Model discussion highlights

[MattH]

2 minutes ago, IDO said:

The trend continues as the models dampen the Greenland heights to more of a wedge and the high sinking for the pattern to relax. The GEFS are poor with no real cold for the south:

...and the trend for rising temps in FI (2m).

The early modelling was seemingly based on the algorithm assumptions of the MJO or SSW, but more recent input data is correcting that modelling.

The ECM:

...shows the trough dropping to our east which was always a relevant cluster. UKMO D7 charts are a simile to D10 ECM charts, and I would not hang my hat on them. THE GEM is the same as the ECM. GFS is not dissimilar in synoptic, just a variation on the theme.

Looking at the GEFS mean, I see no apparent imprint from downwelling, just the tPV and the usual signals dictating the NH profile. The mean suggests that the tPV will begin to cycle back to our NW as we move towards mid-March:

That will drive a westerly flow in our latitude.

We wait to see if the second downwelling can breach this underlying La Nina background cycle of UK high intermittent with westerly push as the tPV cycles to our north.

 

A quick post on this as the second part of this is very misleading to people trying to learn or look at the outlook...

There IS evidence for the strat to imprint on the trop looking ahead, the 00Z EC Det once again shows that. Using the GEFS mean as a foundation to highlight that there is no evidence for a strat to trop imprint just isn't realistic at all. You're also leaving out that if you do want to use ENS that the EC ENS from the last 24 hours doesn't return the main tPV to Canada either.

Use ENS plots and means days out at your peril in times like the present.

Cheers, Matt.

Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4811705

[nick sussex]

It’s one of those set ups where you don’t need to see what’s happening to the east or ne as the upstream trough set up will dictate whether the UK gets cold and dry , snowy and cold or milder .

The models in the key timeframe agree on the PV locating in Asia and the elongated troughing setting up to the east . So west and nw of the UK is what matters .

Essentially  the cold wants to head south with an unfavourable  upstream pattern .

At this point we don’t want cold and dry , and need the troughing further west and that can only happen with help from the upstream pattern .

I’m reluctant to throw the towel in yet because the reversal into the lower layers hasn’t taken place yet and you don’t need a significant change to improve prospects .

Its  a bit more sharpness upstream with more elongation of that troughing running nw se .

If we don’t see an improvement in the charts by Tuesday then I think it might be time to accept its not happening in time to deliver given the increase in solar energy makes it a very tough task later into March .

 

Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4811714

[Catacol]

32 minutes ago, LRD said:

I am certainly no expert in this stuff but that line graph you posted shows a recovery in zonal winds and that, I presume, would lead to a more organised vortex again. I assume that some of the output is reflecting that?

But, as I say, I'm no expert and I might be miles off

Irrelevant. By mid March the vortex is on its way out anyway. What matters is that we have a forecast there of a fortnight of mean reversal when there is just about time to still see deep cold. 2-3 weeks left before the move towards Spring is irresistible. Can we snatch some widespread snowfall before snow chasing ends once again? It will be a close run thing. Note the hemispheric profile is good

and the MJO is where we need it to be

However note that vertical profile forecasts are now pushing the remains of the distressed vortex more towards Asia rather than Europe as was forecast previously. This may work against promoting the gates opening to the ENE and allow the sub tropical high to our SW to keep a degree of influence. Modelling is suggesting as much.

So…anything from a no man’s land omega block ongoing end product to a frigid easterly feed is possible. Having seen these fine line scenarios go the wrong way countless times before I cannot claim to be feeling confident. Hopeful probably best describes it - but that in itself is a distinct improvement after yet another tepid mid winter.

Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4811917

[Daniel*]

6 minutes ago, January Snowstorm said:

Somebody stated earlier how one of the charts was showing the 510 line over Scotland and it was the first time this Winter. That's an extraordinary stat as to how poor the Winter has been. Alas things are looking up now for a last hoorah before Spring proper begins. The mogreps and Met Office being confident is a major plus going forward and you would imagine we are on the cusp of something special. Heads down for the pub run!

No, the 510 dam is rare not even December 2010 achieved that. In last 30 years or so there has probably been two occasions and that’s 1991 beast and 2018 beast. How can we forget the latter unbelievably the 5 year anniversary is very soon, it was likely the latest on record too.

Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4812346

[mushymanrob]

I wont claim to be all knowlageable about these charts, but from what i can gather, the gist of it suggests that theres no (or little) SSW impact here for us within this timeframe, that out to the 15th, in fact, these charts appear to lessen the chances.
 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4812647

[Mucka]

12z Model output analysis

So looking at the UKMO 168 we are in a halfway house.

