Model discussion highlights

[Met4Cast]

Stepping back from NWP model outputs I want to have a quick look at where we are in terms of background forcing, how the GSDM/MJO is progressing and current expectations. 

The MJO is currently in a moderately amplified phase 1, this during El Nino is conducive to blocking & that's why (along with the AAM etc) we're expecting to see blocking patterns in the early-mid January period, beyond this though progression will continue into phases 2/3/4 before uncertainty kicks in and modelling begins to diverge.

Phases 2/3/4 broadly correlate to an Atlantic driven regime across NW Europe, i.e this is a signal for reducing blocking and a returning Atlantic probably in the 2nd half of January, this in itself could give rise to some fun & games in terms of high impact snow events as low pressure systems undercut any blocking and bump into the cold air, but ultimately I suspect the Atlantic will eventually win out. 

Further ahead, with the MJO returning to the Indian Ocean we see a scrubbing of westerly momentum from the Pacific and strengthening easterly trade winds, something we're already beginning to see happening as this MJO cycle ends, -FT is now well advertised and -MT will soon follow producing a sharper dip in AAM tendency. 

Total AAM however is now very high so I don't think we'll return to a strongly -ve AAM again and we should remain in a nino-esq state overall, a more progressive MJO transition eastwards would once again see a return to +FT, +MT and rising AAM tendency but exact timing on this is uncertain & largely dependent on this next MJO cycle. 

Overall then, plenty to be excited about going forward through the second half of winter beyond this next period of interest. There's further potential for a SSW later into January with the vortex most likely remaining below average throughout.

Promising.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987753

[bluearmy]

9 hours ago, Cheshire Freeze said:

No worries. It’s a hotly debated point but obviously any westerly momentum in the strat in the middle of winter (where the strat/trop are most likely to be coupled) is not ideal, no matter how relatively weak.

There have been changes in the modelling. Look at the NH set up. We’ve gone from initial tpv in tatters type charts to a healthy looking tpv atop some modest Atlantic amplification.

I think the speed at which the Canadian and Barents segments separate is relevant. That allows the Arctic high to poke its nose in. If they do it too slowly then the Arctic high will meander elsewhere and we then await another cycle upstream to generate a new Arctic high  

53 minutes ago, Mike Poole said:

You obviously didn’t read my post properly.  I am not talking about the aftermath, I am talking about where the strat vortex was moving in the build up, and the effect that might have had on trop driven amplification.

ec op 
30 hpa 00z day 10                                    Yesterdays chart 00z day 10 
  

yesterdays 12z is on Berlin and I haven’t had a gander yet to see how it manifests through all the levels 

EDIT: having had a look - there is strong support for the Atlantic ridge into the mid to upper start which should give it a chance to stick. In addition, we technically do have a split spv high up with a weak pacific segment which detaches from the Canadian tail and works its way around the NH. the spv continues to be stretched at 10hpa by day 10. I wonder if a strong trop North Atlantic ridge could work a bit of magic from down below ???

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987652

[Catacol]

4 minutes ago, Met4Cast said:

Stepping back from NWP model outputs I want to have a quick look at where we are in terms of background forcing, how the GSDM/MJO is progressing and current expectations. 

The MJO is currently in a moderately amplified phase 1, this during El Nino is conducive to blocking & that's why (along with the AAM etc) we're expecting to see blocking patterns in the early-mid January period, beyond this though progression will continue into phases 2/3/4 before uncertainty kicks in and modelling begins to diverge.

Phases 2/3/4 broadly correlate to an Atlantic driven regime across NW Europe, i.e this is a signal for reducing blocking and a returning Atlantic probably in the 2nd half of January, this in itself could give rise to some fun & games in terms of high impact snow events as low pressure systems undercut any blocking and bump into the cold air, but ultimately I suspect the Atlantic will eventually win out. 

Further ahead, with the MJO returning to the Indian Ocean we see a scrubbing of westerly momentum from the Pacific and strengthening easterly trade winds, something we're already beginning to see happening as this MJO cycle ends, -FT is now well advertised and -MT will soon follow producing a sharper dip in AAM tendency. 

Total AAM however is now very high so I don't think we'll return to a strongly -ve AAM again and we should remain in a nino-esq state overall, a more progressive MJO transition eastwards would once again see a return to +FT, +MT and rising AAM tendency but exact timing on this is uncertain & largely dependent on this next MJO cycle. 

Overall then, plenty to be excited about going forward through the second half of winter beyond this next period of interest. There's further potential for a SSW later into January with the vortex most likely remaining below average throughout.

Promising.

Yes. The goose has done a runner with the golden egg scenario, namely the split SSW that would have produced the potential for a long term cold setup, but we have 2 distinct period of opportunity despite that failed split. The first is the tropospheric driven blocking starting around 7 January which should persist for a week to 10 days, with breakdown events possible as you say. Then - given the particularly swift MJO passage this season a second bite at the tropospheric cherry into early February.

