Model discussion highlights

[Kirkcaldy Weather]

Very pleased with the developments and so much to cover in this post

First we are at the beginning of a super high amplitude MJO progression in phases 7 + 8 

What will the effects of this be? 

As was the case when in the Indian Ocean phases at the end of February we saw a westerly wind burst (WWB) "Many atmospheric weather phenomena act as stochastic forcing to ENSO.  The most effective one is strong episodes of surface westerly wind, often referred to as westerly wind bursts (WWB) 

Role of the MJO

The strongest, long-lasting and eastward moving equatorial WWE are mostly associated with the MJO.  These are all characteristics of WWE needed to efficiently generate oceanic Kelvin waves (Hendon et al. 1998). In addition, WWE associated with the MJO exhibit a basin-scale structure and they undergo strong interannual variability, both essential to ENSO responses.

Many observations have shown anomalously strong MJO activities prior to and during the onset of ENSO warm events " 

Influences of the MJO on ENSO | US CLIVAR

USCLIVAR.ORG

This MJO progression at high amplitude in phases 7 + 8 will be producing a substantial WWB which will bring the rare triple dip La Ninà to an end 

"La Niña has weakened in the tropical Pacific Ocean and is likely near its end."

http://www.bom.gov.au/climate/enso/#overview-section=Summary

Looking at the SST's in the ENSO region from January 

 and progressing forward to May we see the trend into El Ninò 

For Mid > latter stages of March we will see a fresh development of blocking ..be watching for high pressure setting up to the east and/or near to the UK with repeat retrogression into Greenland 

 

 

 

The Pacific Jet also receives a bit of a boost of energy thanks to these phases and those of you who follow my posts will remember that was a common pattern in December especially between Christmas and New Year, I don't think it will be as intense but California again under fire from atmospheric river style conditions which they really will struggle with as they are at the point of having TOO MUCH snow

Pretty cool we will be able to link the patterns a long way round the northern hemisphere thanks to the Pacific Jet 

Certainly opportunities over the next 7-10 days roughly, for snow events in various areas of the UK and frequent frosts some of which becoming increasingly severe particularly with any laying snow.

I created the March 1981 composite which has big similarities 

As mentioned the setup through the remainder of March >>

 

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4816943

[bluearmy]

40 minutes ago, nick sussex said:

Mogreps show the clear split between areas further south and northern areas where the cold tries to hang on.

For London it loses all cold solutions past the 11 th March . For Aberdeen it’s a mixed picture although the trend isn’t as  good this morning even for northern areas although of course still a much better chance of the cold hanging on .

Things are moving in the wrong direction this morning overall for coldies in the medium term although there is still a chance of seeing some snow . 

The models then drop a later tease with a wedge of heights to the nw or taking the GEM that builds a high over Scandi .

At the moment the Iberian slug aka as the limpet high ridging ne looks a temporary affair.

The Iberian ridge looks to be a consequence of the digging Atlantic trough which is in turn, a consequence upstream over n America (and a lot of it related to the speed of retrogression of the block which is a strat related consequence) 

In the first instance, that speed of retrogression doesn’t allow the pattern to dig deep enough into europe to allow the U.K. to get far enough north of the polar jet such that the attempted incursions of the atlantic lw trough are safely repelled within a snowfall context (and areas of vorticity north of the polar flow provide home grown surprises).  Hey ho, it was generally on the cards once the second push of low heights looked to be heeded wsw rather than south or sw with the Arctic trop reversal v evident in the Atlantic sector. 

beyond the will it/won’t it for later in the week, further amplifications and below av temps remain most likely for nw europe. at this range that won’t show anything likely to deliver proper snowfall but we have no idea what will actually end up verifying within the general pattern - todays 00z suites perhaps offer more chances of a scandi high than has looked feasible for some time. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817436

[nick sussex]

NCEP have noted again the uncertainty surrounding the pattern there . The ECM/UKMO and GEM are in the faster progression of troughing there  . The GFS slower.

The former break down the block more quickly and so you lose much of the forcing on Atlantic low pressure . The GFS hangs on for longer as you can see here . Also marked differences in the Pacific .

UKMO day 6                                          GFS day 6

    

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817428

[Summer Sun]

Whilst most of the focus is on snow, it's also worth noting Friday afternoon and night COULD see some disruptive winds in the south

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817550

[Cambrian]

A huge amount to play for yet with the models. Some slightly bolder indications of a gradual change to milder weather, but it’s still fair to say that things are far from resolved even for the end of this week. 

