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Posted this extract below from a post made the last week of November. Against the background of encouraging a-typical Nina signals that have evolved during the subsequent period and been discussed in detail in a variety of subsequent posts, the echoes of this earlier thinking revolving around the North Pacific ridge and the quite different potential downstream solution possibilities that stem from it, have always stayed just below the surface - and as suspected, still cannot be dismissed In terms of the stratospheric feedbacks though, the question mark in my mind is the extended hints of the Pacific pattern starting to show the first signs that La Nina is going to tighten its control from upstream with the building of the Alaskan ridge. On that basis a highly anticipated two week stratospheric forecast is just that, and on the basis of potential upstream changes that ultimately spell return of the jet, it cannot be wholly assumed it will automatically favour this side of the pole as the Pacific pattern starts to take control. So I don't personally agree with some of the views being expressed that this is a "good thing" for downstream prospects in terms of sustaining a cold weather pattern beyond the early to (possibly?) mid December period. Its a typical La Nina development that often happens as wavelength cycles occur with the change of month and season and has the effect of increasing Canadian cold and re-booting the energy into the downstream polar jet. Such inications are minority model solutions at the moment that show increased upstream flow in FI, but I think its not a case that they are automatically wrong, simply maybe on the progressive side Note the reference yet again to seasonal wavelength changes in terms of the tug o' war between the various competing drivers. This, to repeat yet again as in all other posts, is something of a make or break factor in terms of whether the atmosphere continues to go along the Pacific "destructive" Nina tropical forcing path (hence the evolution to a poleward Pacific ridge, westwards shift in the polar vortex to the NW, and ascendant heights to the NE) or the more traditional La Nina Pacific ridge (which pressurises the downstream vortex leading to increased polar jet flow upstream, and increased temperature gradient, and flatter faster northern arm jet flow over a de-amplified Atlantic ridge) There is even a hybrid version of the two that is conceivable - implying some Atlantic trough disruption, heights attempting to rise to the NE, but with angular momentum still not sufficiently elevated relative to La Nina to allow the North Pacific ridge to become enough of a poleward cross polar -AO feature to reduce the influence of the Canadian vortex bleeding too much energy into the Atlantic to back the cold west. I believe a few of the recent winters since the cold period up to 2012/13 have illustrated the latter two less favourable (for cold weather wishers) themes and which have been a bane to the majority of the seasonal audience of this thread. I think the equatorward propagation of the stratosphere>troposphere wave coupling leading to +NAO that is currently discussed in the relevant thread is part of the more traditional Rossby wave propagation that occurs when atmospheric angular momentum is in a cyclical downward phase (which, as totally expected, it now currently is) and effectively, at least temporarily, cancels out any troposphere>stratosphere +AAM poleward rossby wave propagation that has previously been imprinted by tropical convection>extra tropical processes - and conversely attempts to amplify the tropospheric pattern In simple language it explains the mixed competing NWP messages that have been suggesting quite different things evolving - and means : The difference between a La Nina Pacific ridge pattern hybrided by an opposing (almost El Nino like) tropical convection pattern which triggers poleward propagating rossby waves to amplify the tropospheric pattern and further perturb the polar vortex at higher latitude through +MT events and, The much more traditional La Nina rossby wave train which bolsters sub tropical ridges in both the Pacific and Atlantic, increases polar jet flow and an associated downside +NAO signature The question remains, is the current complex competing stratosphere and troposphere processes just a smokescreen hoop to jump through before the east based La Nina Pacific tropical part comes into play once more and leads to the much discussed and anticipated return of -AO, emergence of Scandinavian heights, and cold backing south and westwards - or is there going to be another final stumble again to the traditional La Nina flatter +AO/NAO pattern? No-one, including the wisdom of the Met Office, equivocally knows the answer to this - but I think for now at least that percentages still favour the momentum of the Nina east based tropical pattern disconnect and therefore no reason, at this time, to allow too many longer term conclusions to be drawn beyond the current stratospheric>tropospheric dance and lull in the AAM cycle which is leading to the upcoming +NAO phase That said, I think based on the continued intense focus on this thread of NWP suite ups and downs, it does no harm though at all to keep track of a few grounding realities on the way

Sorry but I have to say this, another single model output at silly time scales and some of you are worrying. Beyond 144h or 168h stop comparing every run to the last. Compare like with like. That way you stand a better chance of picking up any developing pattern and some possible continuity. Neither will the clusters help comparing with the last output. Sorry but it is true. One or two of you will end up with ulcers come the end of February.

Absolutely shocking GEFS. So whats the latest spanner in the works? MJO favourable Low solar activity EQbo Low zonal winds Maybe someone let rip in a dingy in the indian ocean lol. You gota laff or you'll cry...

a zonal flow into Iberia will be just fine .......................