Compare output to see what we need to change.

UKMO 00z V UKMO 12z 120h, note how the UKMO 00z has the core of the high further NW and tilted toward Greenland.

144 00z V 12z And so we get the undercut at 144 on 00z but much less so on the 12z resulting in more energy being forced N thereafter and less E/SE on the 12z and so the high is much further S on the 12z cutting off the cold NE flow

This comes about because the 12 puts more energy and forcing into the Atlantic sector early in the run, see the differences on the W flank of the high at 96h and GFS has the Atlantic even stronger at this point so no coincidence the output degrades for cold accordingly later in the runs

UKMO 00z V UKMO 12z V GFS 12z 96h

For a starker and more obvious contrast GEM 12 with GFS 12z at just 96h and yet again we see GEM went on to deliver.

I think it is pretty clear that how much forcing we get from the Atlantic trough earlier in the runs is going to have a clear impact on later outcomes. 

Which model has this right? 

We can't say because they are all adjusting back and forth and those are large differences at just 96h, likely caused with the models struggling with the impacts of the SSW.

As if that were not enough uncertainly the Operational output of GFS is somewhat at odds with its ensembles at this stage in the amount of energy it puts into the upstream.

In this case that is positive news because the mean has less Atlantic forcing which equates to more ensemble members having the Atlantic low making less inroads at this pint (96h)

GFS 12z Op V GFS 12z ensemble mean

Another positive spin on the output, although based in reality, is that the GFS 12z mean is a little better then the 06z mean

GFS 12z mean V GFS 06z mean 168h

 

All this means is that  FI is basically 96h+ when it comes to whether we get a cold spell and Greenland high post 144.

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4812851

[Cheshire Freeze]

4 minutes ago, RainAllNight said:

The next hurdle to overcome will be seeing if a west based fader can be avoided...i.e pressure pulling too far NW and heights eroding.

I suspect we may actually see a more resilient pressure rise to our NW which allows us to remain cold (with low heights continually being pushed down through the country from the N/NE). All conjecture right now though. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813052

[Catacol]

24 minutes ago, WINTRY WALES said:

Well I think the charts from this morning, plus the last few days, put to bed the theory that a SSW event does not at least hugely increase the chances of cold weather for us.

Would we be seeing these charts on offer if there had been no SSW, I think not.

I agree - though tropospheric forcing from the Pacific is always a key teleconnection too…and anyone wanting to see trop led factors that might contribute to a big high lat blocking episode need look no further than the known MJO context plus this huge bounce in torques, with a significant +EAMT.

Total torque going off the scale. Funny how things align sometimes. SSW plus MJO plus big spike in torque that will fire a huge pacific jet into the system and you have a combined context that really can’t fail to deliver significant disruption to default westerlies.

The only thing to ponder from here is exactly where the block sets up. A cross polar end product looks likely and the angle of that split will be key. Some luck needed. We deserve some.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813277

[Tamara]

6 minutes ago, Met4Cast said:

Thought i'd do another update now models are firming up on what looks to be a fairly significant cold spell for the time of year. Where are we currently sitting & what's the likely evolution?

Atmospheric Angular Momentum has surged into +ve territory led by the 'off the charts' East Asian Mountain Torque (EAMT) event. +ve MT essentially takes momentum away from earths rotation & puts it into the atmosphere, i.e, stronger overall westerly winds within the budget

Following this backwards in time, the above process began with the MJO transitioning through phase 4 > 5 > 6 and now into 7, this created frictional torque (air moves slower at the surface due to friction) which led to the above +MT & eventual rise in overall AAM.

This all serves to add westerly momentum into the atmosphere which results in a more disturbed jet stream. These disturbed waves then travel across the Northern Hemisphere creating peaks (high pressure) and troughs (low pressure). This can be seen on the jet stream chart.

 

So, on that basis alone we have forcing in the tropics creating a disrupted jet stream. This alone would increase the likelihood of blocking areas of high pressure in the higher latitudes. But.. we've also had an SSW (Sudden Stratospheric Warming).

This SSW reversed the usually westerly flow in the stratosphere to easterly, this creates -ve anomalies (easterly) that are moving down through the atmosphere & into the troposphere with the likely impacts being felt in early March.

The first warming didn't propagate downwards for reasons I'm not going to go into here, but the second & most recent warming looks like it will. So, what does all this actually mean in terms of our weather?

