[Winter 2023/24 - A Post Mortem]

I concur with most of the above, the big exception being the stratosphere's role in what took place for the final third of Jan. The 'major SSW' was not a true event, owing to there actually being substantial reflection of the vertical wave activity flux back down toward the troposphere. A large proportion of it did so, while the rest was absorbed.

When that happens, the energy transfer works in favour of the polar vortex - hence zonal regimes for the North Atlantic - and against high latitude blocking. It's rare that this occurs with the 60N 10 hPa zonal wind reversing but on this occasion that was made possible by the minor SSW that took place in early Jan. It seems just enough absorption occurred to nudge the already displaced polar vortex just far enough off the pole to cause that reversal of the mean, before the effect of the reflection kicked in. 

Since then, the middle third of Feb has been all about troposphere-led forcing and, it seems, the discovery that when you have record warm tropical and eastern subtropical North Atlantic sea temps enhancing the Hadley cell, Atlantic trough behaviour is strongly skewed away from tilting negatively and disrupting - the very process required for sustaining blocking high pressure at high latitudes. Never before have I seen so many opportunities for that fail to come to pass within a single season. Even when it has happened it's been at a higher latitude than usual.

There is a very serious implication to this - more than just the UK experiencing relatively little cold, snowy weather: The pattern of anomalously dry, warm weather across Iberia is able to self-perpetuate to a high degree. It may well be that a strong cooling of the aforementioned areas of the N. Atlantic is required for a sustained escape from this pattern. Trouble is, when exiting an El Niño event as we're expected to this spring, there's usually a warming effect on those areas. This is giving me a horrible feeling that the hot season could be absolutely brutal down there this year.

[Model Output Discussion - Spring Has Sprung]

Looking past the finer detail for a moment, the N Hem profile around D10 on the latest two GFS runs is something to behold.

That it’s not readily driving very cold air across our lands (well, not in the model version of events, at least), feels truly peculiar in a winter that’s been characterised by notably cold weather over Scandinavia. Something the modelling has repeatedly proven overly keen to bring to an end, which does make me wonder.

[Model Output Discussion - Spring Has Sprung]

12z det runs of GFS, GEM, & UKM are all interesting to some extent or other, albeit not with exciting surface weather directly on offer. Key aspect is the Atlantic trough disrupting sufficiently for blocking highs to hang about N or NE of the UK with surface cooling over time & the chance of getting an import of cold air ‘proper’ from somewhere to the NE.

Then we have the third ECM run in a row elongating the trough to the NE for reasons we can only imagine. One of a great many scenarios in the ensemble - ECM seems to have a thing for picking a particular option several runs in a row regardless. Sometimes it proves right to have done so, sometimes not.

[Model Output Discussion - Spring Has Sprung]

2nd day running that the 12z UKM is the most interesting deterministic model run of the day so far.

As well as holding back the Atlantic trough more effectively than GFS (not surprising), it disrupts the one over Scandinavia against the high to its east, which leads to a split in that high, one portion setting up over the Barents-Kara seas. This was last seen in a det. run to some extent in yesterday's 12z ECM.

While not directly influencing our weather, such a high can drive cold air from Siberia toward Scandinavia, building up a cold airmass, which would be an interesting thing to see given the propensity appearing in the models for some manner of ridge development near Iceland around 18th or 19th Feb. Even the GFS 12z has got there despite its earlier aggressiveness with the main Atlantic trough. Imagine if it had followed UKM and Scandinavia was considerably colder as of 21st Feb.

[Model discussion highlights]

A QTR is quite probably a misinterpretation of cases where a major SSW occurs at the same time that a -NAM (stratospheric -AO) exists in the lower to mid stratosphere. The -NAM higher up can then, in a sense, connect within less than a day & reinforce the -NAM, hence -AO beneath.

So it’s really a ‘quick tropospheric reinforcement’, dependant on the troposphere being forced the right way (-AO) by e.g. an MJO event in the run up to the major SSW. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034716

[Model Output Discussion - Spring Has Sprung]

 bluearmy Good points. Possibly a reversal never truly propagates down faster than a week of time, with apparently faster proceedings being when troposphere led events have turned the lower stratospheric NAM negative within a few days of the SSW.

