Model discussion highlights

[Singularity]

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

[Cold Winter Night]

 Lukesluckybunch

Let's see, here's EC46 from two weeks ago:

NAO+ until February 10, then a few transition days to an attributed Scandinavian Blocking regime February 16 onwards.

Now watch EC 12z again, that's exactly what it shows.
EC 240h anomaly fits the Scandinavian Blocking regime:

The problem is thinking that "Blocking = Immediately Very Cold and Snowy IMBY," like 1+1=2, but that's not the way it works.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5032052

[minus10]

 Mike Poole It was interesting watching the met deep dive earlier where Aidan stated that the sudden change in the modelling of the low from taking it southeast to stalling it over the uk/ moving it north was resulted from a small increase in the modelled development of the low way back in the Atlantic which then resulted in it becoming larger and extending its influence of mild air thus displacing the cold air further north...again minute changes upstream resulting in huge changes downstream..thus effecting the surrounding synoptic response too...have to say that the gfs saw this a few days before the ec/ ukmo...is it better at picking up developments in the Atlantic...really shows how complex and sensitive the weather processes are...

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5032175

[Daniel*]

45 minutes ago, Don said:

Down to warmer than average SST's?

Unsure about that the North Atlantic is actually pretty close to 1991-2020 average right now, it was freakishly warm last year. So I wouldn’t expect westerly winds to be much milder. Much above average SSTs are seen in subtropical Atlantic off Iberian peninsula and down to NW Africa, more southerly maritime flows are exceptionally mild.

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5032262

[minus10]

Looking at the gesf anoms 0z you could be forgiven for thinking that this is a good chart with blocking to the north west later this month..

...however all is not what it seems...it seems...i think as others have stated the blocking signal is there but it is not in the right place for the uk..too far north to be effective for cold. The trough underneath allowing milder air up from the south...

..the gefs mean also shows this...

More like an artic high ..while the op shows a meaty vortex again for around this time..only the op i know but talk about insult to injury!!

..in addition gfs op has been been at the higher end of the pack for a number of runs now with the mean also getting higher...

...this looks like a trend unfortunately...

Even the eps are milder...

..the EPS anoms (only going out to t270 at present on Titbits) also showing that north west hight rise but again looks like it will not be effective for delivering cold to the uk if it goes the same way as gefs..

..so although from a macro hemispheric perspective it may be going the way as dictated by the main drivers it seems as ever in the uk if the pieces dont fall right it wont make any difference...still can change of course and I really hope it does as would love to see a bit of proper cold before spring...but somehow i just feel it is slipping away...

...noticeable again this year the Daffs are up and out in some places...ground temps warmer than they should be...of course March colder snaps still likely and will cause problems for the flora...

 

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5032526

[Chesil View]

 minus10

An excellent post and a point well made and one in line with what i feel i have learnt about them over my years on here

in my opinion The back ground drivers which I do believe control the general set up can at best only provide broadbrush picture of where ridges and troughs may set up. 

Its easy to get excited when we hear that a certain mjo phase favours Grenland high or Scandi high but worth remembering you can have a high in either of these locations that doesn't advect cold air across our tiny area of the globe. 

In fact the vast majority of scand blocking often just holds a trough in position over the UK with wet and mildish weather as a result.

Similarly with Greeland Highs the vast majority are slightly too far west and simply advect arctic air into the mid Atlantic.

There is no doubt in my mind that the background signal make the general broadbrush set up.  

But getting deep cold to advect across the UK is down to the very exact positioning and orientation of these Greenie and Scandi Highs and that is the detail the no mjo phase or gwo orbit or qbo or eamt event can yet successfully forecast for such a small island as ours.

So whilst I am always interested to hear from the brilliant experts about the drivers that we are so fortunate to have on this site.

I am also aware, as are they. That those drivers simply cannot tell me if its going to snow in my back yard or even my country.

All they can do is give an idea of where the balance of probabilities may lie in regard to getting  the cold to our shores and that is all they can and no more than that.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5032533

[WYorksWeather]

I'll get on with my usual post, but first.

2018 vs 2024 comparison

Seeing a lot of 2018 comparisons being thrown around with the BFTE, so first a bit of a chat about that. I very much doubt this post is going to make me very popular, but I really don't see something similar happening this time. First of all, you have the timing. The 2018 SSW resulted in a zonal wind reversal which began on the 5th February.

Here's the current forecast from ECMWF.

At best, the reversal will happen about two weeks later, if it happens around the 18th. After the usual lag, that would put any resulting cold spell, if the SSW were favourable, into early March. At that point, you need something absolutely extraordinary to deliver. -5C 850hPa won't cut it - you probably need the -8s or even -10s and lower. Which brings me on to the second issue.

The BFTE chart for the 28th February 2018 looked like this. You have the entire UK under -12C 850hPa, and eastern coastal areas under -14C. It's absolutely incredible cold for this country to deliver the result that it did.

So, how did the cold get to that point? Well, in order to reach charts like those, you first need a source of cold air. Here's what the NH cold pool looked like at the same time in 2018 as we are at now (7th February), on the left. And on the right, where we are today, 7th February 2024.

The two charts aren't even in the same league for cold at our latitude. If you imagine drawing a circle over the chart, you can see that today, almost everywhere at our latitude or further south is near normal or above average. in 2018 there's a huge blob of -12C over not just Scandinavia but over Central and Eastern Europe as well. There's also an even more impressive area of cold over North America, down to the -20C isotherm widely.

