Model discussion highlights

[Singularity]

Two separate stories unfolding at the moment.

First one to do with how the Atlantic trough shapes up late this week, which is where the ECM 00z made a notable jump over to GFS with consolidation of most of the positive vorticity into one strong system that moves east, as opposed to two competing systems with the western one later developing into a broad low to our west in the style of the UKM 00z.

Second to do with the MJO signal which has been trending stronger in the modelling. GFS has been responding strongly to that, predicting a rapid response leading to a cut-off high attempt in the vicinity of Iceland. It will be interesting to see how the ensembles behave in the coming few days, because the implication of an active MJO crossing phase 6 is that the high should not sink southeast across the UK. That is, providing the MJO does actually have sufficient amplitude - this underpins the chances, not guarantees, of cold weather that Catacol so brilliantly overviewed earlier today.

The first story might yet prove to be one that follows the UKM/GEM 00z pathway but it's very feasible to get a high north of the UK from that position too - the key difference being much higher UK temps until any cold feed from the E or NE establishes (again, if the MJO forcing proves sufficient).

The Scandinavian High chances will hit peak as we approach NYD. The GFS 06z is a bit west-shifted compared to the composite but I don't think it's an outlandish run given the signals at hand.

Should the MJO forcing prove sub-par, we probably would end up with some manner of high pressure across the UK after all, making the current ensemble signal look rather skilful. In fact, it would be a similar sequence of events to what set up an unusually dry January this year, except the zonal flow looks considerably weaker this time around so there's more chance of the high being in a chilly rather than mild position relative to the UK.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4773981

[Eagle Eye]

Whilst wave after wave of ridiculously anomalously warm air across Europe with possible date records to be broken and maybe some areas around 15°C above average, we're I'm with a chance of fairly decent cold. The interaction of the energy scattering downstream of the EAJR and EAJE have meant that the wave train interacts with that area of warm air Africa drawing up some quite mild air and given the exact angle of attack it can be quite moist air given the moisture right off the coast of Africa. This seems a feed from the Atmospheric River streaming across the Atlantic. So away from southern Europe (more dry African air), it could look like what a  typical zonal Winter looks to us for parts of Europe (including us actually). This Atmospheric River having trended slightly further south recently I would say and Paul Blight's tweet from a few days ago noted, I'd say that we're probably in with the best chance of snow and cold especially in the North of the country compared to about 80% of Europe. I would take that compared to recent Winters. Not looking at Europe with envy as much. Mild and wet for Europe and possibly cold and wet for us but how cold it gets determines what that precipitation falls as, its quite exciting in my opinion. Naturally, you would expect the exposed coastal areas in Iberia and the West of our Country to have the best chance of precipitation, so if we can get it to cool down slightly more progressively there, then they might be in the game too but I'm not sure how well an Atmospheric River event works when we bring in dry air more from the North(ish) if it trends too far South.

Europe

Scandinavia - Bouts of cold and warmth, similar snow chance to us (although they usually get a lot so slightly above ours but that's pretty normal).

Iberia - Mild and wet (that's right, they've taken our weather).

UK and Ireland - Looks cold and wet but how cold and timing?

 

 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4773914

[ITSY]

This is essentially the conundrum.

The left of this frame is an emerging signal for an Icelandic block across the latest GEFS suite. This would *ordinarily* be associated with much colder weather.

The right of this frame is a clear signal for strong heights across the European mainland across the same suite. This ordinary would bring warmer/mild southerlies and SWlies.

What we're left with is a standoff involving mean North/North Easterly signals for the northern third of the country, and mean southerly signals for the southern third of the country. In between this - the middle third of the British isles - is the golden zone for boundary/frontal snowfall. 

For the short-term (aka Xmas-NY period), the question is simply which of these two blocks wins out. We need the low pressure off Iberia/Biscay to travel into and trigger lower heights into Europe if we're to bring the cold boundary south. Alternatively we need a trigger low in the North Sea to do the same thing. 

It's a really interesting setup though and not one I've seen played out before! 

EDIT: The control shows this perfectly as a battle of the blocks (see the one to our NW over Iceland and the one to our SE over central Europe. In this particular scenario most of England is mild but it doesn't take much reconfiguring to turn it all white. It's a tightrope...

