Model discussion highlights

[Mike Poole]

A few SSW realisations showing on Stratobserve today:

Looking deeper suggests the reversals are from the FNMOC model, not sure which one that is (is it NASA’s one? it could even be the NAVGEM for all I know!). Anyway, the minimaps are revealing in so much as an indication of the direction of travel:

The ones with the negative zonal winds are the ones where the vortex is forced right into the Russian side, so much so that some of them split, but not in the usual way, more they are forced to the extremities of the hemisphere.  

At the moment, I don’t see an energetic split that reshuffles the deck with this, so what the ramifications might be are uncertain.  But we’re headed towards a very weak strat vortex, I think it is really important  that we get finally some help on the same timeframe from rising AAM to force a better trop pattern.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976484

[Nick F]

2 minutes ago, frosty ground said:

Not sure how you get to it’s dire, the solution is slower but also colder 

Trouble is, the longer the cold air is delayed getting down across the UK, the more can go wrong with lows developing to the NW or W to quickly cut off the flow. We're talking 9 days away on GFS the cold air pushes across all the UK, that's a long way off in forecasting. so good chance it may change and be even more delayed or not arrive at all.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976446

[MattH]

Just a quick one, but it is worth reiterating the situation with regards to the low(s) over the Eastern Seaboard and just off Newfoundland later next week. As mentioned the other day sort of forget what is going on over in the USA as the N Atlantic, with the cold cyclonic pattern is a breeding ground for cyclogenesis. Models will not yet have a handle on how the low(s) over the far western N Atlantic will evolve and that evolution, IMO, will be key as to whether we get a more wintry, seasonal Christmas per the shift in the 06Z GFS compared with the 00Z run. This highlights this situation and example very well indeed...

What carries high confidence is the low heights and cold, cyclonic air to the N and NE of the British Isles, this is placed well to bring a cyclonic cold N or NW'ly flow, but it is upstream over the W Atlantic that will be the key development as to whether the pattern becomes flatter once again, as per many of the 00Z runs, including perfectly exampled by the 00Z GFS and the EC or, whether, the low pressure system(s) over the W N Atlantic evolve to allow for a greater amount of WAA to allow for more of an Atlantic ridge.

That kind of detail, which is still 5 to 6 days out will not yet be resolved for days. Essentially, for those hoping for a more wintry Christmas period, all is not yet completely lost, again, as perfectly exampled by the 06Z GFS.

Regards, Matt.

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976904

[TEITS]

Not convinced any output is correct at the moment.

I do not wish to give false hope but note the chart below.

Note the low pressure over Newfoundland which develops on the 06Z and is a very different chart to the 0Z. Many times rather than moving E as modelled the main low stalls with part of this being ejected SE. What then happens is you get a weak surface high pressure around Iceland.

I'm not saying major blocking is on the way. However a cold, wintry xmas cannot be ruled out and might not be as shortlived as currently modelled.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976932

[Mucka]

MO watch Synopsis

Quite a few "hindcast" posts today, but even though some make perfectly reasonable points those dismissing any cold snap maybe guilty of doing exactly what they say others are guilty of for getting to attached to FI charts.

That said, there is no way to put a positive spin on this mornings output, indeed the output has in general been moving away from any decent cold snaps in the run up to Christmas and ECM and GEM have always been the most reluctant t show any Arctic plunge.

Counter to that, they have shown the trough digging down as with earlier GFS runs previously and the discussion back then was generally whether upper air temps would support snow and how far S cold air would get and not that the Azores would ridge across the UK as a spoiler although it was noted that upstream phasing could shunt the pattern E.

At that time it wasn't a massive worry to me or others expecting a cold snap because the trough was being programmed too far W if anything and it looked like we would still get a cold snap, just delayed and with shortened prospects. But I did mention it on the 12th and again when highlighting a post by Nick Sussex on the matter.

So what has gone wrong and why is the AH playing spoiler?

As noted above it is all about the upstream pattern and given GFS was giving the most wintry output I will use the 00z as a guide and contrast it with a previous 00z run that was bringing the trough S.

 

GFS 00z (Dec 13th) for Dec 18th.