The unknown bit is how significant the impacts of strat stress are going to be. A technical SSW now looks out of reach, at least for the first half of January, and the tropospheric signals for the kind of forcing required to create one are on the wane. We are moving towards a North Pacific ridge and low pressure around the Urals, and this will serve to halt the warm assault on the strat as well as enhance the sharp decrease in GLAAM from mid month. So… what will the impacts be of the weakened, stretched and potentially wobbly vortex at the time we get that reduction in GLAAM? Way too early to know. If - as appears to be the case - the vortex retains its core then we want that to stay around Siberia and not trot across to Canada. I suspect the low percentage scenario in the Met long term text is built around this possibility because a weakened vortex sitting over Siberia mid month might possibly allow blocking to our north to hang on. Knife edge stuff.

Lots to contemplate over the next month or so, but for the first 10 days of January we have an evolving blocking scenario to follow and snow chances for some…maybe many!

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987772

[Tamara]

11 hours ago, Cheshire Freeze said:

There have been changes in the modelling. Look at the NH set up. We’ve gone from initial tpv in tatters type charts to a healthy looking tpv atop some modest Atlantic amplification.

Emphasis currently focusses on tropospheric led diagnostics and the significant switch to poleward momentum of +AAM anomalies. The starting pistol for this was well advertised & discussed over the past 10 days. Such a process weakening the strength of +ve NAO sub tropical ridging, and slowly but steadily increasing westerly sub tropical jet stream momentum from the Equatorial Pacific downstream across S US and into the Atlantic.

NWP has been erratically attritional with this evolution, and it is still unfolding within the 8 to 15 day period but current EPS and GEPS members seem to be handling this better at present than the GEFS which is not seeing the undercutting processes quite as clearly.

Signs of warmer sub tropical influences across south western European c/o clustering ensemble readings point to slightly better support today for lows tracking well under the developing blocking across Northern Europe and into southern Greenland and a more coherent -ve NAO to come. This signal is still a little disjointed but it has gained further traction in the overnight output suites. My own watch has been on this, rather than SSW-gate. The EC det today is a good example of the type of evolution being looked for.

This tropospheric momentum led transport shows that a classic SSW reversal is not required for this type of synoptic set-up when there is instability supplied from the bottom upwards. On the very large scale, December to January 2009/10 proved a case in point with GSDM led poleward momentum providing the tropopause instability. An SSW did follow that winter, heading between January/February 2010 but by this time much of NW & N Europe had been very wintry already for a considerable time.

And, not to forget that broadscale eddying poleward of +AAM anomalies is a key element of stratospheric pathway diminution itself - looking ahead.

From mine & Portuguese perspective and interest, some sub tropical jet induced rainfall would be appreciated ahead of another Iberian summer.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987895

[Eagle Eye]

Ensembles on both sides continue to go for the anomalously cold temperatures to settle in fairly quickly and hold the - VE AO pattern up very well w/a more Iceland centred high, holding out the Atlantic pattern quite well. Allowing a NE'ly (ish?) flow to push in. To have that on the ensembles is typically a good sign, doesn't mean particularly much in the context of 1 run but I think all ensemble runs for the last couple days have been showing this. Rising heights over eastern Canada and the USA also suggest a weakened zonal jet probably disconnected, allowing for wavebreaking to occur which given the base wave setup and the poleward AWB (anticyclonic wavebreaking) will allow for the undercutting flow to remain intact and be the strongest driver of the weather for us directly underneath the AWB. Hence why the latest ECM looks like this, even if it is on the extreme end it still makes sense as the AWB could well force this. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987891

[Tamara]

1 hour ago, Met4Cast said:

Fantastic post & echos much of what I’ve been saying in the last week or so re: EPS/GEPS broadly leading the way on the expected evolution ahead of the GEFS, though put much more concisely than I! 

Do you see the blocking pattern waning as we progress through mid month on the back of falling AAM triggered by a strengthening of easterly trade winds in the Pacific c/o the MJO returning to the I/O for the next cycle? 

It’ll be interesting to see how far total AAM falls & I suspect much dependent on how quickly this next MJO wave progresses eastwards to restart the +FT > +MT > +AAM tendency cycle all over again, likely this time from a higher total AAM base state than this current cycle had to contend with! 

I don’t see any reason to think a zonal, flat Atlantic regime will take over for any extended period (if indeed at all) with tropical/sub tropical forcing likely to become supportive of higher latitude blocking again later into January/early February (assuming a more progressive MJO, a slower progression still a possibility based on current modelling). 

The westerly wind anomalies which developed over the Equator early December onwards that have been fluxed poleward over the last three weeks c/o zonal mean transports cater for the upward spike in angular momentum. (This also paves the way for a cold pattern to return to N Europe later January into February - see further down). 

Relative angular momentum is up c/o fluxing of westerlies from the tropics into the sub-tropic. This, as we know, can be seen as a +ve angular momentum anomaly on the calculated aggregate tendency plot (shaded red) from 30N

.

Uniform extra tropical +ve mountain torque has driven tendency in relative angular momentum and the GWO well into phases 5/6

With the Nino attractor phases fully engaged, the amount of westerlies propagated into the atmospheric circulation between tropics & extra tropics do, as you suggest, indicate a higher floor to the angular momentum budget in the 'lull' ahead of the next upswing as the MJO traverses the tropics during January. GWO Phase 4 is a good medium base floor level I would think.

If we were to take a diagnostic template of GWO Phase 4 (and account for a likely return shallow orbit through phases 0,8 & back to 4) during an intermediate 'lull', then that very intermediate lull phase you describe will likely feature the return of a flat upper flow extending towards Europe.