Back to the model runs and the deterministic runs on each side of the argument regarding the low by the Azores next week have slightly moved toward each other. Result looks a bit half hearted on both sides but there are more emphatic scenarios to be found in the ensembles.

Really a watching brief for now. What we can say for now is the abrupt change picked up on by UKM & ECM’s 12z runs yesterday does have credibility to it. 

[Model Output Discussion - Spring Has Sprung]

A QTR is quite probably a misinterpretation of cases where a major SSW occurs at the same time that a -NAM (stratospheric -AO) exists in the lower to mid stratosphere. The -NAM higher up can then, in a sense, connect within less than a day & reinforce the -NAM, hence -AO beneath.

So it’s really a ‘quick tropospheric reinforcement’, dependant on the troposphere being forced the right way (-AO) by e.g. an MJO event in the run up to the major SSW. 

[Model Output Discussion - Spring Has Sprung]

As of +120 the ECM 12z has the low by the Azores considerably further east akin to the 12z UKM, with a bit of a high to its NW.

Intrigue increases further.

[Model Output Discussion - Spring Has Sprung]

11 minutes ago, blizzard81 said:

Interesting developments. Always seems to be the 12z runs. Ukmo and gfs control as examples. They want to raise pressure to our north west at short notice

Good spot there with the GFS control. Perhaps a reminder that models, even entire ensembles in terms of the consensus they suggest, are sometimes consistently wrong for days on end.

This has raised interest in what the ECM 12z looks like later. Will it further stir the pot, or leave it to simmer?

[Model Output Discussion - Spring Has Sprung]

That UKM 12z is among the most fascinating runs I've seen simply because it differs so much from the prior consensus from just 5 days lead time. 

The build of high pressure across Scandinavia is just... not there. The polar jet remains on a much flatter trajectory into central Europe and we sort of get around the issue with the lack of trough disruption seen otherwise.

I'm honestly not sure how plausible that pathway is or even how things would then unfold over the following few days. As of +168 hours it seems to be trying to build a high around Iceland but in a seriously messy manner.

[Model Output Discussion - Spring Has Sprung]

For what it’s worth, a 2018 style result is feasible but only with a quick tropospheric response to a split type SSW with a high retrograding from NW Asia to Greenland such that very cold air is drawn swiftly around from the high Arctic.

Not asking for much is it?

We are technically in the game for such a thing, but just barely. More likely we’ll end up looking for surface cold development which is something the forecast models tend to underestimate at more than a few days lead time (insufficient resolving of the boundary layer to free air energy exchange).

[Model Output Discussion - Spring Has Sprung]

About time a GFS run managed to keep the Atlantic at bay later next week. As you can see from the rest of the run, it has a big knock on effect as for a week or so afterward, there is nothing in the pipeline to give the Atlantic another push eastward.

All it took was a slight revision of the trough orientation toward negatively tilted during the middle stages of next week. I was surprised how many of the overnight EPS suite had that negative tilt - let’s see what the midday suite has going on later.

[Model Output Discussion - Spring Has Sprung]

The 12z & 18z GFS runs for next Wed onward had ~16% support from the 12z EPS. So at this stage it seems like a case of GFS’s classic progressive bias at hand - let’s see if that changes much tomorrow.

[Model Output Discussion - Spring Has Sprung]

4 minutes ago, damianslaw said:

 Agree, the jet is southerly, and I'm bamboozled why the models want to move it north. It just seems odd. 

I know what you mean, it doesn't feel right. I've never had a greater sense of 'the models have totally lost it' clashing with 'but surely they can't get it that wrong at 3-4 days lead time?'.

It's especially peculiar how we initially had much more credible looking outcomes from UKM & ECM. Evidently, something very unusual has been picked up on, however well or not its impacts are being resolved.

[Model Output Discussion - Spring Has Sprung]

33 minutes ago, Mike Poole said:

T192-T240 charts to back up your comment:

My main takeaway from these is that the deterministic run probably overdid the firing up of the subtropical jet stream days 8-10. A big deal considering it still managed to disrupt the trough.