As we know, the UK and Ireland, between the latitudes of the Shetland Isles and Jersey, are in a zone of perpetual fluctuation between cold air to the north and mild to the south.

But rarely are those extremes as stark as in the current outlook. Here are the 0z ECM ensemble plots for the far north and the channel. 

For Lerwick, the T850 mean is on a 15-day slog back to the long term average. The 0z op deviates sharply from the mean around day 6, so quite improbable on the face of it that the mild incursion is going to sweep through anywhere near as swiftly or comprehensively as indicated in the recent op runs. Day 4 spread around 5 degrees, mean trend flat.

  

For Saint Helier, the mean breaks through the long term average as early as day 3, so very likely turning mild for a week or so before turning cold again around day 10. Day 4 spread around 8 degrees, trend clearly upward. 

The rest of us are in the same battle zone we were in yesterday, with the reliable limited to the next 5 days or so. For Birmingham, the trend is upward but there is already a spread of 16 degrees at day 4! 

The 6z GEFS mean for day 4, at the beginning of the mild push, shows the troughing of the Atlantic low into the southwest of Ireland and Britain and the core low heights of the PV over Svalbard. Favourable uppers at that stage. 

  

By next weekend, at day 6, the Atlantic trough has moved north, to point at the west of Scotland, and the uppers have responded upwards.

 

But the Atlantic low has weakened, to the order of 15mb or so in just two days, and more to the point, the core PV heights have actually moved south, as if to line up some serious resistance, and the mean uppers over Iceland have dropped to -20 degrees. 

Looking for the geographical areas of most spread, for heights, the greatest ambiguity is the southward extent of the cold trough over southern Scandinavia; and consequently for the T850s, the greatest ambiguity over the north and east of the UK through the North Sea to Central Europe, a 3 to 6 degree deviation either way for the UK and Ireland on those means, with low pressure in the Norwegian Sea holding firm.

 

So firstly, for anywhere north of the Brest Peninsula, the confidence for northward progression of the mild air is relatively low, even in the usual reliable, and the drop-off as we head north is steep.

Secondly, lower heights and uppers are pushing south nicely - once that bubble of Iberian heights passes through, the cold air may very well quite quickly push by the waning Atlantic low and bring us back into a reinvigorated northerly flow, which is probably what the means are picking out with that signal at day 10. 

Thirdly, in the battleground period of several days that we have ahead of us, with such ambiguity in forecast temperatures just to the northeast, and the PV still very much playing ball, what could easily be being underestimated here is the impact of repeated southward digs of cold air on the western flank of passing systems.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817573

[Man With Beard]

3 minutes ago, Tim Bland said:

My analysis is based on needing less than sub -5 hpa for proper snow, -8c or lower for at this time of year 

In March 2018, we had snow on the south coast all the way to 0C 850hpa.

We may not have the same depth of cold this time, but the same principle applies, easterly draw before a front, 850s can be higher than usual.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817812

[nick sussex]

You can see the latest fax charts for T36hrs and T48hrs.

The front moving south slows as that small wave develops over the southern Netherlands . 

You can see just how slow the progress of the front is between those two time periods .

T36 hrs                                                    T48 hrs

That allows the colder air more chance of undercutting the precip .

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817952

[Daniel*]

6 minutes ago, nick sussex said:

Northern Scotland gets some sub -12 850s which is pretty amazing for March from the north .

If we can get a slack flow there and clear skies for several days over snow cover the March record could go.

I saw this map on Twitter recently I found it interesting. March is coldest month of year on North Atlantic side of Arctic above the U.K. no wonder Arctic flows in early spring can be very potent while a source from east we see increasing moderation. Arctic flows remain their potency for longer as the seas become very cold over winter, and Arctic sea ice extent is near annual maximum the 1981-2010 median date is March 12th.

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818177

[Jordan S]

Hi guys, Have been rather busy so haven't had time to really check the weather a great deal, but certainly looking very cold for the time of year for the next few days with around 3 frontal snow possibilities in central and southern England with ongoing snow shower risk in northern and eastern areas, possibly giving a few cm's to low levels..

Certainly some cold uppers over the country at the beginning of this week..

snow probably developing on the back edge of the cold front sweeping south later tomorrow into Tuesday, how much snow differs on the models but I suspect a few locations in central and southern England and Wales could see a few cm's on high ground with a light covering to lower levels before clearing away with isolated snow showers and frost the main theme across the country..

temperatures no higher than 5c with temperatures possibly dipping to -5c in the south on Tuesday night. 
The cold front clearing through tomorrow night starts a series of secondary lows/waving fronts set to move east/southeast through Wednesday into Thursday giving the risk of significant snow at times to southern and southwest England..