The red eclipse shows the upstream area of concern for potential phasing. Green arrow shows movement of low with no phasing (not pulled W). Red arrows and X show movement of shortwave  and low we do not want.

 

GFS 00z (Dec 13th) for Dec 20th

Red Eclipse shows area where phasing was avoided. .Top green tick shows how shortwave held back and low not pulled W. Lower Green arrow shows movement of low. Lower green tick shows AH not forced SW by phasing to NW.

 

The above shows how a cold NW flow was all but certain so long as we avoided upstream phasing.

Now below today's 00z showing the phasing and resulting consequence of a much flatter pattern.

 

GFS 00z (Dec 15th) For Dec 18th

Red eclipse shows area of phasing. Red Arrow shows forcing of the high E due to phasing. Green arrow show movement of low now forced more E

GFS 00z (Dec 15th) for Dec 20th

We can see how the trough has not pushed cleanly through to the E with the phasing upstream and the forcing of the low to the W flattening the pattern and the main trough being deflected more E by the AH rather than pushing SE.

 

Obviously there are many more complications in the detail but essentially the reason the AH is further E and S than previously modelled is because of the upstream phasing and the trough not pushing through cleanly with low pressure developing in the phasing area.

So where do we go from here? "There's life Jim, but not as we know it."

 

Assuming that phasing is now nailed down, then the chances of cold will need the low that was phased to now phase again with the main trough to the E  in such a way that we still get a delayed but briefer cold snap as mentioned previously.

 

 

 

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976988

[Mike Poole]

Yesterday I promised to set out some thoughts about how the rest of winter might pan out, so this post is focussed on the period post-Christmas.

To take a step back, it has seemed to me that the patterns in the NH going into winter were unusual, on the back of a year of weird weather, SSTs that are not within past precedence, so I have tried to be open to the unusual, maybe paying more attention to the models, which continue to just simulate the laws of physics regardless whether the patterns are old or new.  

Much of the expectation around winter has seemed to follow a timetable set out by the MJO, and likely developments in the strat commensurate with El Niño and easterly QBO.  But the early winter cold confounded the first, and the Canadian warming in the strat called into question the second.

So right now we have a weakened strat vortex, and the MJO is moving into Phase 7 with questionable amplitude.  There have been suggestions of a rise in AAM, more consistent with El Niño, but that hasn’t really happened, not yet anyway, it seems to be struggling to get past the half way line!

It would have been nice to have a trop-led amplified pattern deliver cold, then a SSW to come along later and reinforce things when it waned.  Well it might have been nice, but that is now not where we are.  It is looking like the SSW is coming sooner rather than later, but the effects of the Canadian warming are already being felt as the trop vortex relocates away from Greenland/Canada.  I maintain that the NH pattern is still very unusual, so even if mild and rain is what the UK gets (that can occur with many setups!), the NH setup is not normal service.  Why the pattern is so unusual when there hasn’t been much forcing that I can see from the background drivers so far has not been explained to my satisfaction.

An SSW looks most likely to occur last week in Dec or first week or so in Jan:

It is now starting to come within the 16 days range.  Until very recently I would have said this looks more likely to be a displacement, some GFS runs a couple of days ago showed single digit zonal winds, but very little in the way of high temperatures so I was concerned we might get a fairly ‘low energy’ SSW that essentially just means a displaced very weak vortex - although towards Russia which is not a bad place for it to go.  Now there is a more usual pre-SSW signal, I think, from the GEFS:

But there looks to be some uncertainty, today’s GFS 12z showed a possibly more interesting evolution, 14 m/s zonal winds by T384 so it has some way to go, but the idea is:

It is not possible to tell much about the way a SSW will happen from the averaged 100 realisations plots of the ECM 46, but as of right now, I would not rule out the better prospects of a split SSW, I think that signal is in the ascendency.  

If there is no split, we will be relying on a decent trop pattern to take hold while the strat vortex is weak and displaced, with the downwelling of the easterly winds.  That really means Greenland heights if anything is to really deliver.  Given the location of the trop vortex, and the displacement of the strat vortex, I cannot see how a Scandi high establishes itself, it is for a reason that that option has totally vanished from the ECM clusters.  We ideally need some help from this MJO wave (lagged) or rising AAM to aid amplification.  