Things do seem unnecessarily messy this weekend into next week. How much of that is down to genuine issues rather than modelling mishaps remains to be seen.

If we look past GFS frequently doing its usual thing when faced with an Atlantic trough versus Scandi/Russia blocking high setup, the broad scale signal for a large blocking feature retrogressing toward Greenland during 3rd week Feb has been pretty steady for several days now. I view that as the wood that we're trying to see through the trees, so to speak.

It bugs me that we look to be headed for the first true major SSW of the season with some potential for a split polar vortex to result. Bearing in mind the substantial snow events that happened in the Marches of 2018 and 2013 in the wake of a favourable MJO progression and major SSW, I fear spring may have some serious trouble getting off the mark. Of those two years, only 2018 managed to put down lying snow here... though it remains to this day the deepest level snow I've witnessed in my local area (18 cm on 1st March).

On the flipside, this SSW could still be early enough that the polar vortex can recover in its wake and drive a +AO by around mid-April. The blocking signal then refocuses more toward the mid-latitudes which can produce some very fine runs of weather indeed. Again, 2018 comes to mind. Ah, what a year.

[Model discussion highlights]

Some extraordinary scenarios for what goes on this weekend. The GEM 12z has helped me make more sense of what's causing so many difficulties:

Between Sat & Mon, the low that's just off NW Scotland in the left-hand chart travels all the way to Iceland on this particular run. Such a large movement in that direction is rarely seen. It appears to be the result of the mean Atlantic trough position retrogressing exactly when the low over the UK is being disrupted. If it wasn't for that, a lot more of the low would head southeast and we'd arrive an an easterly both quicker and with colder air in play.

Between Mon & Wed, there is another westward retrogression to the mean Atlantic trough position. This is a crucial step that GFS has really been struggling to lock onto (very on-off for several runs now). So begins what looks likely to be a 'slow burner' type cold spell should we actually see any very cold airmass arrive at our shores sometime.

So, I believe we're experiencing a case where the process that encourages highs to build across northern Europe generally is arriving at just the wrong moment from the perspective of those seeking a quick descent into notably cold, snow-inclined UK weather.

That being said, I'm not yet convinced that the ratio of northwest to southeast movement of parts of the low will be quite as high as the 12z GEM & GFS runs depict. On the other hand, it's hard to see enough of a change now to take us all the way back to the quick easterly pathway.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5031822

[Model Output Discussion - Spring Has Sprung]

3 minutes ago, IDO said:

The problem with a Siberian High is that the cold has do loop around the high and takes forever to get the main cold close to the UK

Indeed. I believe getting a deep cool of cold air to our shores will require the retrogression toward Greenland type scenario (not of the entire high, mind - instead we tend to see part of the high do that, with a low cutting through and instigating transport of deep cold air from near or within Siberia).

[Model Output Discussion - Spring Has Sprung]

Talk about finely balanced! Probably a stalemate outcome on this UKM run, with the low swinging into the one just north of the Azores serving to stall it.

A messy picture overall, as with so many forecast model runs of late. Bottom line is, they are, as usual, struggling to resolve how a slowing of the Atlantic polar jet will play out.

Even the GFS 12z slows the jet considerably for 13th-17th Feb, which makes what it does with the Atlantic lows look pretty daft if you ask me.

[Model Output Discussion - Spring Has Sprung]

Some extraordinary scenarios for what goes on this weekend. The GEM 12z has helped me make more sense of what's causing so many difficulties:

Between Sat & Mon, the low that's just off NW Scotland in the left-hand chart travels all the way to Iceland on this particular run. Such a large movement in that direction is rarely seen. It appears to be the result of the mean Atlantic trough position retrogressing exactly when the low over the UK is being disrupted. If it wasn't for that, a lot more of the low would head southeast and we'd arrive an an easterly both quicker and with colder air in play.

Between Mon & Wed, there is another westward retrogression to the mean Atlantic trough position. This is a crucial step that GFS has really been struggling to lock onto (very on-off for several runs now). So begins what looks likely to be a 'slow burner' type cold spell should we actually see any very cold airmass arrive at our shores sometime.