 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774093

[Eagle Eye]

Well, the strong water vapor transport horizontally sourced from the Atlantic should bring cold, dry air on the back edge of it. Considering this is Winter and the long sourced Atmospheric River(AR) and despite the large moisture stream, how much moisture rich it'll be by the time it reaches here is difficult to tell. According to the models it transitions most of the moisture fairly well though. The cold and relatively dry air packet should connect to the back edge of the AR but notice that a little bit of the AR will extend back into the colder air and something like that in an AR if the air is cold enough could potentially dump a lot of snow somewhere.

Note that the AR seems to come in waves that each act like their own little AR with the first one here, the first AR and WAA packet of air. Hence we see the south getting a little bit warmer than average at initial AR landing time ()although the Midlands and further North can still be cold with the AR predicted to move further South each run. Also, the air is quite cold in the first place so WAA will only mix that out sightly and it doesn't look too strong.

Then we see the WAA move further West as the zonal packet associated(ish) with the AR also moves further West and hence the cold, dry air sources itself towards us and there's a decent elongation of the AR into Scotland. In this situation, we could see a lot of snow dumped in Scotland and maybe the North if it looked like this closer to time.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774118

[Eagle Eye]

I think the connect with the Trop and Strat vortex amplifying both into largely zonal patterns was an over-estimate based off pretty unfounded Ocean-Atmosphere coupling. I'm not saying they can't I just don't think that it will, we should stay largely around average and not dip too deep into the -VE AAM for now and some +VE torque events could cause a rebound pr two setting off unstable inertial patterns propping up unstable Rossby Wave thermal transfer. What this means is that, Rossby Waves should be more likely to propagate vertically and they impart a slight Easterly forcing where they propagate so as long as we keep getting these mini rebounds into +VE AAM (or even better hold an unstable inertia pattern in +VE AAM) then the Trop and Strat shouldn't go into the distance of large +VE zonal patterns. Eventually, I would imagine the unstable inertia stops propping itself up but when and how is anybody guess. For all we know, we could technically keep this going on all Winter however unlikely. The more favoured solution is for a better first half of Winter and that's my thoughts, December through mid January for now remain the beat times for a proper cold pattern but let's see how this all evolves.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774286

[stodge]

Evening all

As an aside and it may be no significance at all but I do like t follow the 10 HPA charts and the zonal winds.

Most years, the "attacks" on the PV have come from the Siberian side and what they tend to do is knock the core of the PV back over to the Canadian/North American side firing up the jet and putting us back into the Atlantic dominated westerly regime.

By the way, I think that's what did for our recent cold spell.

The odd thing this winter so far the bulk of the warmer 10 HPA temperatures are on the North American and European side and, as you might expect, this has the effect of sending the core of the PV back over to Siberia enabling our Greenland HP. 

Tonight's 12Z GFS OP (hideous outlier as it no doubt is) illustrates this and what it also shows is, contrary to the normal pattern, the strongest zonal wind pattern is over the Siberian/Eurasian side - the storms are there while under the lighter zonal winds, the Greenland HP is able to survive and occasionally exert its influence leaving the Atlantic its less than zonal stuff and keeping the real storms well away.

Just an observation, may mean nothing.

 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774281

[Rain All Night]

EC46 alongside the GEFS and CFS to look for cross-model agreement. Not difficult to begin with of course. We can expect a mild week for the south, and a chillier week for the north.

Week two maybe chilly for all if we get the favoured ridge to our west.

Perhaps goodbye Euro heights from week three, just don't tell the CFS? Scandi high options starting to proliferate from here on.

Ridges in other interesting locations also available... EPS control run in a cluster with low heights over Europe...

Now what is this... seems like quite a few signals in common at such a distant timeframe?!

CFS not wholly disagreeing with that high latitude blocking signal still present by week 6, not willing to cooperate on those low heights over Europe though.

Scandi high members the most common through much of the run:

I hope I've done an alright job with this!

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774472

[Eagle Eye]

Seeing as this thread's quiet now (and I have definitely not been writing this for 3 hours, talking to Olivia and all that, distractions you know)...

The Atmospheric River(AR) event that takes place across the North Atlantic over the next few days looks to happen I'm two phases (or may two separate AR's). First of note is the first wave rapidly approaching Iberia associated with that long elongation of water transport that moves up and dumps Iberia with a lot of precipitation. It's the back of these that are of note to me though because they are relatively cool and moist compared to the first part of the AR (although that still has the moisture) and as I like to call it now, the loopback feed if timed right can mix in with the colder air coming down from the North(ish) as well and bring potentially a lot of snow to Northern and Scottish areas. Potentially further south depending on how the loopback eventually evolves. Beyond that wee that second wave forming through the mid Atlantic feed and we can see that on the precipitation totals fairly strong just off the coast of Florida and we may want to see how that evolves as well.