Tropospheric led activity (MJO > +FT > +MT > Rising AAM) pre-condition the troposphere to blocking, the SSW then downwells into the troposphere strengthening this blocking signal. High lat blocking has been signalled for a while due to this process.

Blocking will finally come to fruition in early March and will now likely bring a cold period of weather to the UK & NW Europe. Snow is increasingly likely through next week but particularly towards the end of the week.

Beyond next week I think the block will drift westwards into Canada opening the doors to low pressure systems from the SW. As these bump into the colder air they bring the risk of significant snow events. 

 

 

This is probably one of the highest quality assessments of the GSDM diagnostic working in real time that I have read and with well explained and argued prospects ahead (at least for NW Europe).

Very much a bookended season. This winter has seen the Hadley/polar cell dual that I talked about coming into winter back in November. December saw poleward momentum under a destabilised tropospheric lead into the lower stratosphere and thereafter equatorward fluxing took over as angular momentum fell in tandem with the westerly gyre of the upper stratosphere descending into the troposphere. This has largely prevailed through to the end of winter.

Now we see the reverse back in AAM fluxing at the same time as instability in the form of -ve zonal winds descends from the upper levels of the stratosphere c/o SSW into the troposphere- but this time creating a coupled relationship which will enable a more sustained phase of poleward momentum than the early winter with, as @Met4Cast points out (with no further embellishment required) angular momentum tendency surging very high. Make no mistake, when tropics (MJO) and extra tropics (GWO) phase in a harmonious momentum transport wind-flow signal such as at present, in tandem with a highly unstable trop/strat combination than a sustained phase of high impact weather conditions are in the offing.

For my part of southern Europe this significant pattern change will ironically bring to a (welcome) end to anomalously very below average temperatures which have been a nuisance at night for the Mediterranean planting garden. Both late January and now the end of February have seen repeated cold air advection patterns coming around the eastern and southern flanks of high pressure stuck over the UK/eastern Atlantic. With the return of the -ve NAO from the end of the coming weekend and through next week, sub tropical air will bring a quite noteworthy jump in temperatures into the upper teens and potentially low 20s in places  - but also with bands of rain bringing wet conditions. So Spring begins much as to how the winter began.

As for the UK there is a similar parallel to how winter came in (nut with reverse trends in weather). Increasingly  set to be on the polar side of the jet stream and depending precisely where the boundary of southerly tracking low pressure is, high impact weather events look odds on with significant snow events. 

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813471

[Mucka]

1 hour ago, Ali1977 said:

Is that a polar lp heading south ? 

I was going to post about this feature after I read through the comments.

I don't see it on the Euros but it is prominent in a lot of the GFS output and a quite likely scenario.

The timing and how and when it phases with the main trough would be key to any outcome but it does have the potential to be a spoiler.

If it does become a feature then it is probably the last major hurdle between deep cold and us if we get unlucky and it blows up to our N cutting off any Arctic feed and weakening the block.

 

It will most likely phase with the trough without blowing up though and could either accentuate showers if phases to our E or bring some frontal snow if it phases over the UK. The main thing is that it doesn't develop as it comes S without phasing.

It is something to keep an eye on now that the overall pattern is pretty much nailed down for that timeframe because for the UK what can go wrong will go wrong when it comes to cold but it is very much a worse case scenario.

On the other hand it could be irrelevant and be gone in future output or be a potential snowmaker.

GFS has it developing around 120h on the 5th so quite far out for such a fine detail. 

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813509

[Met4Cast]

Thought i'd do another update now models are firming up on what looks to be a fairly significant cold spell for the time of year. Where are we currently sitting & what's the likely evolution?

Atmospheric Angular Momentum has surged into +ve territory led by the 'off the charts' East Asian Mountain Torque (EAMT) event. +ve MT essentially takes momentum away from earths rotation & puts it into the atmosphere, i.e, stronger overall westerly winds within the budget

Following this backwards in time, the above process began with the MJO transitioning through phase 4 > 5 > 6 and now into 7, this created frictional torque (air moves slower at the surface due to friction) which led to the above +MT & eventual rise in overall AAM.

This all serves to add westerly momentum into the atmosphere which results in a more disturbed jet stream. These disturbed waves then travel across the Northern Hemisphere creating peaks (high pressure) and troughs (low pressure). This can be seen on the jet stream chart.

 

So, on that basis alone we have forcing in the tropics creating a disrupted jet stream. This alone would increase the likelihood of blocking areas of high pressure in the higher latitudes. But.. we've also had an SSW (Sudden Stratospheric Warming).