The difference to recently though with the Atlantic forcing will be the jet suppressed further south - and with a cold pool established then attritional forcing of lows against the blocking likely to be a feature. (This accounts for some of the thinking as expressed within my previous posts in respect of the sub tropical jet)

With a cold pool established, any erosion from the south/west into NW Europe is likely to be attritional and not likely to displace cold pooling totally.

Taking into account we are still now in the lag period for the effect of +ve momentum from FT/MT mechanisms c/o the recent tropical>extra tropical cycle, we are seeing the new convective signal come into the Indian Ocean. Sea temp anomalies are supportive of deep thunderstorm convection. This, allied to convection in the western Pacific, will generate further mean zonal west wind anomalies to propagate across the tropics into the sub-tropics. With the westerly wind additions generated in December still in the atmospheric circulation, total and relative angular momentum will increase sending the GWO back into a (potential high amplitude) phase 5 orbit.

This progression & evolution suggests a strong -ve NAO with positive height anomalies shuffling between Greenland > Iceland and Scandinavia. Meantime southern Europe wet & mild under the sub tropical jetstream.

Muito obrigada. Melhores Cumprimentos

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4988037

[bluearmy]

36 minutes ago, northwestsnow said:

I agree with much of this.. 

We need alignment at day 5 -  7 before popping open the champagne  

I've been on this forum nearly 20 years and the lesson learned is trust absolutely nothing beyond day 6 if a cold spell is on the table.. 

Nothing wrong with looking at narnia charts at day 8,9,10 ,but they should come with a Government health warning.. 

 

Good post NWS

the reversal at the top of the strat is now only three days away 

it’s further complicated by the fact that it reverses and then goes back positive and then reverses again 

Can we expect the models to deal with that as week 2 progresses?  - the strat vortex is moving around like a cat on a hot tin roof and it looks well coupled mid upper  strat down to the trop 

Zonal flow today is at 40 m/s at 10hpa which is the top of the mountain on the forecast we’ve been following. So the deceleration starts imminently. With reversal higher up within a few days, we may begin to see the models consolidating on a weaker flow in the trop as a consequence  - of course there is also a possibility that the opposite happens !

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987681

[Daniel*]

I honestly don’t know what people are talking about saying ‘delayed’ and using ECM 00z as an earlier guide which was on extreme end is not the way to go. This is most positive day so far, growing agreement of amplification towards our west pushing north with pressure lowering in Europe, how this amplification behaves is quite uncertain, but the direction is certainly cold if much colder to now. We will need to give it a few days but these colder synoptics are getting closer. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4988436

[Mike Poole]

ECM clusters T120-T168:

You can see on clusters 1 and 2 that at T168, the WAA towards Greenland is now curving back so it isn’t going to properly take hold, this had been the trend on the op runs too.  Cluster 3 still has a decent angle on it, but with 6 members, I suspect that is a ‘legacy’ cluster from which we have unfortunately moved on.  

T192-T240:

T192 - clear consensus on UK high (or slightly to the north).  So that is game over for that attempt at a high lat block.  Not much movement tot T240 on 1 or 2, cluster 3 hints at a move towards Scandi, cluster 4 maybe heading north.

T264+:

Clusters 4 and 5 look messy, clusters 1 and 3 look promising in terms of a migration of the high to the north - these two account for half the members, cluster 2 holds the high close to the NW.

I think the window of opportunity to develop a high lat block (if indeed it existed at all outside the imagination of the models) is limited (maybe up to 20 Jan) before AAM subsides and the MJO moves out of semi-favourable phases.  So now we need a second bite at the cherry, that may well have to be eaten in the last chance saloon to salvage something snowy from January.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4988914

[Met4Cast]

One thing of note; 

Any snow that does fall & accumulate from back edge snowfall of the cold front around the 6th or wintriness from the E/NE before the high builds in could be on the ground for upwards of 10+ days. 

Details of course but worth mentioning. Likely a case of a cold high & cold pooling deepening in situ, when weather fronts return that could be very interesting (assuming we don’t see the currently unlikely solution of a sinking high)

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4989133

[phil nw.]

Still some details to iron out within the operational runs but  the ensembles are starting to clear the uncertainties for the end of this week.The combined 850 ensemble graph for Warwickshire showing the average forecasts from the GEFS/ECM/MOgreps .Good agreement that cold air  is in by day 7 .

So the  suggestion is that the high will build just far enough north west to usher in the first cold wave from the east over next weekend.

Eps T168

I take that as the first hurdle overcome,get the cold in and then accept further forward remains uncertain on how the pattern develops.In the worst case under a UK high we retain quite cold air but  looking a few days further on the eps and gefs we  have another go at reamplifying the Atlantic pattern.

This looks much better with an extension of a European trough underneath so based on this an extension of the cold looks quite possible beyond the initial high.Likely we will get better snow chances from that setup.

The most unlikely scenario is any early return of the westerlies so plenty to be optimistic about going into January for cold seekers.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4989166

[Catacol]

Fascinating model watching ahead. Long term trends are good, becoming a medium range model stand off.