I think it's pretty clear that the Eastern wandering divergent velocity means that from the initial stages and the fact that it seems to be moving quite quickly, the MJO looks to be more progressive and maybe amplified than previously forecasts (like I thought it would, not to brag just a lucky guess)from a few days ago. Suggesting better timing with phase 5/6/7 going into January so we may some renewed downstream blocking around Greenland or Scandinavia depending on how the wave train evolves. Hence the whole EC46keeping the -VE AO pattern in hold. It's a possibility before I think the pattern should collapse by mid January but it depends on how we see the AAM and MJO eventually evolve what happens before that. We may never see the unstable inertia pattern collapse of course but I just find that highly unlikely.

The MJO follows on from the VP and is slightly better than the previous runs although it's still only phase 4 that we see as the most amplified. A good trend emerging to amplify phases 4-6/7 further on the forecasts (especially 5,6 and 7) but I think we still want that little extra kick. It's a good background signal to have and better than phases 2 and 3. Phases 6 and 7 having higher support of a -VE NAO in composites and all that stuff.

It's clear that the zonal wind has recovered and is going to recover further over the area of 180-120W and so the East Asian Jet Extension (EAJE) is fully into place with a possible energy scattering event that may help us or hinder us down the line. This may see a more zonal favoured longer-term or it could also distribute energy and work with the MJO to form another blocking going into January week 1/2 timeframe as my MJO notes also said, certainly interesting long-term and not just a definite zonal forecast. It could also set up a mid-term Californian AR which doesn't have much effects downstream in my experience but I haven't experienced many knowingly to be fair.

The upper troposphere meridional wind also supporting that shorter term Greenland blocking potential (45W) considering the weaker zonal wind but until it actually comes to pass we can't count out the fact that the other models may be correct in that it doesn't form quite so quickly/quite so well so we'll just have to see what happens with this.

Onto the GWO and it's clearly not as amplified into a +VE state than most of November but looking at the forecast there might be a slight rebound associated with a +VE mountain torque over the coming days and if they are more amplified than the GFS forecasts we may see a return to a slight +VE state compared to the slight -VE state we seem to be in right now. At this amplification it doesn't really matter though and instead other background signals look to be the main forecasting tools going forward. Clearly the ocean-atmospheric connect still not properly showing itself and so the MJO is likely to have a fairly large effect so be on the lookout for phases 6 and 7 especially until the AAM becomes amplified either way. Then continue to look for those phases still but with varying degrees of desperation for those phases .

So what do some of these words and phrases actually mean:

I've picked out a few with some of my or others explanations from before

My explanations:

"

Unstable inertia - Inertia is when it keeps it's existing state unless acted upon by another object. Hence unstable inertia is the transfer of energy is between it's existing state and the 'disturbance'.

This is a very basic view at both though and unstable inertia involves a lot more than that.

-VE just means negative (hence, +VE means positive)

Some basics on Atmospheric Angular Momentum (AAM)

A few shortened words as well:

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

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.

"

 

Away from my definitions now

 

Madden-Julian Oscillation (MJO) (Met Office) - 

"The Madden-Julian Oscillation is characterised by an eastward spread of large regions of enhanced and suppressed tropical rainfall, mainly observed over the Indian and Pacific Ocean."

 

MJO phases (Met Office) -

"When the MJO is in its active phase over Indonesia and the West Pacific, it tends to drive a negative North Atlantic Oscillation (NAO) 2 to 3 weeks later.

A positive NAO index tends to be preceded by phase 3 and 4 of the MJO, which brings milder and wetter weather across the UK.

A negative NAO index tends to be preceded by phase 6 and 7, which influences a ‘blocked’ weather pattern and is often associated with colder and drier weather across the UK.

The timescale of the MJO having an influence on North Atlantic weather regimes is usually 10 to 12 days."

 

North Atlantic Oscillation (NAO) (Met Office) - 

"Positive NAO phase -

Winds from the west dominate, bringing with them warm air, while the position of the jet stream enables stronger and more frequent storms to travel across the Atlantic

Negative NAO phase -

Winds from the east and north-east are more frequent, bringing with them cold air, while the adjusted position of the jet stream leads to weaker and less frequent storms."

 

Artic Oscillation(AO) (Climate Prediction Centre) -

"The daily AO index is constructed by projecting the daily (00Z) 1000mb height anomalies poleward of 20°N onto the loading pattern of the AO. "

The loading pattern is just the normal pattern for the time of year to put it basically.