This SSW reversed the usually westerly flow in the stratosphere to easterly, this creates -ve anomalies (easterly) that are moving down through the atmosphere & into the troposphere with the likely impacts being felt in early March.

The first warming didn't propagate downwards for reasons I'm not going to go into here, but the second & most recent warming looks like it will. So, what does all this actually mean in terms of our weather?

Tropospheric led activity (MJO > +FT > +MT > Rising AAM) pre-condition the troposphere to blocking, the SSW then downwells into the troposphere strengthening this blocking signal. High lat blocking has been signalled for a while due to this process.

Blocking will finally come to fruition in early March and will now likely bring a cold period of weather to the UK & NW Europe. Snow is increasingly likely through next week but particularly towards the end of the week.

Beyond next week I think the block will drift westwards into Canada opening the doors to low pressure systems from the SW. As these bump into the colder air they bring the risk of significant snow events. 

 

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813437

[bluearmy]

there is so much good stuff to look at in the 6-11 period that it seems a little churlish to look at the 11-15 extended 

currently it looks like a west based neg nao lifts an Iberian upper ridge and brings less cold air across us with systems bringing rainfall (possibly still a snow line across the north of the U.K. ) BUT I am beginning to see signs that what may well happen is a renewed build of heights to our west or strong enough Griceland wedge to keep the jet far enough south for us to remain on the polar side of the jet. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813500

[Eagle Eye]

Secondary large +VE EAMT likely to cause extension of Pacific jet and cyclonic wavebreaking over North America which is typically good for North Atlantic jet retraction and blocking towards Greenland via RWT induced AWB enhancement.This could potentially form a dipole of blocking to the North. Given MJO staying fairly enhanced in a weakening -VE ENSO state MJO Phase 7/8 with the start of the westerly wind burst into the climbing -VE Nino 3.4 area. Given a cycling more west based -VE EPO cycle and an E US ridge then we could have more chances for cold events up to mid March. Relying on strong MJO Phase and the atmospheric state to remain in the intraseasonal Nino state band (+VE AAM) and w'ly atmospheric drag which cycles around certain areas depending on the RWT. Tropical forcing looks to remain high for now though given the MJO RMM staying around Phase 7-8 and around +2 in terms of RMM 2 strength and mountain torque events. So Wave propagation and jet oscillations suggest zonal wind recovery is unlikely with the mean transport of mass, momentum and energy given the transfer of w'ly momentum via tropical-extratropical interaction under this Nino setup. Frictional torque is likely to repress the mountain torque gain and cause a -VE EAMT event but the SSW appears to suppress that reaction for now.

I realise that's a lot of writing and I kind of just summarised everything.

Essentially, what I expect to happen is that for now we see the first effects of SSW imprinting its effects on the Trop by -VE NAM downwelling (despite non reversal downwelling into the Trop, the Trop and Strat are set up for a reaction anyway) and we see that with the risk of bringing in cold next week (ish) but possibly earlier or later. The exact direction of flow will determine exactly what happens but that depends on small scale permutations where each front has some sort of wider scale effect. Then, we could see a further extension of the Pacific jet and a recycling of the -VE EPO regime but perhaps more west based than last time. Meaning that there is support for blocking to the north to return around mid March. Given typical -VE NAM tripole characterisations of stronger reversals with 1 Upwell and 2 downwells in some or 3 downwells in others as a generalisation; it can be thought of that this one could be similar. Meaning that as the eddies lower down in the Troposphere start retracting, the AWB that caused the norther blocking in the first place may have formed a dipole. If it has then upwelling and Wavebreaking into the <=0 part of the lower Strat is likely and that flushes down another band of weaker zonal winds forcing tropical-extratropical interactions and poleward eddies so downwelling of the -VE NAM is possible. Given the zonal asymmetric response to west Pacific MJO link with AAM budget and the typical response to the SSW I wouldn't be surprised if we did see the upper Tropospheric flow pattern leading surface conditions quite favourably for coldies. With AAM flux converging then we start to see a favourable poleward momentum flux given best MJO convection in terms of Indo-Pacific warm pooling and that grows the strongest extratropical response. A lot is really lining up with the MJO-AAM link with the slow surface frictional anomalies in the tropics leading risk shorter flux in the extratropics and that appears to be linking itself well with the northwards transport of momentum with AAM convergence and the general intraseasonal AAM anomalous movement would suggest the typical response  to the best MJO warm pool in terms of eddies is going to occur here. So we have a double AAM spread with the MJO Indo-Pacific linear time linking. This looks to put tension on eastwards movement in the North Atlantic and drag the jet back. So this is really looking like a sign that this upcoming event may not be it if we can get it all to time together for just before late March though anytime before then is good really if we can get the best fetch in terms of easterly or northerly.