144h is where it begins to get really interesting. Taking the top 4 performing models in order:

ECM

Atlantic approach angled steeply SE, wave break forced up ahead, low pressure previously sat over Scotland has dropped over Europe as pressure rises over Iceland.

UKMO

Less amplified over the Atlantic, different orientation of the Euro low and softer profile over Scandy.

GEM

Similar Atlantic shape to the ECM but again a different take on the Euro low and developments over Scandy.

GFS 00z (taking from same data point)

Similar wave break profile to ECM and GEM but another different take on the Euro low and profile over Scandy.

 

In short - we have cross model agreement on the wave break (no surprise) but quite different outcomes to what occurs downstream of it and, indeed, the angle and composition of the momentum upstream causing it. Consequently anything beyond 144h is in Disneyland and we won’t know for a while yet just how much northerly traction the block will get nor the extent and angle of any undercut.

We don’t get many of these knife edge, high potential scenarios in our part of the winter world so here’s hoping folk can enjoy the ride rather than play the doomsayer “told you so” card that often gets trotted out if a run shows less snow/cold than the previous one. There is a fine line between realism and pessimism, so let’s get it said now: yes…cold options in the U.K. are rare and often downgrade rather than upgrade. There is no wisdom or kudos for pointing this out ad infinitum. But when there is a chance - and there is a chance over the medium term - then let’s keep it optimistic. If it stays dry or westerlies win out then snowhunters can commiserate together (hopefully without the doomsayers playing that predictable card once again) but fingers crossed for some luck, some favourable alignment and some snow by the middle of the month. Remember Scotland has seen some good falls already, and the South got lucky in parts in early December. This has already been a better winter than many so fingers crossed it can continue. Broad scale drivers are favourable.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989158

[MATTWOLVES 3]

I needed to jump out of semi retirement for this post..hope the mods don't mind.

When I joined this forum nearly 5 years ago I was sceptical of back ground drivers as I didn't really understand them..so I just played ignorant to it all....but I never disrespected any of the posters who shared this knowledge! As the years have rolled by the likes of Tamara..catacol...met4cast...chio..matt H.. eagle eye...(I could go on with this list as there's more of you) have completely changed my attitude to such a wonderful science,and the amount of work that goes into this subject with there posts is unreal! Do keep in mind many of the above are not professional! Yes you would think they were...they are that good at it,they simply have a gr8 passion for the subject and spend a hell of a lot of time studying,and then they pass this information on to us giving us the chance to learn something new!

I for one know for a fact some of them in the past have come close to stopping posting due to non stop criticism...and I've actually talked one of them out of it privately on more than one occasion!

Where do you think the Met get there long range forecasts from! A clairvoyant!! The tea leafs! It's all based on drivers around the globe and much of it is inputted into those monthly text updates.. yet when they're talking up cold and snow,everyone loves em! When they get it wrong they're useless! Get my point here!

Let's all dismiss those drivers and base our forecasts on 4 gfs runs a day! One run shows Greenland blocking...the next a bartlett high! Exactly!!! Its not quite that simple.

So I say to all of those that spend so much time bringing us these amazing insights with the teleconnections to please keep at it...most of us on this Forum want to keep learning from it.

Keep the peace folks...no point in dismissing something you either don't understand,or are simply not willing to try and understand.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989205

[Eagle Eye]

All images are screenshots because that's the only way I could get them to work. I think I downloaded them in the wrong format and I can't be bothered to find all the original images. 

Alright, well the AWB appears o have changed to initially an equator-ward evolution on the latest GFS before a secondary poleward AWB evolution occurs. Anyway,  it's sort of an outlier and still results in the same kind of result. However, I've talked about this for a while, lets go to what I feel I know best. The Stratospheric evolution.

Let's talk about general synoptics for SSW's first as there's still a chance long term with about 20% of EC46 models supporting it.

First off, there's a link between extreme cold in Scandinavia and northeast Europe being pre-SSW, especially pre-Winter and then the few days/weeks before an SSW. Generally you want a stronger anomalously cold daily maximum than daily minimum and SE European warmer than average. Also drier along the NE'rn Europe coasts. Generally that seems to be the setup although the daily temperature cycle 

https://repository.library.noaa.gov/view/noaa/25923/noaa_25923_DS1.pdf

I'll initially be using the December 30th 12Z's, will revisit with recent updates. 

 

Generally on the forecasts however, the Scandi cold setup isn't favouring colder anomalies either day or night. So, using the climate extremes here, you need to be wary, it's definitely a cold extreme and a SSW would continue the cold extreme. However, it doesn't look like it's the proper link between climate. Of course, it doesn't ned to be an exact link with the climate extreme and SSW onset, however, I'm still quite cautious of using this.

Explanation of some stuff from my own explanations before the next bit. 

"AAM (atmospheric angular momentum)

 

AMB - Angular Momentum Budget

 

AAM - Atmospheric Angular Momentum

 

MT(mountain torque)

 

GWT(gravity wave torque)

 

FT(frictional torque)

 

If AMB increases in the atmosphere it must have been transferred from another source since AMB is always conserved. The sources for AMB are from the Earth, the Oceans and the Atmosphere.

 

One major aspect in affecting AAM are torques. A torque is the rotational force in the atmosphere.