GFS past day 8 or so makes little sense to me from a teleconnections perspective as it removes the poleward ridging that's meant to set up in response to the MJO forcing. Very strange, that. This seemingly extreme torque event in phase 8 may be the culprit. The t+0 diagnosis is often overblown though - for example yesterday's point was originally almost as far across in phase 8 as today's supposed observation - so if this is the troublemaker, we may see a marked improvement from GFS again tomorrow. Hopefully. Alternatively, having lost it's very fast route to disrupting the push from the west, the model may simply have reverted to getting very carried away with the +NAO pattern via climatological bias. It shouldn't inspire as much dread as it has been doing for some of late, though. Those who state that once a +NAO establishes it can be hard to take down... well. it depends on the background forcing - if it's sufficient to force amplification, the +NAO can break down very swiftly. We've seen in the past few months just a scattering of short-lived +NAO spells for this reason. As a final note for now - phase 8 of the GWO is typically associated with some of the most wildly fluctuating model output in the mid-longer range. Chaos beckons!

Don't usually post in here as I don't have a scooby about interpreting model output, but has the GFS been the model that picks out cold trends this winter, before any other model and the rest soon follow, or am I mistaken?

Dissecting every single frame of every single run... this way madness lies. We went through all this 10-12 days ago. The blowtorch didn't turn up and plenty of places had a snow event. Compare like with like, (ie the JH method) and just let the trend evolve. Honestly you'll lose fingernails otherwise.

They got the idea after seeing just how effective our M4 is.

The Pentland Hills looking stunning in the sunshine this morning!

When you wake up at 3am with the kids and it's snowing

I've narrowed down the Christmas weather: it'll be mild or cold and it'll be wet or dry...?

Not a disconnect, phase tilt. At any time there may be various Rossby waves propagating upwards, and if conditions are right, downwards. The time between 500mb and 10mb may be 6+ days and upwards propagation is indicated by westward tilt with height, downwards by eastward tilt with height. So it is not unusual for geopotential heights to appear out of sync. The chart below shows the phase shift from a composite of downward coupling events (Shaw & Perlwitz 2013 - http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00251.1) and it can be seen that initially there is a westward tilt of geopotential from the mid-troposphere into the stratosphere which becomes an eastward tilt with height as the cycle progresses, becoming quite extreme by the end of the process - A week ago it was suggested that a possible downward coupling event was visible in the Hannah Attard GFS forecast 500/10mb charts, contributing to the ridging off the Pacific coast of North America. This was following a reduction in wind speed in the middle strat which gave a negative vertical wind shear between about the 6th - 8th which provided a potential reflecting surface - It's possible that some of the later effects of this process are what are being witnessed in the geopotential forecasts, and it's also worth noting Shaw & Perlwitz also suggest that reflection events tend to lead to +ve NAO phase which is the case at the moment and in the near to medium term.

I find the difference between GFS & other models fasinating. Yet again we see the GFS Op run supported by the GEFS control. For the sake of everyones sanity it is best to assume the GFS is wrong and that Xmas is likely to be mild. However I certainly wouldn't be willing to bet on this just yet! Beyond Xmas and I shall repeat myself by saying a return to a colder pattern is much more likely to come from N/NW than the NE/E.

Cannot disagree with any of the above - great post and from the perspective of run by run NWP output, totally, cannot argue with it. The imprint of teleconnections, changes in zonal wind profile in the strat, the relocation of the vortex, the continued wave 1 activity, the latent imprint of the MJO effects stalling on the maritimes and the QBO mode view - all suggest at this time frame is a decisive tipping point for impacts on the vortex. Therefore resulting in a couple of different roads for the remaining winter. This time precisely is the window for that evolution..representing the peak wave one activity, the peak heat flux and core shift in vorticity. If that doesn't add up to something of a cliff hanger following the mild blip then who knows !

Looking through various/and supports.. Its quite dire.. lets not polish a turd here!!

As a dry weather fan, not bad though, I am now looking for dry on the models

The truth is we don't suffer from hurricanes, typhoons etc, severe tornadoes, gigantic hailstones, inferno heatwaves, very severe cold, rarely from severe droughts etc We don't suffer like other areas of the world can suffer with the weather. And we don't suffer from active volcanoes and severe earthquakes. We are fortunate in this regard.

I can’t take 3 monthly means as worth anything re detail a single month is difficult enough to draw any conclusions from but 90 days!! we could have two weeks of wintry nirvana second half jan with that pattern shown on the tweet for the remainder of the period and we wouldn’t see it. We also wouldn’t see repeating wedges of heights which could lead to sliders on that chart - you can make a prediction that the three months is likely to return an above average temp profile but that’s about all ! I don’t see a euro high btw !