Here's what I mean with the mid-latitudewards movement of the near surface reversal though we won't see if it'll be helpful for us yet.

Though importantly, we need the MJO to time perfectly in this situation for that second setup to be possible. Essentially, the difference between phase 8 and phase 1 is so big that 1 has Nino reactions and the other has Nina reactions.

Sorry if that post was really confusing, it's right at the top of my ability and there's a bit of conflicting when it comes to Indo-Pacific warm pools and AAM links and also whether or not the frictional torque balances out the w'ly's before or after the RWT sets sail.

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813740

[Ali1977]

Interesting stuff 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813909

[DAVE_ALLEN]

1 minute ago, RainAllNight said:

What did happen next in March 1997?

It was April 1997

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813913

[Cambrian]

Well, our lovely models, would you ever?! It’s crazy in here!

So it’s good now and again to sit back and assess the gravity of what we’re looking at here, in terms of finally seeing the kinds of charts that we look out for over months and months.

These are truly spectacular day 7 and 8 ensemble mean charts from the 0z ECM for the end of the first week of March. We often get snow in March here, normally off a northwesterly, which can be heavy too, but hit and miss as to whether it sticks around. But it looks like this year has something much more sustained and widespread on the way, which would be great for those who haven’t seen any snow through the winter.

The now completely Eurasian PV trough runs from Northern Siberia to the North Sea. Along its western flank, it brings a flow with a long sea track over the Barents and Norwegian Seas, arriving to the UK and Ireland as a potentially very unstable northerly flow, a perfect recipe for the development of smaller low pressure systems, which can really pack a punch. 

  

It’s cold throughout by then, the T850 -6 degree hatching covering nearly all of the UK and Ireland by day 8, and this is just the mean! The Arctic core of cold has also relocated en bloc to the Siberian Arctic. 

The day 7 ECM op is far from alone in reinforcing the scope for precipitation - a stream of snow running from the Barents Sea to the UK and Ireland, with a front running through the middle of us and a significant amount of activity possible behind it with air that cold moving down over the warmer seas.

   

A beautiful jet stream profile, the main Atlantic jet running well to the south, into Spain, and a local trough running down the Norwegian Sea framing the PV and the escaping Arctic air as it makes its way down towards us, the northeast-southwest aligned jet running from Greenland through Iceland providing plenty of scope for the development of more substantial features at the base of the trough.

With this kind of setup, often spoken about but rarely observed, it’s still very much early enough in the year for meaningful wintry weather anywhere in the UK and Ireland. And more, this might just be the starter to the battleground synoptics beginning to set up for the following week. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4813474

[damianslaw]

52 minutes ago, CreweCold said:

A strong Nino would sound the death knell on next winter already. Hopefully it goes strong through summer and autumn then subsides.

I think 09/10 was a moderate Modoki El Niño so that scenario could be workable.

Off topic, but I think the reason for lengthy blocked high pressure features we have had by and large for 3 years since March 2020 with a few exceptions (July- Nov 20 and Apr-June 21) has been due to the La Nina - the last 3 winters have featured unusually long spells of benign dry weather, perhaps we are due another wet stormy one... 19-20 being the last one, an El Nino in summer could mean very wet unfortunately.

I wonder if we are on the cusp of one of those major pattern changes.. to something much more sustained rainfall wise.. a major sudden abrupt change often a sign.. its timing as we move out of La Nina is interesting.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814020

[Cambrian]

Dydd Gwyl Dewi Sant hapus / Happy St. David’s Day. The festival is celebrated on the first day of spring to signify renewal. Its placing might well have been intended as a timely reminder of better days to come as the days draw out, responding to the need for moral comfort during the days of struggle in the face of the notable tussle between established cold and emerging warmth, an important annual passage. 

This year, as in many recent years, we have the feature of it turning markedly colder in the first few days of the new season, which has been consistently indicated in the weather models for a fair few weeks. The ECM pressure anomaly extended forecast from last week for 6-13 March signalled the incoming pattern very well, in the form of a strong Scandinavian trough, a southerly outcrop of a dramatically relocated PV, with the Atlantic trough held out over the western Atlantic, looking set for a protracted cold spell. This compares favourably for example with what is modelled for a week today, the 8th, on the 0z GFS ensemble mean. 