If there is a split, there is probably greater potential for a UK cold pattern to set up due to the strat driving things after the trop response.

Either way, in my view, with all this going on, the next 4 weeks will decide whether winter 2023/24 is one to remember or not.  If the UK finds a way to stay mild, then that’s it really.  The next MJO wave (+ lag) could be well into February and is just as likely to disappoint, and by that point the  writing’s probably on the wall (for the south anyway!)  However, the chance of a locked-in really cold spell in the heart of winter is still very much on the table at the moment.

All the best

Mike

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977190

[Pixel]

5 hours ago, Paul_1978 said:

In the last few runs, Christmas Day has had northerlies, south-westerlies, north-westerlies, westerlies, and now northerlies again. I’m not trusting any run - nothing is any clearer despite the day getting closer!

What we need here on this forum is a dedicated Thread to what was predicted two weeks ago as opposed to what is happening now.

I've been here long enough to know this would make sense and could make for very interesting reading 

Tho I doubt it could ever exist... 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977350

[Catacol]

Lots of fizz in here around Xmas charts - very tough call. I think, given the delay in forcings, if we end up with a snowy Xmas it will be 2 from 2 in the "best case scenario from marginal driver conditions". Happy days.

Looking a little further over the horizon, good news this morning. Mountain torque that has been mentioned a few times recently is now on the up.

...and this will climb further when a substantial element of +EAMT arrives shortly.

This is helping support a rising tendency in momentum, hoped/expected at around this time given Nino and decent MJO amplitude. 

GLAAM has now broken into positive territory as a result. It's a marginal breakthrough - but a breakthrough that fits the analysis and moves things in the right direction.

At long last this is enabling the GWO to climb into the 5-8 Nino orbit, more conducive for cold blocking patterns in our part of the world.

So....where next? Well - there has been some arguing on here about MJO orbit and amplitude. We will watch and wait....but I remain on the positive side of this debate particularly as the CPC - who are the real experts on this - are projecting a reorganisation of the pacific signal over the forthcoming period. With windflows now turning more Nino rather than Nina in shape, and a projected 7/8/1 MJO orbit we can continue to be optimistic about a block establishing itself in the right kind of place to pull cold air in.

Timing uncertain as ever, but I'll stick with where I was before....namely that things have been delayed and therefore peak interest is now more on the New Year period rather than Xmas. If a cold NW can pull a short term blizzard over the UK in the meantime then so much the better (though bitter experience suggests to me it is likely to be so marginal, even if the synoptics line up, that most will not open their curtains to a white out! - hoping to be wrong of course....)

Plenty going on.

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977554

[Kasim Awan]

9 minutes ago, northwestsnow said:

UKMO is very brief..

By 168 the pattern is collapsing

 

I have to disagree. Between 144h and 168h there is barely any movement, we've seen much faster evolving northerly patterns before.

It is looking more likely that this will be a temporally stretched wavelength. This largely due to stronger phasing of the main low in the 3 to 4 day time frame and the resultant lack of left over low pressure over southern Greenland (where we would look for a breakdown of this wave) in the 5 to 8 day time frame. The result is a very large and as a result drawn out pattern, with rather little energy being shunted out of Eastern Canada. This is good news for the UK - because it gives time for the low to become entrenched over central Europe around 6-10 day time frame aswell as allow moderate builds of mid atmospheric pressure around southern Greenland. The JMA is still going for this aswell - such significant phasing / low amplification can result in alterations upstream into the Arctic. I know it's the JMA but it has been solid and rather accurate in it's phasing. GEM/GFS are similar. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977434

[Mike Poole]

ECM clusters T120-T168:

A lot of uncertainty, the ES low behaviour seems critical at this timeframe.  Need to maintain this feature west of the ridge for the better solutions.  ECM op is in cluster 1 with 16 members, and most of the others are better.