 

Atmospheric River (I say AR) (NOAA) -

"Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow."

 

Global Wind Oscillation (GWO) (a NOAA paper on it by Klaus Weickmann and Edward Berry) - https://psl.noaa.gov/map/clim/wb08_final.pdf

 

Anyway, back to my thoughts for the future...

So pretty much my thoughts for now are that it's a possibility that's on the cards that we could see a fairly substantial Greenland blocking going into later this week and onwards associated with some sort of zonal wave train but beyond that there is so much more difference. This one should bring cold somewhere in Great Britain even if the Greenland blocking doesn't form and if it does it could connect up with an Atmospheric River to dump a lot of snowfall somewhere depending on the timing I think eventually we could see another renewed Greenland blocking as long as we keep the pattern away from a large zonal state (which I think would have to mainly be powered by the AAM dipping quite deeply -VE after connecting with the La Nina base state) sometime in early(ish) January.

So a fair few cold chances long-term and mid term I would think and compared to recent years we take that. There needs be some caution not thrown into the wind as we simply don't really know how energy scattering events will happen in the future if their related torque events haven't happened yet and so wave trains that bring us into a zonal pattern cannot be ruled out. As well as that the -VE NAO isn't a guarantee of cold to be long lasting and it could be west based as a basic example.

The EAJE could also connect up with the PNA(Pacific/North American Pattern, another pattern that doesn't directly affect us but I think it's an alright indicator of possible colder air when it's amplified away from neutral but it depends on the pattern) and sort of hijack our weather as well, I think it's a possibility that over time if it keeps being over extended we could see it's downstream effects 'phase out' the Atlantic -VE zonal repeating pattern. Though it can still interact with the MJO fairly well in my opinion to just about distribute the energy right as long as timing is done well which seems to be getting better run by run with a more progressive and amplified MJO phase 6/7 pattern possibly taking effect.

The thermal Eddy's 'retracting' due to the loss of AAM doesn't seem to have happened as quickly as I had initially thought a week or so ago (possibly longer). This is shown in a propped up -VE AO phase and so supports the fact that a loss of AAM does not necessarily mean that northern blocking has to go if you can hold the Eddy's in place with little events and if the loss of AAM is fairly minimal then the 'retreat' does not really seem to happen. This has sort of helped my mind thinking things through as to my post this fine morning. We could see these Eddy's 'retract' quite quickly but I don't see much sign of that unless a quick Strat-Trop downwards connecting forcing the Trop to go largely zonal but I think the models initially over-did that and now are trying to bring it back down. As seen by the connect only slightly adding to the +VE zonal forcing on the Troposphere compared to what I've seen on previous runs. This sort of "Saving" us potentially, for now though.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774485

[MattH]

Morning.

This is, without question, as I think Mike Poole mentioned a little earlier, one of the most difficult and volatile periods in model data for some considerable time, believe me it is making it extremely difficult for forecasters with regards to 'fine tuning' the details for the Christmas Weekend, but even now that remains near impossible to do, given the ongoing oscillations with the model data for this weekend and into early next week.

This is, once again, highlighted perfectly by the comparison with the GEFS which lower 850mb temperatures from the N through Christmas Eve and into Christmas Day, yet the ECMWF is still holding a more developed low over the British Isles with higher 850mb temperatures - Quite remarkable and tiring to say the least. As I mentioned a few days ago, there is no 'quick fix' here and it is still a case of waiting for NWP to sort this whole mess out, but with clearly much shorter lead times than would otherwise be the case for any sort of detail. Overall, I haven't changed my stance on the colder air getting into Scotland by the 22nd/23rd with reasonable confidence, then filtering southwards through Christmas Eve and into Christmas Day, perhaps with the exception of the far south.

There also remains a risk that somewhere, through central areas, depending on how Friday's low evolves and tracks eastwards through central areas then a significant snowfall event remains possible before the low clears into Christmas Eve, something to keep an eye out for too.

Where we go from there is equally uncertain - The 00Z GFS highlights yet another extreme evolution with significant winter synoptics through late Dec and into the New Year period, all very possible, but, equally, there remains a number of solutions that provide greater emphasis on a more unsettled, zonal pattern becoming established as well, this is less-likely, IMO for the reasons below, but certainly has to be considered.

The long-predicted next passage of the MJO is underway, most recent OLR anoms showing it is now approaching the Maritimes (phases 4-5) and we continue to see some interesting AAM plots making an appearance now as well...