 

The 3 main torques affecting the atmosphere are:

 

Mountain torque (MT) - The turning force applied with the effect of pressure systems on mountains.

 

Frictional torque (FT) - Boundary layer dynamics.

 

Gravity wave torque (GWT) - Subgrid (local) torque, upward movement and downward movement to do with the buoyancy.

 

If there is a net westerly surface wind, the atmosphere speeds up the Earth's rotation and transfers angular momentum to the earth and so there is a net decrease in AAM. The reverse happens with a net easterly wind.

 

In the NH Winter:

Global MT anomalies are constructed by sypnotic waves that scatter energy across Asia and North America. These sypnotic waves trap the SLP anomaly and push them towards the south. Driven south and east of mountains. Sypnotic wave centres amplify aloft. MT associated with anomaly air parcel transportation in terms of momentum into latitudes its next to. More angular momentum is therefore moves to the 20-20N band. This is where the AAM anomaly appears significant. The anomaly quickly becoming equally uniform leading to a global FT that weakens the AAM anomalies. An anomalous distribution of mass accompanying the MT acting to balance the zonal winds in the 20-30N region. In the mid latitude, Eddy's are the physical link between those two torques."

Unstable inertia - Is the tendency to keep the same atmospheric state with a different disturbance replacing the initial one in the same rough area. So a Greenland high weakening that is fed into and is 'replaced' and strengthens again for example.

 

Eurasian influence

 

Then looking at the Eurasian signal, there's a clear retraction of the wave as the AAM falls especially in the area w/an AWB equatorward and appears to help force the return to a -VE EPO. Until another omega high is forced up through the Ural area. Generally, an initially Tibetan forced wave can rise vertically and the polewards cyclonic wavebreaking occurs. That retraction isn't necessarily bad for the surface pattern as it could help uphold/force a return to the -VE NAM due to pushing up the north Pacific +VE heights and the extension of the -TNH pattern occurs. The north Atlantic jet gains AAM from this energy distribution w/the loss of AAM over Eurasia transferred west and that in turn disconnects the Pacific and Atlantic jet streams further when combined w/the natural jet retraction and split over the next few days to further amplify the AAM pushing into the north Atlantic jet by day 10. With the jets seperated, retracted over the north Pacific and the energy over the southern USA jet backing east w/the transfer of energy over the Pacific, it's complex. However, that could be what the 18Z GFS was realising.

Modifications of momentum

 

The recent +VE EAMT and the slow reaction to the wavebreaking is clearly going to have a large effect along with the MJO phase 3 to drive the pattern. The energy distribution of both the MJO and AAM are obviously going to modify the pre-set pattern but unironically, the unstable inertia rules here due to the distribution of AAM appearing to continue to force the -VE NAM imprint on the Trop. Generally, a supportive sign for Stratospheric imprinting as well. As we go into mid-Winter, the built up momentum through the north Atlantic holding up the -VE NAM pattern is opposed by a loss in momentum budget over the Pacific where we see flip of the north Pacific mode also flipping the north American pattern. This recent evolution appears to be occurring backwards to usual with the AO leading the north American pattern (linearly, it's more of the AO being flipped before the north American pattern due to the pre-supportive AO pattern flip that managed to work through the base US pattern due to the north-south split and the US pattern flips to an east-west distribution that works to being a positive feedback loop for the Greenland-Iceland pattern.

 

So as the AAM weakens in response to the frictional torque loss after the +VE EAMT  event, the momentum that's been forced poleward and is cycling for the next couple weeks, holds the -VE AO pattern in place and forces poleward waves. However, there is therefore less built-up wave distribution as you go up in the atmosphere. Leading to an upper-Trop to lower-Strat recovery for the time being, sort of disconnecting with the upper vortex? So the recovery of the Strat despite the flipped pattern is possible. The recovery will either budge up to previous levels or overcompensate and go further towards the average. That should mean that, they become increasingly -VE NAM and w/being downwards forcing is easier than upwards forcing, the -VE AO could well lose it's inertial grip on the surface-mid Trop.

However, w/rising AAM possible and an already set up Omega block rising over Eurasia right at the end of the 12Z (30th December run) then the recovering tropical-extratropical forcing could eventually re-force it's way towards the north Atlantic. Taking this run at  face value, the re-cycling of the Eurasian wave could lead poleward forcing over the north Pacific and pushes cross-polar air towards the western and central US and the re-retraction of the US zonal jet pushed rather far south. That could slowly recover momentum into the north Atlantic  and depending on how much built up momentum was left (e.g. less if the upper Trop pushes down earlier and/or if the AAM recovers slower) and so the  stress on upper Trop and lower Strat for upper Strat re-connection will increase and the Rossby wave momentum should transfer back up and that's where our next SSW potential comes from. However, taking a mean at face value is still rather risky, it's the best we can do at the moment and given the AAM cycle makes fairly good sense in terms of the connections between Trop-Strat especially. The problem w/AAM amplitude being lowest in the important trading area of Rossby waves and an area especially where the swap in regions of the EHF occurs, it needs a good tropical drive to force that exchange in energy. I suspect we'd have seen a lot more SSW's if it didn't do that.