T192-T240:

The big news here is that the majority cluster 1 with 27 members goes with the GFS/GEM solution.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977543

[Midlands Ice Age]

4 hours ago, carinthian said:

Morning all, UKMO run this morning produces a good example of a " curveball low " as I highlighted a couple of days ago ( above post )  Winds could be dangerous with this type of development , especially over The Southern North Sea. Still of course a bit early to be sure of its intensity and track as shown by ECM run. Whether it allows the cold to spread further south towards the end of next week cannot yet really be determined but now the potential is increasingly being shown. The "day after tomorrow " runs ( Monday ) with the weekend data  added, will I dare say, cast some more light of this development and get some sort of consensus from the main models for a Christmas forecast, hopefully ! Enjoy your weekend.

C

Carinthian..

 Your mentions of a 'curveball' low crossing the north east of Scotland and moving down the North Sea and its impacts reminds me of one of the really horrible moments of the 20th century. Particularly to the East Coast and the Dutch coastline.

Not many people will be aware (or more probably - see it happen) that the event caused the greatest loss of life in the UK of the last century.

It has become known as the North Sea disaster.

That of late Jan 1953 when a low formed between Greenland and Iceland and ran SE towards the northern tip of Scotland. It then went for an explosive cyclogenesis (bomb in todays terms ), as it moved SSE down the east coast and in towards Holland.

Nearly 3000 people were killed in total on both sides of the Channel, with around 300 in the UK. Whilst not wishing to spread alarm, I do feel that we need to watch this situation very carefully over the next 6 days. It could be  a Christmas wrecker. 

I remember the night/evening well in North Nottinghamshire out delivering groceries with my father and being invited in for drinks, and at one house they had a tele (wow)  and were showing scenes of the conditions on the east coast with nearing 100 mph winds and  with us a raging northerly and occasional snow flurries.

A few links which may be of interest below -

 Great North Sea flood of 1953 remembered 70 years on - BBC News

 https://www.bbc.co.uk/news/uk-england-lincolnshire-64414388 

The Met Office will be aware of this situation I hope. 

I have included the re-analysis for the event so that one can see the similarities.

 https://www.meteociel.fr/modeles/archives/archives.php?day=30&month=1&hour=0&year=1953&map=7&region=&mode=2&type=ncep

PS Perhaps the Iberian high made the situation even worse by squeezing the pressure gradient.

In fact there were a lot of similarities world wide with the main vortex transferred over to Siberia. 

MIA

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977669

[phil nw.]

I know Mike Poole posts the ecm clusters regularly but it is good to get the Icelandic ones back again and they do seem to show the start of a slow down to the Atlantic train and a confirmation of a cold north westerly  setup for Christmas.

Christmas day                                                                                T360

             

 

The later clusters showing a trend to raise height anomalies further north with only a minority for holding on to a south westerly setup.

These are only a guide of course but a promising trend, probably reflecting the forecasted reduction of zonal winds and the displacement of the pv towards the Siberian sector going forward.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977684

[bluearmy]

18 minutes ago, TSNWK said:

Thanks., I had thought thst was a busted flush when the early December cold spell fizzled out

The Canadian warming has knocked the strat vortex off the pole and will continue towards the Barents/kara and possibly further. Hence the predictions of a tpv on the Asian side and the evolution we see play out over the next ten days where part of the tpv heads to n scandi from ne Canada 

But what we’ve seen is the zonal flow in the trop stay strong and whilst that continues we can’t get any HLB established. It would seem inevitable that at some point the trop zonal flow will drop off to allow blocking to establish 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4977677

[AdrianHull]

Wow that run had everything.

50 onliners , toys starting to come out of prams.

Then someone reading out model runs like they just came back from the future.

To end in a decent run for the Ecm and coming into line with the mighty big two JMA and NASA models.

Got to love this Place

*And just to add I'm sure BartyHater drives a gritter for his local council and doesn't want to work this Xmas ...

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978058

[Catacol]

1 hour ago, Allseasons-Si said:

To explain why the ECM is good,...yes it is at day ten

this is just an example 

pv lobe diigging south creating amplification ahead of it,there are weaker heights in between the two pv lobes(one over West Greenland and one over Scandinavia/W Russia) for this amplification to exploit and this would send more energy around the eastern flank of the high had it gone a few more frames.