So, I believe we're experiencing a case where the process that encourages highs to build across northern Europe generally is arriving at just the wrong moment from the perspective of those seeking a quick descent into notably cold, snow-inclined UK weather.

That being said, I'm not yet convinced that the ratio of northwest to southeast movement of parts of the low will be quite as high as the 12z GEM & GFS runs depict. On the other hand, it's hard to see enough of a change now to take us all the way back to the quick easterly pathway.

[Model Output Discussion - Spring Has Sprung]

I think we’re being reminded what happens when the usual westerlies take a vacation - the model accuracy drops substantially.

For what it’s worth, runs that lift the low well to the N or even NW during this weekend look pretty bizarre against historical precedent - usually when there’s a trough over Scandinavia, the low either stays at a similar latitude or heads southeast. Yet we have some model runs suggesting that a small low to the northwest becomes the dominant ‘anchor point’ around which the low over the UK moves. I suppose it must be plausible but seriously unusual nonetheless.

How it pans out has implications for next week too, although typical model bias seems more influential by then. GFS throwing overly broad and progressive lows at a high over Scandinavia, now there’s a familiar story.

[Model Output Discussion - Spring Has Sprung]

No doubt about it, seeing the 12 ECM take a route that none of its 00z ensemble suite or the 00z UKM did at just 4-5 days lead time is astounding. Should the 12z prove nearest the mark, it will have furthered a growing sense I’ve had these past two years that it’s no longer the top model for Europe by any appreciable margin.

Yet even with that large shift it has still cleared the low eastward from the UK by the start of next week, and that remains a critical step that separates it and all other deterministic model runs of late from GFS. 

Reason being, that’s when the next bout of pattern slowing and amplification is expected to arrive, initiating the move toward the HLB regime that’s long been expected to characterise 3rd week Feb.

If we’re under a ridge at that point it’s an easier, quieter route to a cold continental feed than if we’re under a trough GFS-style, let alone one centred so far northwest that it holds the door open for further lows to swing up from the southwest.

[Model Output Discussion - Spring Has Sprung]

What turns Thu-Fri into a more interesting setup for southern UK is a more west-east elongated trough, which limits the wraparound of milder air from the southeast while increasing the cold near surface inflow of air from the E/NE.

The latest two ‘main’ ECM det runs have made substantial strides in that direction & the ensemble is starting to follow suit.

On the other hand UKM hasn’t adjusted as far & GFS even less so. Yet again we find ourselves looking at a snowy frontal zone in 5-6 days time that could well be anywhere from northern England to northern France. A reminder that the models still have considerable limitations in certain setups.

[Model Output Discussion - Spring Has Sprung]

5-6 days lead time is when southward adjustments to the path of a low tend to kick into gear (if they happen at all).

The usual process now begins: wait to see if adjustments hold or continue further across the next few runs.

[Model Output Discussion - Spring Has Sprung]

The low crossing next Thu-Sun is a transitional event, occurring as the pattern shift starts. Still far enough ahead in time for substantial path revisions but probably a long shot for southern UK from a settling s snow perspective.

After that, we have a strong signal for a highly blocked pattern with sluggish polar jet. Meanwhile, the polar vortex looks to be under assault again, limiting its ability to force a +AO.

So the stage will be set for 2nd half Feb to be characterised by a classic Scandinavian to Greenland high sequence. As ever, the finer details will determine how much the UK is affected by resulting cold air masses & snow events.

[Model Output Discussion - Spring Has Sprung]

Timing i.e. pace of change is usually the bigger challenge than direction of travel when it comes to looking 10+ days ahead.

Main reason being the wide variation in lag times between the behaviour of the MJO & the mid to high latitude pattern response.

Cycles of AAM are connected. The slower the MJO moves, the more of the time AAM is likely to be out of sync, inhibiting pattern shifts. Seems on this occasion the result should be a pattern shift sometime in 2nd week Feb.

Then we run into the question of how pronounced that shift is. Recent modelling has hardly been emphatic, hence I’ve kept expectations low lately, not talking up much cold weather wise. It kind of feels like building a shed using several different schematics - the end result may loosely resemble a shed but it will have a propensity for deficiencies!