so long as that remains, so the outlook is as before - sinusoidal, mobile, changeable and 'mean'.
The warming at 10hPa over S Greenland starting on Friday raises temps from -80 to -7 by early next week. I guess this could be classified as one of the categories of SSW, but what propagates down and how it affects 500mb heights has I suspect yet to be resolved. The optimistic part of me will accept that for now it is beyond the model range.

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HERE IS MY LATEST ANALYSIS FROM THE OUTPUTS OF THE WEATHER COMPUTER PREDICTION MODELS FOR TWO WEEKS FROM 8AM TODAY WEDNESDAY DECEMBER 31ST 2014.

NEXT UPDATE WEDNESDAY DECEMBER 31ST 08:00

THE CURRENT GENERAL SITUATION Fronts will move SE across the UK in a freshening SW flow today and again tomorrow with a colder interlude of Westerly winds across the NW tonight..

MODELS-2 WEEK HEADLINE Becoming unsettled and windy later this week and onward with rain or showers at times and snow on Northern hills. Some drier periods likely in the South.

THE JET STREAM ENSEMBLE FORECAST The Jet Stream Forecast shows little change from that of recent output, remaining flowing in a relatively flat pattern West to East across the UK or close to the South between Low pressure to the North and NW and High pressure in a belt from Southern Europe and the Azores.

GFS OPERATIONAL The GFS operational this morning shows a prolonged zonal spell of weather coming up with just brief chillier and drier spells in a NW flow alternating with much longer periods of relatively mild but very windy weather with spells of rain and showers with snow restricted to Northernmost high ground where the heaviest rain is also expected.

THE GFS PARALLEL RUN The Parallel run is very similar to the operational this morning with the emphasis on the heaviest and most unsettled conditions lying across the North of the UK as High pressure comes very close to Southern Britain at times pumping mild and rather cloudy and drizzly conditions here rather than the heavy rains of the NW. Some short cooler periods with showers, wintry on Northernmost hills is shown but this covers the minority of time within the period with ni imminent change indicated by Day 15.

THE GFS CONTROL The GFS control run is very akin to the parallel run with days of mild SW winds and occasional rain while short colder interludes with showers occurs more likely at first in the period. As with the other runs the wettest conditions will be in the North with the highest of the rather mild temperatures in the South.

THE GEFS ENSEMBLES The GFS Ensembles show strong support for a Westerly flow to dominate the UK through the period. With Low pressure biased to lie to the North and High near the Azores and Southern Europe Westerly winds, strong at times will follow their normal characteristics of spells of wind and rain for all with slightly colder and more showery interludes in between, these most prolific across the North while the heaviest rain occurs across the North and West.

UKMO UKMO shows a mobile Westerly flow developing across the UK from today onwards. With Low pressure to the North and High to the South there will be fronts moving East in the flow bringing rain at times and short brighter, slightly colder conditions with showers perhaps wintry over the Northern hills.

THE FAX CHARTS The Fax Charts follow the raw data quite well today with a milder SW flow replacing the cold High across Southern Britain over the New Year. A cold front in the stronger SW flow will clear SE by the weekend with another set of fronts approaching the UK from the SW later in the weekend following a brief chillier ridge on Saturday.

GEM GEM today shows a sustained period of strong Westerly winds with gales possible in all areas. It will be generally mild with just short colder and more showery phases, chiefly across the North whereas most of the time outside of this will be cloudy and dull with spells of rain, especially across the West and North.

NAVGEM NAVGEM also shows the well worn track between the models recently of replacing High pressure with a milder Westerly flow between High pressure to the South and Low to the North with rain at times with some brief drier and chillier days in between, these mostly restricted to the North.

ECM ECM this morning shows High pressure to the South or SE and SW of the UK driving mild and strong West or SW winds across the UK with rain at times especially towards the North and West. Later in the period as the pressure gradient strengthens across the UK gales and heavy rain can be expected for all areas with some colder and blustery westerly winds with showers wintry in the North becoming more dominant.