So a mini-recover in the NAM feels unavoidable due to the Tropospheric connections. However, doesn't rule out the possibility of a transient cold-mild-cold scenario, w/an especially transient mild if the AAM can recover quickly. Wouldn't be surprised if we don't even get a proper AO recovery, given the quick AAM rise. The shorter the frictional response the better and the wavebreaking is the starter of the eventual re-rise in AAM that feels likely to me, just that, it would be preferable for the initial wave base of the north Pacific to be similar to that at the end of the run.

The GWO continues to favour what I was saying before about the loss of frictional torque into the earth, however the release of +VE EAMT looks to be overcompensating and we could well see a large response. I suspect the response will be quicker with this, generally, the CFS has felt a bit behind with these forecasts to me. So we should be circling towards a phase 5-6 where we gain momentum and  there's southwards momentum transport mid-latitudes, forcing a more southern and weaker jet, building up a poleward jet and the general model is a flip of the pattern.

 

However, it's not as simple as there, there's generally as may southwards moving warm episodes due to the initial wave setup and the energy distribution. Along w/that, that's a top end +VE EAMT event w/large kinetic energy. These are generally weak events compared to what's theoretically possible. So the energy burst cannot really change the entire zonal pattern and is more renowned for re-distribution of momentum. Generally, the pattern is for redistribution of westerlies southwards into the equatorial jet with the flip of the zonal anomalies.

 

The transitional between tropical and extratropical momentum tends to be having the sinking jet exchange rebounded by a symmetrical response of a weakening jet which leads to a complete flip in the previous frictional torque momentum exchange. However that is not a flip of the pattern, as there has been time passes and the wave setup has differed, not only that but a complete flip would require a lot of time to occur and AAM is not the only input into the atmosphere that there is. So you always have to remember to think about how the initial setup will move based off the changes the rise in momentum makes into the atmosphere.

Generally, the northern response to the event is stronger and more variant over Eurasia which leads down the line the north American variance in patterns. That Pacific area in general is extremely important because there are a few different scenarios w/jets and waves that lead to vastly different patterns to a change in AAM. The main area of variance for future forecasting is the Eurasian area as it's the first area affected.

The Japanese signal is important to the evolution of the Pacific ocean as it's the transitional area between land and ocean and is also important for determining the distribution of energy between the Pacific and north America. Given a similar strength AAM to what we're likely to see, I'd rather an extended north Pacific jet and a retracted north American jet ripe for AWB just off the eastern seaboard which pushes the -VE NAO and Greenland high again. That's what's currently being modelled on the mean but it's low amplitude as expected and still a fairly weak winning signal.

 

When combining a potential rising MJO w/the AAM signal then it becomes a bit of a problem. As the MJO interactions affect both the tropical forcing and extratropical interactions, they can constructively or destructively interact sometimes w/the building of waves and that becomes important for the waves amplitude. That is likely to have less of an affect on the north Atlantic signal. However, as the Pacific leads most of the rest of the wave interactions, it's still very important the initial wave interactions. Generally, this is an amplitude issue and so using regimes for forecasting and general responses, isn't necessarily that bad nor is the teleconnection forecasting if you can imagine interacting signals.

 

Along w/that, the use of pacific jet modes to forecast with extreme composites can also be a help. So, navigating this site, can be of use.

North Pacific Jet Phase Diagram

WWW.ATMOS.ALBANY.EDU

 

Back to the Strat.

 

The driving long-term does look good. However, the disconnect from the upper Trop to the mid Trop stopping the influence on the Strat may undo quite a lot of work. W/ the loss of heat flux into the Strat, there's a good chance that the dynamical recovery can happen quickly due to it not being a major SSW and so we see a recovery to pre-minor SSW. That recovery is unfortunate but likely given the strength of the polar-night jet over Winter, that's why it's so hard to get a SSW. 

 

The vortex finds it easy to return unless it's constantly connected up to when it's Winter.So, that mini-disconnection essentially loses our progress like it's a game but because of the AAM recovery we saved a few steps before we got there.

 

However, towards the end of the run, the AAM rises again and potentially that's enough to push into the Strat again. It could bring us wvebreaking into Greenland, a well known Strat driving wave that can do a lot of damage. It's something to keep an eye on but until the Ural Omega block can return (our best chance at another strong wave) then I don't think it's worth much. The problem is that I think the unstable inertia hides this well enough that people will be wondering why the upcoming wave (not the one past day 12 on the mean but the one between about day 7-10) isn't going to be able to connect up to the Strat.

 

However, thinking right at the end of the run, a weak Wave-3 rising has been supported and if a stronger Eurasian and Alaskan wave can materialise then yes it's possible. However, that's all it is for now.

Beyond modelling, we should push into an MJO phase 4 for early February. The general composite does suggest for a continuation of the -VE AO mode. This combining w/the AAM is another good sign away from the Stratospheric signal. However, you have to take these composites a month away w/a pinch of salt. It's just a general signal for a continuation of the Greenland high that mostly seems to just be an extension of the +VE EAMT signal so perhaps that phase doesn't have a particularly coherent signal though the specific week composite is a lot more zonal, I suspect the lowpass analogs will have more of the other signals taken in to account so as long as the MJO isn't the biggest signal we should be fine.

