David Morse

I'm sure that a SSW event precedes a cold event in the troposphere by 12 days(AVG)? So when looking at the Strat temp and location of the PV at the end of the 12z GFS run you really need to see the trop 12 days after to know if it causes a cold event over NW Europe...

For the first time this winter the GFS is forecasting a major SSW event at the begining of Feb and this appears to be in line with Cohen's latest AO post. If this comes off then seasonal forecasting seems to have taken a mighty leap forward in both accuracy and precision.

http://www.chorleyweather.com/strong-arctic-summer-heights-linked-to-cold-uk-winter/

The jet is strong in the latest 06z GFS run, but its showing a divergence to the south at 90 hours which remains so well into FI. In North America there has seen a recent change to cold in the south from a very mild winter so far (t-shirt weather in Atlanta on xmas day at 71F) this is at odds to what a normal El Nino winter should be with nominally warmer to the north and colder to the south which should weaken the jet? I wonder if this change will see a weakening of the jet now?

The latest GFS looks very confident that the cold air encroaching from the east will be easily displaced. Would it be correct to say that the GFS always underestimates how hard it is for cold blocking to be moved out of the way with a return to more zonal conditions? I remember in 2010 that the GFS kept predicting the end of the cold only to have to back track in later runs and try again, but the block remained! Probably because very cold air at the surface under cuts the encroaching warm Atlantic air and it just stays put?

Some snow forecasts giving falls up to 1.5 meters from Friday to Monday in the Portes du Soleil!

I though this was an interesting piece: http://www.weatherscientific.co.uk/2015/10/10/winter-2015-16-analogue-years-assessment/

East or West based NAO: what effect does El Nino have on its position. With today's BBC report about temperature trends over the next two years (http://www.bbc.com/news/science-environment-34226178) I've been looking at seasonal prospects for Alpine Snow in the winter ahead (as I'll be returning to Avoriaz in March). According to several sources I've read; in recent history there has been a steady fall in winter snow depth over the western Alps: rising global temps have been pointed to as the main reason. The BBC news article mentions that the UK MO thinks the next two years will be the warmest on record, but with the cooling trend in the North Atlantic our summers may remain cool for some years. This though, as I've read, is against a background of a "low fequency multidecadal" cycle of snowfall which signal is sometimes "leveraged" or "dampened" by other oscillations like El Nino. I know something like snow depths in any region is very difficult to forecast at any time; so I was more interested in what were the general potentials. I was interested to find out if under the current conditions of a developing and potentially (and forecast) strong El Nino what effect that would have on snowfall levels on the western Alps. I've scanned through several academic papers looking at historic snow depths in the French Alps and also years with bad avalanches and compared those years with years of a strong El Nino; now there was not a great correlation anywhere. But as I read on and looked for more articles I did find several which showed a good correlation between years of strongly negative NOA and high snowfall depths in the French Alps. There was a good correlation between these. I did; however, see that although a winter with a strongly negative NAO gives cooler than usual temps in western Europe along with high snowfalls over the alps, the East/West placement of the NAO anomaly is crucial as there seems to be (particularly for the US east coast) considerable difference. A good example was given: winters 1963 versus 1966 Both had strong -NAOs 1963 was a record breaking year for cold in the UK, but snowfall over the French Alps was higher in 1966 with a -NAO which was west based as opposed to the east based 1963 -NAO. So the question I had was what effect do you think a strong El Nino would have on the location of the geopotential anomoly for a negative NAO this winter?

Hi Bluearmy; I was looking back at previous posts and this chart was posted on the 21st dec for the 28th dec, its a cold set up, but low is to the north east and its polar flow. http://www.meteociel.fr/modeles/jma/runs/2014122112/JN168-21.GIF?21-12

I'd be delighted to look for you if you point me to the link. But I would have looked at the outputs on GFS, PFSP and ECMW plus more every day myself but never saw this set up, most of those were northerly set ups, polar express solutions.

I think this is the first time we've seen this winter the GFS & the GFSP show a serious set up for serious cold over western Europe. a blocking ridge extending from the equator to the Pole across the Atlantic with a large low to the south east over Spain. In my view its similar to the run-ins to 2009 / 10 and 2010 / 11 where a period of northerlies bringing snow to the north coasts changed to a period of easterlies bringing the widespread cold and snow streamers. I've not seen this appear in any operational run this winter; if it has I'd love to to when?

Hi SK, Thanks for the reply; I suppose what I'm looking at is something which I'd think should be more indicative across the years. I was talking about the axis along which the low anomalies lie. In my mind these indicate the nature of the development of depressions in the lower atmosphere over time; in other words they are a pattern of behaviour. We know that just small changes in the direction of any factor can impact the localised weather massively. It would be interesting to see a correlation between the exact axis along which anomalies lie and their location and the consequential seasonal averages following. What we've seen recently is a disrupted PV and weak blocking from the east; but the track of Atlantic depressions has continually brought warm air across these Islands and frustrated the easterly blocking. I see this as a battle between the easterly blocking which has simply not been strong enough to over come Atlantic depressions that have been persistent, slow moving and often deep and large which are forming anomalously across a north west / south east axis and acting like a wall preventing encroachment of the cold east. In detail its because the depressions are moving air up from the far south east and keeping temps high here; and the East has to over come that amount of energy to bust through. In the historical averages you put up, the low anomalies in the Atlantic are most definitely concentrated in a different and cooler direction; lows tracking in a direct west to east axis would be more readily deflected to the north by easterly blocking allowing ingress of those easterlies. One indicator that I saw consistently in 2009 and 2010 was the strong development of a the "Polar Express" caused by the Greenland High and an intense Scandinavian Low. Both cold winters were preceded by this and I remember seeing it appear consistently in the charts in FI and far in advance of the cold arriving; and in both the years the cold arrived from the north first and then the intense cold followed from the east. These lows were from the Atlantic and were getting deflected north to settle over Scandinavia and then sending cold air south. So I really don't see this same set up or even close to it and I feel that is a paramount detail that needs to be in place before true cold arrives. Yes the disrupted PV is a good indicator; but I'm sure people are choosing to overlook the details which I feel are a better signal for timing. I can't see any chance of cold for the foreseeable future because in my view; the details aren't in place.