  

The ECM pressure anomaly forecast from this week for the same week (6-13 March) was quite different though, reflecting the trend in the modelling towards a stronger Atlantic influence much earlier, perhaps significantly curtailing the length of the cold spell. The GFS mean for day 10, the 11th, also picking up on this much swifter progression. However, though we pick up an influence from the southwest, the Scandinavian and Atlantic troughs have hardly moved. Typical battleground setup.

  

Perhaps only a brief cold spell then - but this is the point, just.as validly, perhaps not. The 0z ECM ensemble mean for day 10 displays similar marginality. 

 

And the Atlantic, it would seem, has only a very finite amount of time to make inroads on the back of any west-driven energy, which would appear to be shortlived. Monday’s ECM weather regimes extended forecast shows a speedy waning of the Atlantic after the 13th, with the following week after that indicating the re-establishment of northern blocking with heights into Scandinavia. 

  

In the coming days, the models will continue to grapple with the degree of certainty, rapidity and ultimate extent of any breakdown, a particularly challenging setup to capture reliably, but even more so with what is still a relatively week jet stream trying to grind away at such deep cold to the northeast. And so they will indeed struggle, much like the spring. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814183

[Battleground Snow]

Just a quick one on the MJO,

ECM really pushing high amplitude 7 and 8 now, with an average lag of 10 days this would indicate the affects of this would be felt around the 13th to 18th.

Generally in the winter months this would promote Scandinavian heights in phase 7 and griceland in phase 8,

But we are now in March and there are different seasonal wavelengths at play, there is also no composite for both 7 and 8 in Nina years for March, so I assume it's very rare and not enough data can be drawn upon to generate one,

Also complicating matters will be the ongoing reversal.

My point is that the modelling for this period should be treated with much caution, because the nwp has so many conflicting bits of information to calculate. The GFS probably just gives up and resorts to the default pattern.

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814256

[Met4Cast]

11 minutes ago, Battleground Snow said:

Just a quick one on the MJO,

ECM really pushing high amplitude 7 and 8 now, with an average lag of 10 days this would indicate the affects of this would be felt around the 13th to 18th.

Generally in the winter months this would promote Scandinavian heights in phase 7 and griceland in phase 8,

But we are now in March and there are different seasonal wavelengths at play, there is also no composite for both 7 and 8 in Nina years for March, so I assume it's very rare and not enough data can be drawn upon to generate one,

Also complicating matters will be the ongoing reversal.

My point is that the modelling for this period should be treated with much caution, because the nwp has so many conflicting bits of information to calculate. The GFS probably just gives up and resorts to the default pattern.

 

Just in terms of this - I think it's relatively safe to use the Nino composite as the likely sustained and high amplitude +AAM essentially pushes the atmosphere away from La Nina & into a more El Nino esq state

Quite a messy composite so it's difficult to gleam a huge amount from it. My expectations are that we'll see a waxing & waning of high latitude blocking through the first half of March, perhaps extending into the second as the MJO transitions through phases 7>8 & we see a "drip drip drip" of -ve anomalies impacting the troposphere from the SSW.

Towards the end of March, with changing seasonal wavelengths as you mentioned at play & the trajectory of the MJO, I wouldn't be surprised to see this high lat blocking beginning to lose influence & the Azores high taking over. Depending on how things play out this could suggest a rather warm start to April with March overall more likely to come in below average.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814266

[Eagle Eye]

33 minutes ago, Met4Cast said:

Just in terms of this - I think it's relatively safe to use the Nino composite as the likely sustained and high amplitude +AAM essentially pushes the atmosphere away from La Nina & into a more El Nino esq state

 

Absolutely the best composite to use. Late season Nino MJO Phase 8 I think is clear that it's a transitioning type phase and given the lower Strat/Tropopause driving of Troposphere weather zonal wise (localised 0 or below zero zonal wind states appear to have even more affect on surface than the Troposphere; given drag and boundary layer viscosity dominating over inertial forces, I'm not surprised that the 'skin' of surface weather is more variable zonally than the Troposphere and so momentum budgets are amplified surface wise) and that depends on the transfer of baroclinic eddies. Went off on a bit of a tangent there, in other words what I'm saying is that the ENSO state is having little impact on the weather and given the westerly wind burst there by early to mid March (ish) it should continue climbing towards neutral and Nino. For now though, the atmosphere is acting as Nino with some sort of Indo-Pacific warm pool convection type and a strong reaction. Composites may not be enough though with likely strong tropical forcing for interactions of momentum, energy and mass states so whilst I agree with the composites used here, it may be harder than using just the composites for surface patterns (though the Troposphere still drives the surface and theoretically is affected less by the SSW and zonal fluctuations but is still affected), anyway that's enough of my waffle for now.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814282

[Methuselah]

I really do think that this ought to be in here:

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814333

[Catacol]

36 minutes ago, Mike Poole said:

A few questions running through my mind as we await the 12z suite, which I think might firm things up a bit.