I take increasing comfort from the ensembles, many of whose members are cold to very cold. Plenty of encouragement still:

A great post I vote we allow more edit time

The SS Save Christmas has hit rocks and is fast taking on water. Its been all downhill since yesterdays 12hrs runs with the pattern flatter and the phasing of shortwave energy with the upstream low making things even more difficult. The limpet high can't retrogress as shortwave energy blocks its path. Save Christmas has collapsed from 4/10 to 2/10. We need to see the rot stopped asap !

Well i dont think there will be much going on in this thread until probably the new year so with that in mind i hope all my fellow north western peeps have a joyous and happy christmas

E,ly inbound later on in this run!

Just to offer a bit of hope - this is a chart from 20th Dec 1970 - there's a few similarities there in terms of the ridging on the US/Canadian West Coast and the Atlantic pattern too - relative to GFS forecasts (Joe B has been talking about 1970 a bit and you can see the broad-scale similarities). BTW - I don't bother looking at the ECM op past 144-168h these days as rightly or wrongly I no longer trust it at this range since the last upgrade. Here's how Christmas day 1970 looked: I think what's been hampering us for a while though are the SSTs around the Azores which are still a bit above average - that just doesn't help when it comes to splitting the jet and getting energy South into W Europe. I think we have the atmosphere wanting to play ball, but for the moment Atlantic SSTs are working against us being able to lock in a cold pattern.

Always an issue when the models are opting for the most efficient energy transfer possible and producing very large, intense storm systems as a result; they produce regionally flat-looking outcomes even when the broader-scale pattern is nicely amplified. It'd take a few days to push such a broad trough southeast and get back to a colder regime from the ECM day 10, but I question whether there will really be a large, deep trough to begin with; often enough I've seen these fail to come around, with the actual outcome featuring multiple smaller lows zipping across. In this case they'd do that and then merge with a trough to our NE, in a similar fashion to the past few days. It doesn't sound that exciting for southerners I know, but this is why the Scandinavian high signal is of interest longer-term (v. late Dec/early Jan) as it can potentially convert the polar maritime setup into a polar continental one, which works much better for those who've missed out so far this winter. It's tricky I know to get the sense of balance having already come so close to low-level snow in the far-south this winter. Fact is, snow to below 200 m asl in that region is very rare indeed in December - as I know all too well!

Some of the more seasoned members on this forum will have noticed a trend over the years that is still very evident. The gfs ops pick up the baton with a cold trend in the medium to long term but is not followed very keenly by the other models, especially the ecm. The gfs ops then drop the baton only for the ecm to pick it up. The result can be a big win for the cold after all. Just a little later than what the gfs was initially progging.

As usual a raging PV over Greenland is a complete winter killer for us, obviously it's only December but the longer we see this strengthening the longer the agony will continue. We have all seen this before that's the issue, we reassure ourselves that it's only Decembers plenty of time! But that vortex has killed so many winters. Not much to say on the 12's having looked through... some amplification in the mid range but whilst that PV is there spewing energy out it kills it, it always bloody ends up in the worst possible place for us! And for us snow starved southerners it's getting more annoying

Neither more nor less plausible than the runs that showed a stonking great Beast from the East...? We can hardly be judging models on how much snow (I mean potential) they seem to promise...

Looking at the NH profile later on, I would say this gfs op is for the bin after day 10

It didn't last winter, zonality struggled last winter. There were all sorts of predictions including weeks on end for how long a zonal spell was going to last winter but the fact is zonality will last as long as long as the circumstances allow it. To know how long zonality lasts is like knowing how long a piece of string is.

I’m baffled and this isn’t the first time. What on Earth are you seeing? I see no difference.

I don’t see a backtrack as such still interested in amplification in D7 now even the ECM clusters are solidly behind it that is a GFS win here immaterial to final outcome. GFS 06z ends on an interesting note again height rises to N/NE. Reasons to be optimistic IMO.

The progression we're seeing the models explore (somewhat inefficiently perhaps, but getting there) is one in which the Canadian vortex has a window in which to venture towards Europe via Greenland, but then comes under attack from upstream changes with the Pacific ridge heading back poleward and initiating a buckling of the jet pattern that should encourage the vortex lobe to come apart a bit, with some getting pulled back into N. America while the rest either heads into Europe (as either a large organised trough as 00z ECM tries for or a smaller one following trough disruption as per the 00z) or digs south in the middle-N. Atlantic. The former resembles our predominant pattern of Nov-Dec so far, but tweaked to allow for more the way of Scandinavian height rises as a follow-up to the trough dropping in. The latter is a different path, more like we had to go through in Nov 2009 - and laden with the risk of some mild, wet weather while we wait for ridging to make its way sufficiently north from Europe to turn the tables in favour of cold. This manner of evolution has a habit of drying out just as it turns cold enough for snow, though. GFS 06z is very much the latter scenario. This setup is also easy to tilt unfavourably with a bit of climatological bias and/or over-deepening of troughs, which I believe explains some of the less than inspiring ensemble behaviour of late. It is for reasons like this that the festive period has a valid stormy weather risk, but given the (at least) equally plausible alternative scenario of a trough setting up over Europe again, I'm not inclined to expect a windy washout route at this stage. GFS still managed to get quite close in the end on a hemispheric scale - a nice interpretation of the Pacific-Scandi relationship, shame about that dartboard low!