Then the next risk of snow after that in the south being later Thursday into early Friday with a low moving northeast into southwestern areas, giving the potential for several cm's of snow in places, moving erratically northeast so precipitation probably more back to rain in the far southwest..

Then by late Friday early Saturday the next frontal system tries to move in, this will probably make more progress so a transient period of snow seems likely for now for the Midlands,north Wales and anywhere to the north and east, with rain for western and southern areas introducing more of a thaw, Scotland keeps the snow showers..

On balance I think Scotland keeps the cold for the most part over the weekend and into the early part of next week 12th -14th march with further persistent snowfall most likely here due to frontal systems coming up against the block of cold to our north and northeast with the risk of gales and heavy rain in the south.

But around mid march as recent output has shown, a trend back to colder and snowier conditions further south are a possibility but featuring more from the east as high pressure could develop over Scandinavia for a time bringing possibly frequent marginal snowfalls to eastern areas but less cold in the west and far south with rain for the most part being the most likely outcome overall with winds tending to be from the southeast I think, probably not as cold as this spell coming, of this doesn't materialize then a very unsettled milder period with rain and strong winds frequently crossing the UK is the lesser likely outcome but with heights still relatively high over the north west and also to our south this time.

An interesting couple of weeks to say the least with most weather types being featured, a proper mixed bag but overall a cold/chilly mixed bag.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817956

[bluearmy]

21 minutes ago, EML Network said:

Wondering why both the Met office and most of us on here arent taking this seriously ?

Is it because its during daylight hours ?

Just wondering if some of us are looking too far ahead so notice what MIGHT be a memorable day on Tuesday ?

Maybe theyre seeing it as rain/sleet ?

Looking at Tues morning gfs snow and dp’s - doesn’t look likely to be sticking snow to me 

   

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4817489

[Alexis]

Just looking at some of the high res precipitation models - Arpege, Harmonie, Icon, UKMO.

Three of the four don't even have the rain on the radar we are seeing now in NW England, even on 18z runs.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818201

[DAVE_ALLEN]

So much information to digest on the fax chart for next Friday. Whoever does the week ahead forecast for the Met office today. Will have a lot to include this week  

On the chart blueline shows an approximate limit of the snow coming south. 
The Midlands looks to be particularly wet with possibly some very high rainfall totals. If  that warm front waves over that region for long. This in part due today huge difference in temperatures to the north and south of the front.
 

After probable heavy fall of snow, in southern counties of England, on Wednesday and Thursday, there could be a great deal of fog and a rabbit thaw , if the warm front moves north into northern England, as shown on the fax chart. 
The incoming low over Biscay has some very warm air behind the warm front, possibly up to 15 C. So there could be a particularly warm day one day this weekend down south.

And finally, the low with a purple arrow could develop into something of a beast, as the jet is still to the south, and the low will be developing in the colder air close by the UK. If this happens, there could be some very stormy conditions along the south coast and in the south western parts. 
 

All of that follows the snowfall that we see in the days beforehand. If you check the radar this morning, you will see, see it is already falling across parts of Surrey and the borders to Sussex and there has been some wet snow passing close to Cambridge, and heading towards Suffolk and Essex. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818337

[That ECM]

The only accurate model this week.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818418

[nick sussex]

You can see the wave and how it ripples eastwards here .

So the wave develops towards Bristol on the T24hrs chart .

 

In the T36hrs chart you can see the wave towards the Low Countries .

The wave slows the front and also helps to intensify the precip .

You also have a slack flow north of the front with little mixing so evaporative cooling comes into play.

In 12 hrs the front has only edged about 50 miles south.

 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818493

[Cambrian]

Two aspects of today’s output are striking. 

Firstly, the formation of that cut-off low across the UK and Ireland and its subsequent movement into the North Sea around day 4/5 puts a local cat amongst the global pigeons. 

   

Here are the last four GFS operational runs (12z, 18z, 0z, 6z) for the early hours of Saturday. The low appeared with a bang in the 18z run, was less of a feature in the 0z, but is shown deeper again and closer to the east coast on the latest run. The westerly in the south / easterly in the north has been replaced by a northerly flow through all parts. 