First, do we think the models are still playing catch up with the SSW, have caught up, or have overshot?  (And any other signal that is promoting amplification.).  When they have overshot, one sometimes sees features crop up that degrade the pattern.  I still think the pattern (~T144) is still strengthening - will be interesting to see on the next runs…

Second, I think it is harder to achieve cold in the UK than the models predict sometimes, but conversely, once cold is entrenched, it can be harder to shift than the models predict - will we see this?  We have some confidence we’re going cold, the question is about the longevity.

Third, this relates to the MJO - I’m still trying to disaggregate the effects of this (and AAM more generally) with the SSW.  We know the MJO was headed towards favourable phases for blocking at the start of this (16th Feb), which the effects of the SSW seems to have constructively interfered with.  But to what extent does the SSW affect the MJO?  Looking at the forecast sudden amplification in phase 7 and 8 (at least in the RMM plot, which I know can conflate things), is this due to the SSW?  Surely a major relocation of the PV might have some effect.  Anyone have any insights here?  Which is the dog and which is the tail, here?  (I think by this point it is the SSW that is the dog but could be wrong.)

I don't think the 12z will firm much up at all. To my eye NWP is really struggling beyond about 144h to resolve what is happening way up top with what would happen out in the atlantic in a "normal" scenario. The volatile model journey will continue for a few days yet and I would say again that upgrades rather than downgrades is the way it will go. Not the usual direction, but that is the joy of a significantly disruptive SSW event. We have seen a gradually upgrading picture overall in the last 10 days - let the same journey continue.

I still see a potential danger, namely that the disrupted vortex edges out to Asia rather than holding its ground closer to Europe and that this changes the picture to a less cold one. See here the 72h vertical GFS forecast

followed by the picture at 192h

You can clearly see the forecast retreat eastwards, very different to what we got in the heady days of 2018. However - and this is the big imponderable I think - if vortex energy leaks away and dies a death over Asia what replaces it over the NE atlantic sector? Logic would suggest a ballooning Azores high scenario which could potentially give us a collapsing block or perhaps another Omega scenario...

BUT look once again to the injection of momentum on the back of a significant +EAMT

and consequently a rising AAM profile that will promote a more meridional pattern to the sub tropical circulation.

What this will do is enhance the pacific jet once again and this opens the door for poleward eddies (blocks) to appear in favoured zones - eg the North Atlantic where the vortex has ceased to be a player.

Much of this will be about timing. And NWP is poor at getting block behaviour correct without an SSW in play - it will be pretty damned hopeless at nailing it in a scenario where an SSW is impacting at the same time as a big spike in pacific energy arrives....and this before we even mention the favourable MJO orbit.

I'm getting a bit confused in my technicality - but I think in a roundabout way I am suggesting that clarity will not arrive anytime soon, the first half of March will become a murky forecast zone but we are holding some very good cards indeed which could offer us a big win when the hand plays out. I'm getting pretty excited now. Are we in ramp mode yet, or playing the safer waiting game while we wait for unreliable NWP to come fully onboard? I'm tired of waiting for NWP approval....I'm all in for a cold and snow party come the end of next week..............

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814348

[Met4Cast]

32 minutes ago, Catacol said:

A really great question - not a million miles away from the chicken and egg conundrum.

I've thought about this a lot over the years, not that we have been lucky enough to have a large sample to test it with. SSWs of a bottom up nature are trop led - and so in that context pacific background drivers lead the way. Look at December just gone - we had a bottom up split created by pacific drivers that led to a period of genuine (if disappointingly dry) cold. Once those trop factors faded the vortex took back control despite some signs of stress at the upper levels which provided hope of something more long lived.

However the current SSW scenario looks to be the product of a blow in late January followed by a double blow at the current time. And it appears to be a top down event - trop led wave activity faded as we entered February and I cant quite see where trop led activity led to the sudden weakening in late February which arrived. MJO was not favourable through the early part of the month and we didn't have the classic precursor pattern embedded for any length of time.