An additional interpretation using vorticity charts. 

While the Canadian trough does indeed drop south it is not the instigator of the change - it is a consequence of it. What we have is a burst of momentum through the Atlantic and its peak strength on this chart is heading through the mid Atlantic - and such is the strength of that burst of energy that it starts to throw an eddy up ahead of it. That eddy is a ridge that then gathers strength thanks to energy to its south, and that same ridge is what then forces troughs to move. This is how ridges are thrown up to high latitude - they don’t just happen.

It would be better if I could build a movie of these slides but I haven’t learned how. But you can run the sequence on tropical tidbits just by sliding the slider….

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978166

[Blessed Weather]

1 hour ago, Mike Poole said:

The much delayed JMA maintains the cold plunge timed for Christmas Day, T192:

Earlier in the run it kept separation of the ESB low for as long as possible.

As there's been a few people knocking JMA without providing any evidence, if I may Mike, I'm using your post as an excuse to show the latest day-5 model verification graph for Europe from the ECMWF website. On balance it's a respectable performance from JMA, mostly in the middle of the pack. I've highlighted a couple of days when it was the top performing model and on one instance (the 22nd Nov) whilst it was top the ECM was bottom. Mind you, equally there are some days where JMA clearly loses the plot and plummets to bottom. If only we could dig deeper and establish the reasons behind the various models good/bad day performances.

A final point of interest - over the last couple of weeks the performance of NCEP (the GFS) has been less than spectacular for Europe, often bouncing along the bottom. The higher score the better performing on the following 500hPa anomaly correlation chart.

Source: https://charts.ecmwf.int/products/plwww_w_other_ts_upperair?area=Europe&day=5&score=Anomaly correlation

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978195

[bluearmy]

6 minutes ago, Kasim Awan said:

Can you post it?

         

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978096

[Retron]

49 minutes ago, MJB said:

A debate was had on TWO years ago, I think Darren ( Retron ) provided stats on it 

Indeed, it wasn't a myth. ECM used to provide data input charts (which you could access even if they weren't the "free chart of the day", if you knew the URL) and they showed a massive reduction in aircraft data from late on the 24th until late on the 26th. There would invariably be a reduction in accuracy on all models showing up in the verification charts a few days later.

The sort of reduction we're talking about, FWIW, was something like 240,000 aircraft obs going down to 80,000 or so each 6 hours. 

In recent years it's been much less noticeable as there has been a marked increase in satellite data at all levels. It seems that's enough to overcome the loss of aircraft data which, of course, still goes on over Christmas each year. 

EDIT: Forgot to mention, the most common side effect of the data loss was the sudden appearance, from the 6z Christmas run, of implausibly good blocked charts for us! That usually lasted a few runs, then gradually went back to normal. It was after a run of such Christmas wonderland charts that I started to look into what was going on... must have been over 10 years ago now. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978355

[bluearmy]

Getting up after the runs were all out gave me the option of viewing the ens anoms and mean on the three suites before the ops

that should help deal with the disappointment of very underwhelming ops - but I was actually surprised to see them all like that 

the ens indicate that we will continue to see attempts to drop the jet south and bring low dams across the U.K.   end this week (which is already looking like it will be too far ne for most) and then again middle Xmas week and then around new year 

that’s decidedly mobile and whilst one of those latter two incursions could deliver more widely, it would seem that the north and especially elevation are in line to see something wintry on the ground but unlikely to stick around unless you are pretty high up. Obvs the Scots have a chance of remaining under the low dams for longest each cycle.  So whilst it’s a poor start to today, I suspect we’ll swing back and forth with decent ops from time to time to raise the mood over the next few days. We still have a ticket but looking less like Xmas times well with the ebb and flow 

and the geps and eps have a late ens tease at the very end of their runs with a possible Atlantic height rise. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978349

[Tamara]

On 05/12/2023 at 11:02, Tamara said:

Unless something rather unexpected happens across the tropics to derail the wavetrain and the angular momentum response, again, somehow falls short..