Secondly, to the west, the edge of the lower heights running along the bottom of the Atlantic low to Ireland and the UK is just getting flatter and flatter for around day 5, the shallow undulations being stretched straight, reinforcing the idea of the models picking up on more scope for disruption of the advancing lows. The 12z yesterday had the ejection of a 990mb trough across Ireland and the UK. Today’s 6z keeps the 995mb isobar about 200 miles west of southwest Ireland. As well as its trough being held off further west, the Atlantic low itself is being edged south to the east of Newfoundland. 

To the south, the 584 gpm height contour over Iberia is slowly being adjusted south since its zenith on the 18z run, and to the north, the 512 gpm line from the PV has steadily been pushed further south too, running through northeast Scotland, so the overall distance heights gradient is similar but the pattern is being shunted south. The core PV heights in the Norwegian Sea continue to lower, and moreover, the Norwegian Sea trough is well and truly holding its ground in the reliable. 

 

As a result, by day 8, early next week, we’re back to square one, returning into the Arctic air, 512 gpm low heights digging south into the middle of the North Sea, possibly even colder in the south of the UK than this week, and a channel runner thrown in for good measure. The Atlantic trough has by then given up the ghost. 

This all still has a great deal of firming up to do, and we still don’t know where the most intense battles will line up over the next few days, but we are beginning to get a much clearer idea of which side will ultimately win. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818591

[Mike Poole]

When there’s lots of uncertainty like this, I like some of the probabilistic output, here’s the % risk of snow throughout the week from the PEARP (ARPEGE ensemble):

Most of us in with a shout there at some point!

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818585

[Catacol]

I'm not going to be popular....but I think a lot of folk are going to see cold rain for the most part this week - and nothing close to approaching 2018. At elevation a different story but unfortunately for me the setup has gone awry. Blocking to our north is set to transition westwards very quickly and this will allow the atlantic to come in much flatter than we want. There is no block for these systems to bump and slide against, instead a cold trough pushing in from the NE but the cold trough itself isnt making sufficient inroads to get entrenched cold in place as we did at least manage to get in early December.

Why? The origins of a less wintry setup I think lie in the worry I had last week, namely that the remnants of the vortex setup too far east. This was a change to what was modelled initially and it has worked against getting artic air to flood far enough west. You can see it clearly here - setup last night shows the block in place and the deep trough over Scandy

and then wind forward 72h and the trough has essentially gone nowhere - stalled due to lack of momentum inspired from aloft because of the positioning of the SPV

Heights pulling very fast towards Canada, atlantic pushing in a merging with the Scandy trough - and for me this is classic nasty cold rain conditions for the vast majority away from Scotland.

To labour the point a bit - here is the vertical profile of the vortex right now

way - way out east whereas back in the days when the SSW was getting exciting this was where it was projected to be:

Location and angle have changed and absolutely not to our advantage.

Note also how the MJO has panned out: we hit phase 7 in good time.....and it died. It dropped into the COD taking away the signal for northern blocking when we wanted it and is now surging dramatically once again into the West Pacific - but much too late to give support to our current pattern this week.

This suggests a chance for renewed blocking in the second half of the month, but I think in truth by then it is too late for any deep, sustained cold. Spring will be on its way.

Not the right time for an end of season reflection yet - things can and do change and I hope this post is left looking daft in 3 days' time as people wade through the drifts...but for me another opportunity looks set to slip by. I have seen more frost this year than in many years but for swathes of the south I think this winter is going to go down as a "so near and yet so far" kind of season. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4818628

[nick sussex]

The NAM the USA mesoscale model which covers just enough of the Atlantic to get an idea regarding the much talked about low has been trending south since last nights 18hrs run .

The 06 hrs run is even further south . We await the other 06 hrs outputs !

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4819736

[Cambrian]

It looks to be setting up a meridional pattern over the Atlantic and Northern Europe by day 9 looking at the 12z ECM operational run. 

There is however a more extensive Eurasian block and blocking towards Greenland too, with the Siberian PV showing a bit more stomach for the fight compared to that shocker of a run of 12z yesterday. 

 

Potentially important for us as the UK and Ireland are near to the dropping point of the trough with a similar Siberia - Norwegian Sea - Northwest Europe alignment as the one we’re in now. 

As we progress past day 10, looks like we’ll soon get the low to the east of us and return to an Arctic maritime flow sourced from the pole via Svalbard.

 

A broadly slow-moving setup too, so a further notable cold spell around mid-month a distinct possibility. 

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4821212

[Mike Poole]

1 minute ago, sheikhy said:

Just a quick question!!if the ukv is a high resolution model why is it playin catch up with the ecm?!!you would think being a higher reso model that it would perform better inside 2 or 3 days?!!