So....top down warming of a very different nature has brought this one, very different to the scenario in December. In this scenario is the SSW a creation of an amplification signal already in place? I'm not sure. I think this current event is being caused by the SSW and lag effects of MJO through 6/7/8 with the spike in EAMT is providing the back up. But I'm at the outer boundary of my understanding here. 

All fascinating stuff. Always so much to learn.

You know, I've not actually stopped throughout this entire episode to consider what actually triggered this SSW and you're right, does seem the late January blow massively knocked the wind (ha, puns) out of the SPV allowing for the events in February to essentially knock it out entirely. I don't think the top of the SPV really recovered & that subsequently then "wobbled", propagated downwards triggering the SSW in February with help from increasing poleward flux, particularly this second round of warming that we're seeing now.

 

Interestingly - If this current period of blocking is mostly trop led on the back of the MJO triggering a rise in +FT, now +MT and overall surge in +AAM then the SSW impacts may yet to be felt. 

Fairly good agreement that the SSW will be enhancing the early March blocking rather than "causing" it directly so I think we can probably expect another round of blocking towards the middle of March or sometime in the second half of the month. Where that sets up/how it impacts the UK is a different matter entirely, but does lend to the suggestion of March coming in below average. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814377

[Mucka]

12z Model Output Analysis

Overview all models.

All models have the UK turning colder from the North and East from 5th March with very cold air established by the 7th.

The 5th will bring light showers into the NE turning wintry through the day from the NE with snow showers for high ground.

GFS snapshot 5th

The 6th will bring sleet and snow showers and snow to low ground in the North and East with that potential snow line moving SW through the day.

UKMO snapshot. 6th

By the 7th snow showers may be heavy and prolonged or there may be more organised bands of snow even to low levels across England and lying snow is quite possible. High ground will likely see significant snowfall, especially across wales and the Pennines but snow could crop up anywhere.

ECM snapshot 7th

The 8th will remain cold but dryer with snow showers more likely from the NW but still with the possibility of some snow moving inland into central areas.

From the 9th the models want to attempt to bring in low pressure from the SW as high pressure to our NW wanes. This could bring a brief period of snow to rain before milder air moves in or a period of snow for the S or low pressure may move into Europe with it staying dryer and cold for all the UK with snow showers from N and E once more. The third scenario seems more likely to me than a rapid breakdown as depicted by ECM and GFS 180 through 192 with low pressure disrupting SE into N France.

GFS 168/192

ECM 168/192

 

UKMO 12z that has a more rapid breakdown looks unlikely at this stage because the breakdown is initiated from the N with low pressure that travelled up the W flank of the high and then moving E through Greenland rapidly weakening high pressure and the blocking pattern.

UKMO has pressure drop to 1020mb while ECM has the low all but dissipated with a brief low of 1030mb before rising rapidly once more while GFS differs completely and does not take the low  E  but phases it with the Atlantic trough and moves it NW

UKMO/ECM/GFS 120h

ECM is the best outcome because we don't get the extra energy fed into the Atlantic trough and have it dissipate feed out to the E side where it has little effect.

GFS is next best as it does not weaken high pressure over Greenland but does give the Atlantic a small boost meaning disruption of any Atlantic low later is a little less likely than ECM

UKMO is a worse case scenario and the least likely IMO although obviously plausible. Something to watch for tomorrow morning.

Naturally having all but resolved the potential GFS shortwave drama quickly cutting off a cold NE feed, we need a new one to keep us on our toes. But, at least UKMO would still be cold for a few days.

 

The coldest and snowiest of the Ops is clearly ECM, but I believe also the most likely outcome, and say without my cold bias.

I think UKMO will correct itself tomorrow and we will see whether GFS or ECM is correct with that low heading to Greenland 96h through 120.

If ECM is correct with Greenland pressure profile then I think any milder air and Atlantic systems attempting to push in are going to get shredded out in the Atlantic and move SE, probably into N Europe but possibly further N with snow for the S rather than across the UK bringing snow to rain breakdown.

As with ECM I would expect a quick return to Northerly regime by the 10th.

Out into crystal ball territory.

From here further attempts to make inroads would likely be made by low pressure to the W and that is where it would get tricky with milder air having a better chance of winning out but by this time the Atlantic trough will also be running out of steam.

This may result in low pressure stalling and high pressure rebuilding in the Atlantic maintaining cold weather at least through to mid month. 

TLDR

Cold and snow for some is on its way, starting from the 5th/6th and will last until at least the 8th/9th but more likely 11th or longer. Possibility of milder air from the SW from the 9th but even here it may turn colder again for at least a couple of days.

 

 

 

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814601