If we use the regression plots as a guide, then compared with phases 3 and 4, phases 5-6 creates quite a significant change to the N Hem profile, as exampled above. Obviously, and as usual, there are other factors to consider and there is not an A+B=C outcome here...but...it increases confidence that the MJO and resultant AAM changes will, IMO, give yet another boost to blocking patterns looking ahead and but the brakes on any sort of arrival of the usual winter-time pattern that is a dominante +ve NAO regime. It is also interesting to note that the overnight EC Monthly AAM plot has shifted quite significantly higher as well than recent runs (see below) while we continue to some very interesting frictional torque plots, while we also now see tendency increasing, and with the GWO heading over towards phase 4, which I mentioned was likely in a post few days back...Plots below highlight all of this.

At the moment there remains little evidence for any significant downwelling or influences from the strat on the trop either for at least 2 weeks, if you ask me. As I mentioned the other day mind it would be extremely rare (near impossible!?) to go a whole winter without some sort of connection at some point. However, as previously discussed this up-coming MJO passage could be arriving at the right time, not to only reinforce tropospheric blocking, but to potentially aid in the development of an SSW as well, but that is even more problematic. Some model data suggests that another significant MJO could occur through late January as well, so I'm still in the 'camp' that an SSW will eventually happen this winter, but, as ever, time will tell on that one.

Overall, the short-term term remains very problematic - The overall suggestion for Christmas would be the further north you are, the greater risk that it'll be a cold and seasonal one, than compared with further south. Where we go from there, between Christmas and into the New Year is equally troublesome as I fully expect further amplification to the pattern and, of which, will continue to bring about some further very interesting, but also conflicting NWP data. As usual in an amplified pattern wherever there is a significant ridge, there is always a trough, etc.

Fascinating watch and output as early winter progresses, but definitely a case of not getting caught up on each model run, more-so than ever is this currently a perfect example of that and we shall see where we go from there.

Best wishes for Christmas.

Matt.

 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774546

[Catacol]

So - here we go. Biggest increase in relative AAM since the start of November. 

Torques increasing too - East Asia on the up.

MJO emerging nicely - note the text from the CPC yesterday

• Although the RMM index indicates a poorly organized MJO, upper-level velocity potential anomaly fields reveal a more coherent, eastward propagating intreaseasonal signal over the Indian Ocean during the past week.

• Dynamical model RMM forecasts are in fair agreement with the MJO signal remerging over the Maritime Continent, and propagating eastward into the Western Pacific through the end of the year.

• While there is much uncertainty on the strength of the MJO due to high ensemble spread and the potential for destructive interference with the low frequency base state, OLR and lowerlevel zonal wind forecasts focused south of the equator evince a more coherent MJO signal in the outlook.

 

Long and short of it = chances of a high lat block emerging in the medium range are there. And we need to hope that it succeeds, because if not then the waning phase that will follow will be flatter than now and turn mid January into something unremarkable. 

Meanwhile the upper strat is intensifying

Still not impacting the Trop but the descending nature is now clearly visible. The pot of liquid gold scenario here continues to be that height rises will be over Scandy with the jet on a southerly arc while low pressure emerges in the North Pacific once again. This would cause the strat vortex grief. Ensembles are showing that low pressure. NWP and ensemble data for the atlantic/UK sector continue to be all over the place while models try to resolve just how big a shift in momentum and MJO forcing is underway. 

Nothing resolved yet. Fine lines.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774601

[Mike Poole]

ECM clusters, well - there are 5 in the earliest timeframe!

I’m not going to dwell on the detail, or the difference - which is in the lows in the atlantic mainly, just to note the upfront uncertainty at short range in all of this.  T120-T168:

Cluster 2 brings the cold further south in the UK, but as cluster 1 is still OK for Scotland it does seem the dividing line will be somewhere across the UK.  The all mild option seems to have gone.  

T192-T240:

Cluster 1 has an atlantic ridge avoiding Greenland to the west, cluster 2 looks to link heights over Greenland to the euro high through the UK, we’ve seen this one before.

T264+

The takeout for me here, is cluster 1 with 19 members, has a block including Greenland, whereas the other two put low heights over Greenland, with a scandi high in cluster 3.

I guess the bottom line, the mean looks uninspiring at T240, but given the variation in the clusters, it is not surprising to see the spread indicate the uncertainty is big upstream of the UK, and there are some good solutions in the mix:

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774607

[Singularity]

3 separate sources of uncertainty are interacting to keep us on our toes.