Reanalysing based off the 00Z EPS

 

There has been some cutback on the initial and secondary setup. The initial wave setup has become more UK based but it's still rather close on the mean. Could well become a big snow event w/a few changes and it only takes a few miles (relatively) of difference which won't be resolves till maybe a day or 2 before and almost certainly won't be resolved on the mean yet and a fair amount of the Isles could well get snow. Pretty much the same w/the next potential showing up. Like I've said before, we are surely unlikely to get through such a long time w/good driving factors w/out having a decent cold and hopefully snowy spell. The upcoming period is almost certainly going to be cold, however, the snow from the direction of flow and amount of moisture is clearly a potential issue.

There's not actually a big change in the overall idea of the pattern however, it's only localised changes.I wouldn't be surprised if we do see it delayed again but just because the main snow potential has technically been delayed, doesn't mean it actually has when you think about. The wave has just moved, the delay is not a result of a slow down but it's a different wave altogether that could cause the return to the potentially snowy pattern on the models. We're still in w/a chance of snow w/shortwaves but I'm not overly confident on this. Still, a repeating pattern over the next few weeks, wouldn't be that surprised to see snow turn up. Surely, we can't go that many chances w/out snow. Well, now that I've said that ha ha.

I'll stop there for now, not going to wait for the 12Z EPS's to add here. 

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989400

[phil nw.]

Over the years it is uncanny how many times the split flow of the jet upstream causes disagreements within the operational runs.

Further it is uncanny how on many of these occasions the GFS is proved to have thrown too much energy into the northern arm of the jet.

Just saying...

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989478

[bluearmy]

10 minutes ago, northwestsnow said:

Looks like EC taking the MJO into COD early January ..

Not ideal..

We probably need to make hay ASAP.. 

better it stalls in 3 and goes into cod than heading through phases 4 and 5. 
it can then re emerge into 7

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989591

[Kasim Awan]

6 minutes ago, Scott Ingham said:

That’s how I see that frame as well. This is the period of interest for me. Wednesday next week will feel a whole lot different on here

Personally I'm absolutely sick and tired of waiting, delays and pushbacks. A lot of us have been relentlessly watching since the start of Winter for nothing but day 10 charts, pushbacks and downgrades. It is wearing thin and I think what I say about the Hadley cell / Iberian high needs to be taken more seriously going forward.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989774

[phil nw.]

Still some details to iron out within the operational runs but  the ensembles are starting to clear the uncertainties for the end of this week.The combined 850 ensemble graph for Warwickshire showing the average forecasts from the GEFS/ECM/MOgreps .Good agreement that cold air  is in by day 7 .

So the  suggestion is that the high will build just far enough north west to usher in the first cold wave from the east over next weekend.

Eps T168

I take that as the first hurdle overcome,get the cold in and then accept further forward remains uncertain on how the pattern develops.In the worst case under a UK high we retain quite cold air but  looking a few days further on the eps and gefs we  have another go at reamplifying the Atlantic pattern.

This looks much better with an extension of a European trough underneath so based on this an extension of the cold looks quite possible beyond the initial high.Likely we will get better snow chances from that setup.

The most unlikely scenario is any early return of the westerlies so plenty to be optimistic about going into January for cold seekers.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989166

[Catacol]

2 hours ago, The Enforcer said:

I am sure GSDM is a useful forecasting tool, but there hasn't been a cold period yet.

I’d describe it as a diagnostic tool probably more than a forecasting tool. The fact the MetO don’t use it illustrates its one weakness, namely that the GSDM approach is itself the product of other forecasts. So…if pressure patterns don’t develop to allow a mountain torque spike, or if the pacific wave is derailed or lowered in amplitude then GSDM forecasts, like any other, can bust. The ENSO profile can suggest the likely direction of the GWO orbit, but - as we have seen in recent years - the atmosphere can fall into a pattern that disconnects from this forcing and perhaps most significantly the GSDM approach produces a shape of wind flows to the hemisphere and this doesn’t factor in locally relevant features that can overtake it on a local level.

However, what it does do is provide the context within which NWP forecasting sits. So….we can be pretty confident in a period of low momentum and coupled Nina season that amplification will be significantly less likely to occur than a higher momentum profile coupled to a Nino ENSO. We know that momentum spikes produce meridional responses and the phases of the MJO help give a clue as to where the peak/trough pattern will setup. So GSDM diagnostics help provide an understanding of higher or lower probabilities of events occurring.

Finally - to show its value the GSDM approach correctly “forecast” the cold at the end of Nov/early Dec. It also correctly forecast the flatter period through the middle of the month. It didn’t quite get the latter third of December because the MJO didn’t play ball…so 2 from 3 there. And now we have a period of blocking on the horizon that GSDM analysis diagnosed as likely a fair while ago. 3 from 4? Useful.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989213

[Catacol]

Happy New Year everyone. Top 2 models going for the higher latitude option this morning - a good way to start the year. Not much more to be said right now. The scene is set, we know which drivers are in play for this upcoming phase and we wait for the models to move towards consensus. 