Always expect the unexpected.  What doesn't make sense, usually gets supplied with an answer why if one waits for it.  

Equatorial fluxing between the tropics & extra tropics has created intense sub tropical ridging and a vastly inflated Azores/Atlantic ridge. That does, contrary to prior expectations in absence of it, make complete intuitive sense. With this, and stubborn legacy of -ve inertia within the extra tropics on the back of two large -ve MT events across both Asia & NA in mid late November and again in the early part of this month, then angular momentum falls shorter than it otherwise would and much greater polar jet energy is present as a consequence - in tandem with the strong sub tropical ridging.

In the absence, at this time, of any switch in fluxing between the tropics & extra tropics to boost amplification in the extra tropics than a continued mid latitude ebb & flow of ridges is well modelled by NWP and the polar front restricted with its inroads in longevity towards NW Europe. SW Europe stays under the influence of the resident anticyclone.

On 15/12/2023 at 12:31, northwestsnow said:

Your post the other day was so so true with reference to teleconnections ,they have limitations insofar as our tiny island is concerned

.The biggest elephant in the room is GW ,and the Hadley cell, its a winter killer for Europe..

Looking at the MJO in isolation is not a magic bullet, especially to try to fit to idealised synoptics. As seen in the ongoing scenario where the tropics & extra tropics are so out of kilter and producing a-typical Nina-esque patterns in what is an El Nino background. Any fluxing from tropical eddies into the extra tropics is muted and rather than eddy waves propagating poleward, they serve instead to boost the Hadley cell and bottle up the pole. In this context also, any displacement of the polar vortex is equally subdued by the ferrel cell asserting itself over the polar cell.

Macro-scale meteorological tools are not intended to micro scale forecast for a very small geographical area in the first place. They assist a framework for the larger hemisphere scale pattern to be evaluated within which that micro scale area exists. But more than anything else, as is always on auto-repeat - no diagnostic approach is intended to be skewed into fitting an idealised weather preference & outcome. Which is why so much mood driven emotion consumes this thread between about November and March.

11 minutes ago, Kasim Awan said:

Often happens in El Nino years, warm September was a giveaway aswell. EQBO + El Nino combination works in our favour a little though.

x+y= theory does not work in straight equations like this. The precise atmospheric circulation in existence at a given time has to be taken on its own merits - there is no given linear response. This post detail, again, tries to illustrate that. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978487

[TEITS]

1 hour ago, northwestsnow said:

Looks like the operationals are leading the way.

I had been banking on the ens/eps but it doesn't look like thats going to pay dividends.

Back to feeing utterly deflated there is nothing worse ,for me anyway,than wind and rain over Xmas and that looks clear favourite this morning..

Moving forward I do hope some of these positive background signals come to the fore ,soon.

You are indeed correct. As is often the case the operationals always prove more reliable than ensembles at short range. For example the 0Z GFS OP was much warmer than the GEFS mean on the 27th Dec and yet the 06Z GFS is the same. When this happens and is supported by the GEFS control run then the GEFS mean slowly increases towards the operationals in future runs.

As for positive background signals. Funny how during the past 10 years despite all these different positive signals we always end up with the same winter weather pattern i.e SW,lys with pressure relatively high to the S of the UK with very little signs of any blocking at N,ly lattitudes.

Maybe these teleconnection signals may of proved correct and gave the UK the promised winter wonderland many years ago. However are these positive signals being overridden due to changes in the global weather patterns due to a warming climate. If so then at the moment nobody fully understands how climate change will affect our weather patterns. You cannot make a successful prediction if you do not understand the cause.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978478

[Cambrian]

Focusing on the more reliable timeframe, the 0z UKMO gives a balanced take to my eye on the developments over the next week or so in the run-up to Christmas Eve at day 7. 

High pressure to the west peaks at 1050mb at day 4, a remarkably high surface pressure for an Atlantic cell at any time of the year. By day 7, it’s pushed a long way southeast to the east of the Azores and is much weaker, at 1030mb. 