1st what goes on in just a few days time - how much of a secondary low develops on the western flank of the main Atlantic low. Significant differences evident as soon as +72 hours between GFS on the left and ECM on the right. The latter outcome allows most or all positive vorticity (rising air, cyclonic spin) to clear E or ENE during the weekend, whereas the former leaves more behind to the west or southwest. Even within these two scenarios - but especially the 2nd one - there are a range of outcomes for positioning of lows and resulting flow and temps across the UK.

Then, the way the intense storm over the US behaves as it matures and decays, which others have discussed already this morning. GFS continues to be the only real advocate for it stalling with a dispersing warm sector becoming the seed for a blocking high over Greenland. EPS aren't as disinterested in the 00z set as they have been previously, mind.

 

Finally, whether the MJO drives sufficient amplification to the N. Atlantic pattern for high pressure to establish in the Iceland-Scandinavia area by NYD.

The outcome of the 1st source determines how mild conditions in the UK, especially the south, are during the early stages of next week.

That of the 2nd (the intense US storm aftermath) dictates whether we then see a significant and sustained push of cold air south toward the UK (which either battles with pre-existing southerlies or sweeps in easily, depending on the outcome of the 1st), or a transient mid-Atlantic ridge with low pressure establishing more generally on the Atlantic side of the Arctic.

Then, if the MJO drives sufficient amplification of the pattern, we see build of high pressure in the Iceland-Scandinavia region that either ends up more toward Greenland due to a pre-existing blocking high there (so extending the high-latitude blocking spell), or stays put as a blockade battling with low pressure areas near or over Greenland while others head for SW Europe.

Otherwise, we either see blocking across Greenland collapse or retrograde to N. America, or establishment of a 'traditional' zonal flow regime across the N. Atlantic and N. Europe, bringing changeable, mainly mild conditions.

 

You see, that's a lot of possible combinations for the broad scale setup - let alone the finer details!

Currently, the MJO-driven Iceland-Scandinavia high scenario with no Greenland high has surprisingly little representation in the various ensemble sets.

That tells me that the models are leaning heavily on the outcome of the intense US storm for producing sufficient amplification in the N. Atlantic sector. The MJO signal is currently being diluted too much to noticeably influence what they show around NYD. Unfortunately there's no way to be sure whether that's because the MJO won't be strong enough, or because of over-suppression by the models.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774621

[bluearmy]

1 hour ago, CreweCold said:

As Bluearmy has pointed out, we can't see if there's a coupling with the ECM output because the strat charts aren't available. However, my bet would be that the ECM is seeing coupling and that's why it's throwing out the output that it is.

I take your mind back to 2016 when all the experts were saying there was a strong disconnect when in reality the coupling happened very very suddenly and the output flipped to +AO (just like the UKMO and ECM have now).

Often the simplest explanation is THE explanation. If it walks like a duck, quacks like a duck, looks like a duck, it’s probably a duck.

 

Perhaps we should look at this upcoming period in the same way as we do ahead of a predicted ssw. We often find a very poor continuity of output until the reversal actually  happens or is virtually upon us and the starting data absorbs the fact that’s it’s happened or is inevitable. That’s doesn’t really make sense because the models are predicting the reversal but they never seem to come to terms with what it actually means until it’s here.  Whilst a recoupling of strat and trop is much more normal, it seems the event is possibly causing the shenanigans we currently see.  you would likely place gfs well down the list of models to trust in the strat/trop. Ecm has the best resolution in the strat

gfs is still taking the increased flow back up once it’s arrived below 1000 hpa so some encouragement that any recoupling may not leave any long lasting damage. 

as far as next week is concerned on the eps, the lee northerly looks to be in two clusters - one at the end of the weekend like the op and one a couple, days later as the op has previously been showing. So the ensembles will drag out the cold signal for longer on graphs.  Also, a cold mid lat high to follow the northerly fits with the colder outlook for the week. 

there is still plenty of HLB showing on the extended eps clusters so perhaps the ec suite is indeed seeing a short term coupling over the polar field and associated uptick in AO. 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774599

[bluearmy]

eps mean and gfs op for day 15 on the same page with the upper spv located on the Asian side, having drifted across from its current position n Greenland 

that would say the week 2 output from gfs op which tends to take low heights away from Arctic Canada are reasonable. 