ECM 10 DAY MEAN The ECM 10 Day Mean Chart shows Low pressure to the North and High to the South the most likely scenario at Day 10. This would indicate a strong Westerly flow with spells of rain mixed in with slightly colder and more showery conditions in between, the latter most likely over the North.

NOTABLE TREND CHANGES FROM PREVIOUS RUNS The trend this morning remains unchanged with all models supporting a sustained and little removed period of Westerly winds across the UK, quite mild at times especially across the South with spells of rain and a few brighter and colder interludes especially at first and again later over the North.

MY THOUGHTS To illustrate how set the models are in their output currently I could of cut and pasted a lot of the models text from yesterday into today's and it would not of looked out of place. The theme remains for a strong Westerly component to the weather in a persistently zonal period driven by a strong belt of High pressure from Southern Europe to the Azores and Low pressure to the North and NW of the UK. Differences day to day will depend on whether we are under warm sector conditions of cloudy and drizzly weather with temperatures near or above average and this looks more likely for longer periods across the South where not too much rain is likely. Alternatively there will be some chillier and clearer weather on occasion, these most likely at first almost anywhere and later more likely over the North where a little snow will continue to be a risk at times over the mountains. The long term pattern of the next two weeks depressingly looks unlikely to change at the end of the period either reading from today's output as GFS and other long term models support little if any significant change in synoptics at Day 14-15 with the only trend that I can see in being a deepening of the unsettledness across the UK with all areas coming under Westerly gales and colder showery periods in between, a point at which all areas could share in some wintry showers over the hills but this is a grain of sand in a desert of damp and mild Westerly winds when few of any of us living in lowland Britain will see any snowfall this side of mid January at least. Frosts will also be relatively scarce and slight with no chance of any as sharp as those of recently being repeated within the period and conversely night's could be unusually mild at times in the warm sector conditions across the South expected.

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There will be no SSW in the near term. There is a general warming underway which is very marked. There is now no model which generates a proper strat vortex split and reversal of flow at the right lat and height to be a technical SSW.

There has to be some consequences to this warming but they are currently unclear and may not affect nw Europe anyway.

Like I said yesterday, I believe that the trop is the main driver at the moment and in the absence of a strong strat reversal which would drive a neg AO response in the strat and trop, I don't expect a change to the zonal flow we currently see modelled. The envelope allows for a drifting north of the AH to bring a more settled period to the south of the uk or a sinking south of the PFJ and the trough with it. still zonal across this side of the NH, even if either of these two appear.

Agreed. SWW isn't actually defined solely by temperature but by flow reversal from W to E and that as you say is not being modelled - not today anyway.
We'll just have to rely on a general relaxation of the surface mobility into February to rescue what could turn out to be another huge disappointment of a winter

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Run of temps IMBY

6c 10c 8c 4c 7c 8c 5c 7c 9c 6c 9c 11c 6c 8c 4c 3c

I cant see you 3 weeks of temps at 12c Tally

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ECM and GFS have finally met at halfway house in terms of strat. output this morning. The result is sufficient to greatly weaken the vortex, but not enough to maintain a decent split or generate a new one within the next 10 days.

The current Canadian vortex falls apart but a new one arrives from day 10 onward, this being a weak feature but still capable of preventing a total shutdown of the low pressure chain from the U.S./Canada into the Atlantic.

This is far from ideal, falling short of the major SSW as defined by Cohen (see my post yesterday evening).

Still, with the warming near or over Greenland persisting out to day 10 and beyond, I can see some support for a more amplified Atlantic pattern later in time, which could lead to some interesting results in 2-3 weeks time if the Atlantic jet is weak enough. In terms of the potential for a more sustained mid-Atlantic ridge, that's like an upgraded version of what we went through earlier this month.

 

Really though, I'm not very encouraged by this current ECM/GFS consensus, given how stubborn the pattern has been to allow such changes so far this season. Sure, the far better placement of the persistent warming that improves our chances this time around, but I'd much rather see a major SSW event as that would shake up the pattern beyond recognition, which from where we look to be in a week's time would be most welcome.