So wet and changeable and stormy....I have my t-shirt on.

Hi SK, I'm sure you've read much deeper into this than I have; but I do have a few questions about your analysis of the two 500mb charts. On the face of it it looks convincing; but perhaps you could support your position more by showing composite anomalies from Novembers preceding a mild winter and an average winter so we can see the differences? Secondly you say that the "key features are in place"; but even a casual glance shows remarkable differences between the long term composite and this November's composite? For instance the Atlantic low anomaly is aligned along a North West - South East axis. In the long term chart the low anomaly is aligned on an East-West axis. A substantial difference would you agree? Thirdly the high anomalies are in quite different positions: the Siberian high anomaly is much further west and there is an intense high anomaly in the high Arctic centred on the Canadian / Alaska border; this has no equal in the long term chart.

Firstly thanks for all the support in ref to the last post I made; much appreciated. Sorry I don't couch my comments in obscure forum jargon or include all the things I look at or take in to account before posting; I'm sure I'll get better as time goes by or be beaten into submission just so I can fit in :-) . Just to put the record straight: I LOVE SNOW! I'd love to see 3 foot of snow fall here in Dublin so I can get out my brand new Salomon Rocker2 90s and scoot down Bridge Street. But I'm a complete pragmatist and have been watching weather models for years and I simply don't see the weak blocking pattern developing into a "snowmageddon" or anything remotely like that; not for another few weeks at least. Now considering both GFS and ECW are showing an Atlantic feed of low pressure systems winning over the weak blocking to the east and this is reflected in the ensembles....sorry for my naivety of my posts; but looks like I was right ...is it too soon to dust off my "I told you so" T-Shirt... :-) (that's a joke, don't worry I'm often more wrong than right) and Mods...in future I'll keep my comments strictly on topic!

I just feel that the majority on here are interpreting this so wrongly (and very recent events support this). With a lobe of the PV over N.America bringing cold weather there this is the ideal set up for a conveyor of low systems to develop over NW Atlantic and head for the NE Atlantic where the temperature differential across the N.Atlantic is accelerating the Jet Stream and this will deepen any lows. Something the models proved last December / January that they cannot see very well. There is a slight blocking pattern; but this will be overwhelmed by the low altitude effect of the heat of the N.Atlantic (where SST are anomalously 5C higher than average over Nova Scotia) driving the creation of low pressure systems under the anomalously low temps at high altitude. Its stormy weather not snowy!

But SURELY, that Low to the south west is pumping warm air across the UK, not blizzards but buckets full of rain and winds! What is all the cold outlook hoola for?

LOL OK Paul, I was being some what facetious with my earlier post; I'm well aware of what happened in Jan '14 with the PV. My issue is that the same scenario in late November 2014 doesn't inevitably lead to the same outcome but it is the most probable outcome, i.e. that the USA will see an cold period and we will get lashed out of it with storms. As opposed to the very early onset of an exceptionally cold spell which is not indicated at this point in the ensembles, when the majority of them start aligning I'll start to consider it; but the outlook says mixed and stormy not cold and frosty.

I'm signed in with FB and I'm posting from Dublin.

@Nick sussex, Captain Shortwave and Banbury: if the PV was better organised in January 2014; where did all that cold weather in the USA come from and why didn't we get ice boxed in? What is clear is that a disrupted PV does not inevitably lead to backing up Exacta Weather and Daily Mail / Express sensationalist headlines. We are only in November, sea temps are warm and current conditions simply make it HIGHLY unlikely that there will be ANY serious cold in the foreseeable future. SST's are so high that living on these islands they impact the surface temps so massively that no matter how strong a blast of cold we get; low altitude temps will always be too high for wintry precipitation until the end of November / early December. Instead I expect that the pressure charts will start to show up the development of some intense low pressure systems. There is NO aggregate signal in the ensembles for cold; but lots of signals for unsettled weather.

I think there's a strong tendency to be running away with ourselves here; last year the PV broke down giving North America some of its coldest temps, in the UK and Ireland we experienced the worst storm period on record and temps were higher than avg. Therefore a disrupted PV does not mean cold weather for this part of the world! A lot more has to happen before that comes in to play in my view. What this scenario suggests to me is a replay of the storms that affected us and the cold that affected The US last January?

Interesting to see that the ECMWF is now reflecting the GFS run from earlier today showing HP off the Irish west coast and no low to the south of the UK at 240hrs; giving cold but dry instead of the 00Z much wetter set up? So the GFS not so uncertain?