  

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774697

[bluearmy]

3 hours ago, Bricriu said:

If people are convinced the Strat and Trop Vortex are connecting, then what is the EC46 at with its blocking signal persisting through  most of January. Why, in particular, does it have a strong signal for Scandi height rises? Is it the case that the coupling might not happen at all or be very brief. Of course the other option is the EC46 might drop this signal in the next update. 

the ec46 is an ensemble product - hence it can show different mean features but often we won’t see both verify. 

I wouldn’t be getting over excited about this evenings 12z runs - the holiday period could still throw out some surprises and I’m not convinced yet that this flattening, if it does indeed occur, will be sustained. 

the eps extended clusters certainly don’t look like a rampant vortex. Let’s see over the next couple of runs if the higher res ops are leading the way with the upcoming recoupling. 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775129

[Kirkcaldy Weather]

OK I will give it a bash to try calm things down from this "zonal" obsession that pops up most winters faster than Usain bolt getting past the finish line

Taking winter 2013/14 which was one of the most Atlantic dominated we've seen (I'm not a believer of the Z word let's be fair it's definition basically equates to Westerly / Atlantic sourced conditions that are dominant) 

20th December 2013 

20th December 2022

 

Might appear like there are some similarities but overall its actually significant differences 

With 2013 no blocking really to speak of, TPV lobe setting the scene and we continued that theme for months. So far in winter 2022/2023 we have seen plenty of interaction between various teleconnections (+EAMTs and MJO cycles specifically) resulting in a number of blocks developing.

What really gets frustrating is the amount of expectation from some members that weather patterns will basically unfold within a few days, you have to let the weather patterns develop hence why patience is one of the most important qualities in the world of meteorology  

Using 2018 to give a great example 

12th February 

2 weeks on ... 26th February 

Or the pattern we've just seen 

25th November 2022 

Jump two weeks on ... December 9th 2022 

I mentioned previously that our next period of focus links to MJO movement with any cold and blocking before weeks 1-2 of January a bonus 

 

 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775350

[Nick F]

Re: 06z GFS Boxing Day snow event, in reality the air probably won't be cold enough in the end to support snow on northern flank of the low, and western flank as it clears. Often the model is a little too optimistic with how cold the air is with cold air advection at range. But the track of that low moving up from Biscay and the CAA from the NW may change over coming runs - so we'll see ...

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775473

[Eagle Eye]

The track of the -VE zonal wind anomalies has shifted East over the last few runs (at around 45W to 30-15W)instead of becoming a shored up Greenland zonal setup early link, its just going to become another Eurasian signal (possible Euro ridge repeating pattern and could draw up more WAA into the Eurasian area/Scandinavian ridge or more hopefully block). I've noted the zonal packet transitioning over the North American area (this being just above the Tropics) from 120W to 30W that was probably not handled well and admittedly it probably wasn't handled that well before now but I just assumed that the way the GFS was forecasting it was how they evolve normally but I guess not. This may act to fire up the ridge/trough/ride pattern as the Atlantic jet tends to try and extend. I've also noted around 120E as the initial East Asian jet. Right now it's extending in reaction to the remnants of the 2nd wave train moving on which did scatter some fairly good energy but it was slightly too over progressive hence we didn't see a proper Greenland block being the most likely outlook right now. It's extension doesn't look too sustained and may just act to amplify the American pattern come Christmas time but beyond that how it scatters energy will be hard to tell. Like I said, it doesn't look to sustained and towards Christmas time we should see it slowly retract to around normal so outputs should become a little less complicated once we go into the New Year as we rely a little more on local dynamics. Still a lot to be interested about and note that the Greenland (area, the actual zonal is well below Greenlan) +VE zonal wanes towards the end of the run so possibly we may see another chance. The pattern doesn't look like going fully Nino-esque nor does it look to go Nina-esque plus the MJO is progressing fairly well and perhaps with more amplification of the MJO we could once again see a sustained signal for a +VE NAO. Its asking for a lot of little changes but if we get a few things in our fortune, we could be back in the game for long-term significant cold potential.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775633

[Eagle Eye]

50 minutes ago, trickydicky said:

Is Atmospheric River not just another name for trailing fronts or pineapple express or whatever else? And is it not just what happened in November 2009 and December 2015? And rather than burying us in snow do they not tend to just drown the west of the UK?

Not really, it's a tight corridor of concentrated water vapour transport (that can be very long in length but its not that wide usually). The pineapple express is just an example of one really, it depends more on the initial state of at Atmosphere and what air source it mixes in with. In a large zonal system it does create that pineapple express but in something like this, if it times itself with the cold(ish) air then the potential for snowfall will arise with it. Its not really a weather system in itself, its simply a large transport of water over time in a narrow corridor.