 

You never know though, NAVGEM might have it nailed:

 

 

A word on the 500mb output... still quite a good vacation of low heights from Canada going on days 9-10 on the ECM run and GEM is similar, GEM not quite so keen though and GFS/GFSP really not keen on such a fast response to strat. forcing - you're looking some 4-5 days later for such changes on those runs and even then its a bit half-arsed. We're left hoping that GFS/GFSP are being too flat with the U.S./Atlantic pattern... a familiar story!

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One for Tally

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Actually Nick from the data I have looked at it does look like a minor SSW (not major) event has/is occurring. Whether that has any impact whatsoever on our weather is an entirely different matter.

That aside, the output is still rather westerly and mobile with everything one would expect from that. Once again in FI the GFS op run continues this unsettled theme but with lower heights and a chilly edge to it.

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Might as well start looking at the next OPI

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No technical SSW Michael, and as Neil observed earlier, the warmings just relax the zonal flow whereas the reversal would generate a neg AO response.

just think the trop is making the running and the strat would have to make a huge intervention to change things. it's not yet jan so plenty of time, given the weak strat vortex wouldn't hold back a downwelling for very long at all.

A refection of desperation about current output that we have to go to +800h to find eye candy!

The more amplified ridge around 144h confined to just 4 ENS on GEFS.  GEFS  overall supports the mobile westerly theme described in METO extended foprecast

 

 

The definition of an SSW is:

A sudden stratospheric warming (SSW) is an event where the polar vortex of westerly winds in the winter hemisphere slows down or even reverses direction over the course of a few days. The change is accompanied by a rise of stratospheric temperature by several tens of kelvins.

http://en.wikipedia.org/wiki/Sudden_stratospheric_warming

Therefore, surely what we are seeing or about to see fits this description? The flow doesn't need to reverse.

I also note the Met say SSWs occur every couple of years on average. Given that I'm on my 44th winter and have seen significant cold spells in perhaps 8 of them the maths isn't too hopeful.

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Or perhaps just some comedy value

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Yes Brian, you are correct, AFAIK it is a minor SSW.

It is only a major SSW event that needs the reversal of the zonal winds, the criteria (and therefore probable impact) of a minor warming are different to that of a major event (which as far as I know has not occurred).

There is a lot of discussion about SSWs here and elsewhere and a lot of hope placed in them but, as your stats suggest, major events do not always lead to cold wintry conditions in the UK and minor ones certainly do not. The former can seemingly lead to hemispheric pattern changes but we should all know in our little maritime temperate islands that may not end up equalling a spell of wintry weather.

Having said that it would not surprise me if somewhere in N America got a bit of a pummelling in coming weeks - whether that is connected to the minor SSW or not is another matter entirely!

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Well if you assume that a given cold block affects maybe 40 degrees of longitude (one ninth of the globe) then that skews the numbers a bit - even if you assume an SSW sets up, say, three cold blocks spaced around the globe :)

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So, in essence, it's like a massive high-level high pressure cell sitting over the Arctic circle roughly in the 1-30 hPa level?

Anyway, back to about the models. Tell you something though, that Arozes ridge, along with the associated rising heights certainly look like putting up a good fight. Keep it up, I'd say as that would have the effect of pushing the jet northwards out of harms way.

And then all that will never occur come high summer and we end up with northern blocking. Don't need no CFS charts to tell me that.

 

 

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If GFS is correct certainly a few cold days are in store at the weekend

http://www.wetterzentrale.de/pics/Rtavn7817.gif

http://www.wetterzentrale.de/pics/Rtavn10217.gif

http://www.wetterzentrale.de/pics/Rtavn12617.gif

 

Run of temps IMBY

10c 8c 2c 2c 2c 4c 3c 7c 6c 9c 9c 4c 8c 5c 7c

 

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I agree. The trend over recent model runs is for the Azores ridge at 144 to become more prominent. The odds are still very much against it becoming significant in terms of stopping the Atlantic train of course. Trends are trends though, and it will be interesting to see if the 12zs develop the ridge further.

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