Finally woke up to a white world this morning, just a couple of inches but no sign of thawing though. Husband oot playing happily with his snow shovel lol

Operation Save Christmas suffers a heavy blow this morning! The GFS 00hrs is at least better than the previous 18hrs run but not as amplified as the 12hrs. The ECM is flatter than yesterdays 12hrs run. The UKMO is dismal from a cold perspective, the flattest of all. Okay that's the bad news out of the way! The ECM ensembles show a spread through the UK in terms of temps and the pressure ones show the spread just to the east suggesting theres support for more amplification and low pressure digging south. The problem remains in terms of Christmas that the models agree in pushing the PV south again and you need to have a strong ridge to the ne to help disrupt some of that upstream energy. Save Christmas drops from 5/10 last night to 4/10 after the early outputs. We need to see more amplification within the T168hrs timeframe as quickly as possible!

To some extent i agree but there is a pretty strong PV setting up exactly where we don't want it (Greenland) and the ext eps go +NAO so it doesn't sound good , at all.

Think average for the EC00Z is being generous- looks flat as a pancake by day 10- we await the Ens - Knocker in the other thread seems to be of the opinion ec46 is poor judging by his pic of sidney doing cartwheels - sorry, should have said poor foe coldies...

The departing of the Arctic sea.

thats good news SS. i'm glad you told us that....

Quite right about zonal but this isn’t like normal winters for us, so far anyway so I don’t think any zonal will be as stubborn to shift as normal. We shall see

UK Outlook for Saturday 30 Dec 2017 to Saturday 13 Jan 2018: Through the final days of December and, perhaps, the start of January it is likely to be unsettled and often windy across much of the UK. Temperatures will be near to or below average and snow is likely, at times, especially across northern parts of the country. Through the first half of January a gradual reduction in the frequency of Atlantic westerlies seems most likely, this will be coupled with an increase in the frequency of colder and drier periods, although confidence is low at this stage. Milder, wetter, and windier spells should become more short-lived during this period with the colder and drier spells of weather bringing an increased likelihood of overnight frost and fog, as well as some snow, wintry showers, and below-average temperatures. https://www.metoffice.gov.uk/public/weather/forecast/gcpvj0v07#?date=2017-11-14 just posted by summer sun , to my untrained eye it does not look to bad if the meto are right

Aye, merry Christmas my southern friends preferably not like larrys greek turd of a Christmas

may all your christmases be shyte

Aye...A shyte Christmas is now odds-on. Nowt unusual there then!

Yes scotland gets a waft of -5 uppers for about 12 hours lol. As you say gfs looks less amplified - shame we came very close to something really good around the 120 mark but yet again the vortex spewing its ugly energy towards Southern Greenland it flattens everything out.

My tuppence worth (not that it’s worth much ) is the models continue to struggle with the upstream pattern post Xmas offering a variety of solutions within the ensembles. One only has to look at the MetO updates to see the wide range of scatter on offer so it’s no good following every single run as if your life depended on it, just sit back, watch with interest and try to spot these upstream signals without the emotive outbursts on each and every run. Back to the sidelines for me.

I think we're all at that crossroads where we're waiting for the background signals alluded to, to manifest themselves in a favourable model output. It's interesting but frustrating all at the same time. Cant help but thinking something is going to change suddenly but it's getting nerve-wracking!

Sidney having just looked at the latest EC46 update. Positively glowing

You wouldn't be throwing balls of this stuff at your girls - they'd be on to social services! Or making snowmen... Ms HC and younger HC both came close to a cropper earlier just getting to the gate, and don't say sweep it up, this stuff would need a kango hammer to shift it and then the friggin neighbours would be on my case for the noise.

Time for some realism All this talk about amplification to our north circa 23rd December is a bit of a red herring. Yes, the ecm op at face value has given way to the gfs op persistence. However, this raising of heights/hopes is nothing more than a glorified toppler. Nothing to see here prior to New Year. Ensembles back this up. New Year onwards - still up for grabs but with the obvious disclaimer that we have all been here so many times before. That's my take on the models today.