Anyway, away from that and...

As @Mike Poole says, it's not a 'roaring' +NAO that seems to be setting up here just yet. It does initially connect up with the Trop and we get a slight +VE zonal state going into the New Year. Yet, it doesn't look to be really strong and I think the AAM becomes fairly significant here. If we can get a significant rebound of AAM then the Trop-Strat connect up probably won't be long enough to build itself strong. Instead, thermal Rossby transfer poleward from fairly unstable inertia in a 'marginal' situation like this should probably be enough to swing us back into a slightly -VE zonal anomaly state.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775779

[Cambrian]

12z GFS ensemble mean - it’s striking how quickly the storm exiting northeast Canada on day 5, Boxing Day, flattens the Atlantic pattern.

 

It meets the Greenland high, gets squeezed and split at day 6 - the energy divided, some goes to the north to rejoin the core PV, and some to the east, to feed the Atlantic flow. 

By day 8, we have a mean westerly so looking mild, eh? But if we continue to give ourselves a break from looking south for any elevated heights over Southern Europe, we can look north to a PV bang over the pole, pointed our way, extending through Svalbard to the Norwegian Sea. The anomalies chart shows our polar trough is more than that, it’s a single cross-polar trough. 

  

If it wasn’t for the westward extension of the trough to the south of Iceland, i.e. largely shortwave miscreancy based on the remnants of the energy propelled into the Atlantic by the American storm, we wouldn’t be worrying about whether we squeeze a white Christmas, we’d be looking at a vortex-driven direct northerly hit straight off the North Pole. The satisfying irony is that the emerging depth and orientation of the polar PV to point our way is also caused in part by the energy that headed north from the same American storm, and this might well in time push more directly through the Icelandic barricade. Even with the pebbles in the stream, the mean T850s are on the cold side, reflecting the polar origins. 

12z ECM op at day 8 / uppers day 9 - the same broad idea. 

  

Nothing much mild here, and some real potential for an Arctic or a distinctly cold polar maritime spell moving forward into the New Year. 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775873

[Cold Winter Night]

The EPS on the ground out to day 15 is the poorest I've seen this season. Especially in De Bilt.

So I was curious if there was at least something of interest in the extended clusters, and well, there is.

41 out of 51 members are in a cluster that amplifies over W-Europe, starting from those Euro heights, developing into a block over the UK or Scandinavia.
A BLO+ (blocking regime) is evident here, with just one 10 member cluster of continued mobility.

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775885

[Daniel*]

The stratospheric polar vortex while strong on ECM 12z seems to be heading for northern Russia from Greenland. That might well open up opportunities for height rises in North Atlantic and weakening westerly flow/jet stream into early 2023.

 

Notice some forecasts are taking NAO negative…

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775875

[Daniel*]

I noticed EC control run while garbage in European sector hemispherically was quite interesting Arctic high and weak ridging to our north and Greenland, intense polar vortex well compacted in Siberia. This is a direction of travel we need to follow. 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775992

[bluearmy]

12 minutes ago, Daniel* said:

I noticed EC control run while garbage in European sector hemispherically was quite interesting Arctic high and weak ridging to our north and Greenland, intense polar vortex well compacted in Siberia. This is a direction of travel we need to follow. 

Now this has taken my eye - I noted a cold Arctic surface high around nw scandi to svaalbard at the end of the ec op. It’s showing with lowIsh heights so may not have been picked up by many. Your post reminded me to check out the control and it has a similar feature at day 9/10 and goes on to produce a very disturbed polar profile as week two progresses. A straw indeed! (And a look through the NH profiles late week 2 reveals the picture not nearly as grim as seemed to be the case). 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4775996

[Eagle Eye]

1 hour ago, snowbob said:

Haven’t got a clue what your talking about 

But A I want to learn

and B it sounds promising

NAM is the Northern Annular Mode or otherwise known as the Arctic Oscillation (AO)

What it's associated phases look like (as a general rule).

VI is vortex intensification

So essentially what Chio says here is that it might just be that the Arctic Oscillation is returning towards the positive phase in the Troposphere and instead of the Stratosphere (which happens to have a positive Arctic Oscillation already) 'coupling' with the Troposphere, it's just the AO in the Troposphere recovering itself. Meaning that the Troposphere going positive AO is not necessarily due to the vortex intensifying in the Stratosphere. 

Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4776043