All the high resolution models don’t model the entire global domain - they are for a local region and take their boundary conditions from a larger or global model.  So if the reasons for inaccuracy aren’t local, they won’t perform better, even with the higher resolution.  And we’ve noted with the current setup, the track of the lows is complicated and related the train of lows on a larger scale.  

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4821325

[Catacol]

4 hours ago, bluearmy said:

The dp’s are generally fairly borderline where the main precip occurs so I wouldn’t be certain that we won’t see more of a slush fest than a winter wonderland away from >400 feet.

its very tricky  

Yes. Don’t be dazzled by the pink. This setup is very marginal and parts will see rain or sleet - possibly many areas. There is likely to be a sweet spot that is impossible to identify even at this short range that will see a good snowfall for a time - but my instincts looking at the charts and the daytime temperatures are that this is a scenario where people are best to set the bar pretty low and then if you are in one of those fortunate areas where the conditions click be pleasantly surprised. Expecting this setup to deliver widely in terms of an all snow event that will stick and survive I think might lead to widespread cursing. I’ll be interested to see tomorrow if the Met turn any of the warnings amber because so far they are clearly not convinced of significant impacts and have left everything yellow. The media are fanning the flames - even the moderate minded BBC were getting all excited on the news - but I can see a lot of mixing going on given the angle of the Atlantic attack and it might end up being a scenario where experience trumps models.

Hope very much to be proven overly pessimistic still.

The overall pattern sliding away as the block hotfoots to Canada. I would not have expected this ridge to be so quick to retrogress and collapse when looking at the shape of things at the start of the month but without the vortex split perhaps it was naive to expect a -AO to hang around for long.

Bring on 2023/24….winter of eQBO, Nino and the dramatic late December split.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4821855

[Eagle Eye]

I'm going to have a look back at how I did for now as I'm too in love with a girl to get mixed up in the will it and won't it snow stuff .

February 15th

"

This time, the SE US ridge might work in our favour without a -VE EPO so strong and no exiting wave of warm air meaning that the lag in about 10 days plus with a quite strong phase 5-6 lag suggests a much better North Atlantic and perhaps Pacific signal as well. So more poleward advection of energy transport and with that comes Rossby Waves carried by the eddies.

To put it bluntly, the first SSW is unlikely to affect surface temps however the second warming might be able to downwell easier. Looks very much to me like were probably seeing the uncertainty of if the first SSW can affect the Troposphere at all and the lag of the MJO movement rather than the direct affects itself yet. 

I feel like this will get pushed back till the second SSW time as the models start to model the zonal wind structure throughout the atmosphere better when it gets closer to time. Essentially this is a learning curve with so much going on and of course the poor modelling between the Stratosphere and the Troposphere. We've also got to be careful of conformation bias when looking back at how this eventually worked out as I think this could be a good event to look back on to understand everything."

February 25th

"Looking at the background signals and everything is lining up fairly well for early to mid March for now as the general signal for the best transfer of momentum via mountain torque temperature. Heat over the Himalayas so upwards wave transfer of momentum into the Strat and circulating into the Trop also exciting a Rossby Wave packet which breaks cyclonically and extends the Pacific jet but also the Tropopause to Strat part of the Wave could really help bring down the SSW quicker than expected."

February 28th

"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 think overall I did quite well but I was too excited about the second chance to properly mention this one which is looking to be much better than any chance later into March though this is from the second part of the SSW downwelling and MJO excitement with Tropics momentum and convection modification. The Nino modified w'ly eq-lat with Rossby Wave dispersion events leading to better MJO CCKW modification between tropics and extratropics for RWT events in the meridional circulation. With the extension of the Pacific jet, the energy and momentum changes between Eq-lat and the Tropics dynamically shifted. Imagine a scenario (in a Global Synoptic Dynamic Model (GSDM) ) where you have a w'ly momentum bands exchanging energy up from the lower tropics and it's induced by a large double +VE EAMT which retracts the Pacific jet. That frictional-rising AAM double response between the two to balance out the laminar flow in the atmosphere as a fluid flow tends to do (despite inertial forces tending to dominate and a strong jet stream favoured above boundary layer area, the viscosity and resistance of a fluid to being misshapen helps with the development of that RWT) helps with southerly jet push and the momentum fluctuation in the jet back up towards the north in the form of a Rossby Wave. The setup in this situation happened to favour North Atlantic AWB with a retrogressing Greenland-Scandi dipole that eventually let's our best angles cold flow fall eventually as the energy is transferred towards the Alaska area of the Rossby Wave.

Enjoy the snow if you get it you all.

Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4821953