This movement is being driven by the same core development that we’ve watched come in from the day 10+ timeframe, now at days 4-7, namely the relocation and reorientation of the PV away from northeast Canada and Greenland to Scandinavia. 

 

This north-south orientation of the polar trough is the core development that opens the door to incursions of much colder air from the northwest and north, a good push at day 6 and another looking likely just beyond the run heading into Christmas itself. 

These feeds from the northwest will be moisture-laden and potentially rich in spawning features. There is one at day 4 / 5 to the southwest of Iceland, that swings southeast, bringing that colder air south at day 6, and another ready in the wings at day 7, looking like it has our name on it, for bringing the colder air wrapped around it our way. 

These blasts of cold air will find it easier to make southward progress as time goes by, due to the steady encroachment of notably colder air much further south over the western Atlantic later in the week. 

For the UK and Ireland, precise timings, how cold and how far south will be firmed up over the next few days, which will make for very interesting viewing. Plenty to look out for, slight variations either way on this core theme to keep us on our toes and anything beyond day 7 to be taken with a huge dose of salt.

And no doubt about it, with the reorientation of the PV and the steady southward movement of the polar front through the North Atlantic, this is a fascinating approach to Christmas, much more than is so very often the case. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978551

[near northampton]

9 minutes ago, TillyS said:

Yes and Paul B******* (almost a swear word on here!) said he once counted 8 anticyclones one after another in a single winter that were collectively eponymous. They move around a bit as you say but the blasted thing keeps re-appearing.

Still early days and it may all change but after a very promising start to December it’s an incredible turnaround. We’ve lost all of the cold pooling and northern blocking is currently absent. 

Someone mentioned climate drivers, which is a different topic but relates to the model outputs. How often in recent years have we seen all this model promise get destroyed by warming?

 

Indeed, and before the the euro high was established, Spain broke its all time December high on the 12th December with 29.9c in Malaga. Much is talked about scandi cold pooling earlier in the December, no one it appears was looking the other way. Month after month of record high breaking temperatures across Europe. Of course that in itself is not a driver of our weather now?

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978474

[Met4Cast]

6 minutes ago, Northern Sky said:

Just to clarify for people trying to learn - the other day (may have been yesterday) you mentioned a number of factors that were in the favour of cold weather which were now about to occur - GLAAM/Mountain Torques etc. Are they now not in our favour, or has something overridden their signal? 

Is anything left in our favour for cold over the next few weeks or have the background signals turned against it? 

In terms of the AAM/Mountain Torques, that's going largely to plan. Following the recent +MT events we've seen the AAM increase into a weak +ve state. In terms of this, everything has progressed as anticipated. Instead of moving poleward though the anomalies have mostly been moving equator-ward, Tamara explained this well in her previous post.

With the MJO collapsing we've lost a key ingredient to the expected pattern. Whilst we'll still see the high push out into the Atlantic and amplify, a response to the Pacific jet extension & movement eastwards of the Canadian polar vortex lobe, we wont have enough "oomph" to amplify that high from a mid latitude block into a high latitude block. Hence the switch in expectations re: the Christmas period in the last week or so from "proper cold" to a more transient cold snap.

Instead of tropical & sub tropical forcing working together we now see them out of kilter with each other. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4978616

[Cold Winter Night]

The output for Christmas remains interesting.
It’s precisely around Christmas Eve and Christmas that the ensembles show a bifurcation: milder or colder.
Visible on the latest GEFS 6z (London):

And also visible on EPS 0z:

The problem with the colder options could be a lack of precipitation.

After Christmas, the extended ensemble output is now more convincingly pointing at the combination of a Pacific Coast Ridge-Labrador Trough-East Atlantic Ridge pattern I mentioned in earlier posts.

This pattern is now well reflected in the extended clusters:

The panel at 336h shows many variations on this same theme, some very clean and promising, like p32:

Some are messier, some very unfortunate, for example these two:

It’s a setup that initially always brings milder SW-lies to our shores, that’s why the interest is visible on NH charts and not yet on temperature graphs.
However, it’s also one that takes just a nudge of amplification to develop into the output most of us would like to see, with the risk that this amplification never comes.

Oh well, we'll see.

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4979230