Longer term relative AAM has started to reverse but the GWO orbit is high and overall GLAAM is very high. We therefore have a cushion to work with from around mid month, but chances of longer term cold I think do hinge on the extent to which the block can get onto the northern side of the jet. The chance is there, the MetO text alludes to it - and EC and UKMO this morning look onboard with that idea. Going to be a close run thing and we dont have a split SSW to fall back on.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990236

[Mike Poole]

ECM clusters this morning, T192-T168:

Starting with a high centred just NW of the UK, there is a gradual drift west on all 3 clusters, and - importantly - also a drift north on clusters 1 and 3, although not discernible on cluster 2.  It is good to see this into the 10 day timeframe, yesterday it was an option in the extended.

T264+:

I think having just 2 clusters is an improvement (over 5 yesterday) as it shows a reduction in uncertainty.  

Cluster 2 is strong on moving the high north to leave an Atlantic ridge through Greenland and into the pole - great signal, detail to be decided.

Cluster 1 has potential because the shift north is also clearly there, but by day 15 things rather diffuse out and it leaves a bit of a mess.  

Onwards and upwards…

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990265

[CoxR]

A point on ensembles:

Ensembles are a powerful tool most of the time for gauging the spread of uncertainty and the possible solutions going forward. However, in reference to the GEFS, there is a reason that the control and the operational (low resolution and high resolution) run on the same initial conditions are included in the ensembles, and that is to determine whether resolution of the model plays a key role in the outcome. In the past 4 or 5 days or so we've seen consistent disparity between the operational (high resolution) and the control (low resolution) runs and to me that suggests one thing - that the resolution of the model run has a key role. 

When both the control and the operational run are broadly in sync then resolution doesn't play as big a part and therefore you can rely more on the mean or the spread within the ensemble suite, and the operational run (particularly at longer lead times) does not hold as much weight. However, when the two (operational and control) are out of sync, then this starts to suggest that the resolution of the run is playing a key role and for me you can then place less trust in the ensemble suite, as all perturbations within it are run at a lower resolution. When the operational run is consistently diverging from the control run, for me, you should place far more emphasis on the operational run (depending on the model and the model ensemble suite) than you otherwise would do with respect to its ensemble suite, when the control and operational are both in sync with each other. 

Therefore, I think and have thought for a long time, that it isn't wise to post a control run when the operational isn't showing what you want and isn't worth highlighting the ensemble mean and suite as much when the control and operational runs are not in sync. I think it would be generally helpful for this forum if people were mindful of this going forward

 

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990200

[bluearmy]

Incidentally, not many ens members across the three suites with anything resembling an organised tpv end week 2 

just got to hope that the jigsaw pieces fall for us 

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990368

[Cambrian]

A week ago, this is what the 0z ECM ensemble mean modelled for the 4th of January at day 10…

 

…compared with what it’s modelling for the same date today at day 3. The comparable GFS means we’re similar.

 

We can forgive the models for not modelling the wedges, although the ECM did a great job of indicating the polar high. 

The greatest difference lies with the PV, in that the Barents / Kara lobe is missing altogether and also that the heights to the south of Greenland were underestimated. 

So where would we put the jet stream on today’s chart? Somewhere in the belt of greens heading into France? Mais oui, bonne année. This compares with the modelling of it a week ago further north over the UK and Ireland. 

The models have been underestimating the Siberian lobe of the PV and the high Atlantic heights, slightly overestimating the strength of the Canadian lobe, all resulting in the jet stream being modelled for us, quite significantly further north than later transpires. 

Interestingly, looking ahead into week 2 from here, there are some signs that the same trend isn’t on the cards. The 0z ECM ensemble mean for week 2 (day 8-15) hands the dominance to the Siberian lobe, introducing a surface northerly flow into the UK and Ireland as heights lower from the northeast over Scandinavia. This is a very positive sign for cold in northwest Europe. 

The last few runs of the GEM have also been very supportive of this trend in week 2, nice because we also have the anomalies to show the westward then northward migration of the heights from the UK and Ireland, and the low pressure working southwestwards through Scandinavia in our direction. 

 

And if we have learned anything from the above, the Siberian lobe may be even stronger and the pattern may be even further south than currently modelled. 

I can not recall a New Year with such firm indications for a forthcoming cold spell.

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990389

[Catacol]

25 minutes ago, WINTRY WALES said:

So is the spilt back on ??

It is possible for a vortex split to occur without a reversal or technical SSW. This is where the latest EC run is. We had one of these last year in December which gave us the very cold spell which was dry for many but still featured some severe frosts.

The issue with splits like this is they are very different to the one we got in 2018 where the split was representative of complete vortex disintegration. If we get one in 7-10 days it will be with the vortex still intact but certainly weakened. That means no nirvana synoptics as we got back then, but still very possible for things to align with the block dragging cold in from Scandy…but we will need a dose of good fortune. And we could do with a swift MJO passage through mid Jan in order to sustain any such cold. The MetO are keeping their options open on this one - clearly their super computer is suggesting that the block might possibly entrench and then be reinforced though they see that option as low probability. For amateurs like us, keep checking the strat forecasts, keep a beady eye on the MJO and we will cross fingers that the dip in GLAAM isn’t too long lived. We have falling momentum now (well - 29/12 anyway)

and the first sign we will get of a move back towards reinvigorated support will be when the gradient on this frictional torque graph turns positive.

Bear in mind all of this discussion is impacted by lag. Add 10-14 days approx for impacts near us, though I have never quite been able to nail down the lag period with any precision. 

 

Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990455