With the cold pool like it is this year, I don't even see how a BFTE event is possible. If we got the identical synoptic pattern to 2018, the air source would only be -12C 850hPa or so, it would moderate as it headed our way, it'd be happening a week or two later than in 2018 due to the SSW lag, and we'd end up with marginal snowfall at best, probably a similar event to this week, frankly.

I think this explains why we've had a lot of good looking charts this winter, and very little in the way of outcomes. Milder cold sources means that even good looking charts are underwhelming in terms of the resulting temperatures.

Tomorrow's snow

Here's UKV again. To add to the frustration, this looks like a further downgrade, to be honest. More of the initial precipitation band tomorrow looks to fall as rain, except over high ground, where there will of course be the potential for some disruption, hence the Met Office warnings. 

By Friday afternoon, even the -5C 850hPa line is pushed back to central Scotland, and further rain will probably wash away any remaining snow, and temperatures well into mid to high single figures away from high ground will aid in melting.

Medium range outlook

The medium range just looks relatively boring. We may get some sort of easterly flow, and it's not looking overly mild, but there's a distinct lack of any cold to work with, as shown in the above charts.

You can see this in the ECM meteograms.

I mean it literally is like the models are trolling us at this stage. We finally get the much promised easterly in the extended at day 10 and beyond, and it brings average to above average temperatures! It's an absolute joke.

Long-range

As for the longer range output to whether we might expect a change beyond the current NWP horizon (so into late February or early March), it's a mixed picture.

CFSv2 is going average to slightly cooler than average for the most part for the next four weeks, but it's not a notable enough anomaly to really amount to anything.

The GFS 35 day mean is notably above average after the coming week until early March, before then dropping back below average.

The ECM extended for week 2 (19th-26th), week 3 (26th-4th), week 4 (4th-11th), beyond which possibilities for cold and snow are pretty much done, also show nothing of any note. If anything, slightly on the mild side. Also worth accounting for the fact that ECM is using the 20-year average in these charts, so near average conditions on these charts are still mild.

Summary

It's not a good outlook. Possibly the best we can hope for is some overnight frost and fog under areas of high pressure, and maybe some sunshine by day in gaps between the rain.

I think the most probable outcome is just average late winter conditions - a few colder spells with overnight frosts, interspersed with milder, wetter periods, but with no cold or snow of any note away from the usual prone spots on high ground and in Scotland.

I may turn out to be wrong, but we're rapidly running out of time here, and we need a strong signal to appear in order to change things around before its too late. That means a strong cold signal beginning to appear in the ensembles within the next two weeks. Extended range model output is currently not supportive of such a change either.

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5033197

[Blessed Weather]

10 minutes ago, Met4Cast said:

The MJO runs on a cycle of roughly 60-90 days, it won’t be back into favourable cycles again until April (ish). 

AAM tendency likely won’t return in any meaningful way until late March (assuming a coherent MJO cycle). 

So in a word.. no. 

Whilst certain phases of the MJO are known to assist with 'bringing on' a SSW (and the MJO has recently been in phases 6 and 7 which do), there is also research suggesting that - acting in the reverse direction - the SSW then impacts the MJO. So maybe it's 'all bets off' with regard a continuance of the normally expected MJO and AAM cycles? Extracts from a 2020 paper:

Influence of stratospheric sudden warming on the tropical intraseasonal convection

.....analysis reveals that SSWs also exert considerable influence on tropical intraseasonal convection. The occurrences of MJO phases 6 and 7 significantly increase during around 20 d after the onset of SSWs...

Full paper: https://iopscience.iop.org/article/10.1088/1748-9326/ab98b5

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5033893

[Cambrian]

Day 4 / 5 looking a bit chilly on the 0z ECM ensemble but otherwise above average temperatures for the foreseeable.

However, the very definite southerly track of the jet stream on the 0z ECM op - often heading into Iberia - would suggest that the mild outlook is a fairly precarious one. 

 

The day 10 mean anomaly gives the impression of being very blocked through the eastern Atlantic, good heights to the north and south but the heights through Western Europe are only marginally positive. The Atlantic trough will eventually try to head for the gap, but ironically looks that it will be blocked by the heights to the south.

   

0z EPS shows that the low is no longer being directed from Greenland, rather from a centre over Newfoundland, where the heights are lowest, all the while pumping heights north into Greenland and Iceland.

0z GEPS sends a ridge up through Iceland into Greenland at day 12, with a surface high over the Greenland plateau and a notable polar high.

Along with low pressure over the Barents Sea, this is feeding a cold northerly flow down into the Norwegian Sea. That is how we can get cold air back into Europe. The 0z ECM control is at something similar. Don’t worry about modelled uppers at this range, a path to insanity.

  

The Atlantic low can’t go north and is being blocked from disrupting southeast into Spain, so just stalls over the Atlantic, leaving the UK and Ireland at the southern edge of an Arctic block with easterlies or northerlies an option from there. 

The resilience of the southerly running jet stream and looking out for the Atlantic low to eventually pump enough heights north are the main aspects to watch here.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5033825

[Tamara]

 Met4Cast

Based on a likely spontaneous tropospheric response to what looks like a very robust SSW reversal of -ve zonal winds upcoming, the timing of tropospheric switch to a falling angular momentum regime implies that returning polar jet energy faces a roadblock across higher latitudes and result in actually perpetuating blocking effects across the NH.

I say this from a purely diagnostic viewpoint of a macro scale synoptic response & objectively as to how such a blocked regime may play out - without regard to surface weather responses at any given geographical location. 

So protracted +AAM transport c/o +EAMT and tropical convection rossby wave driving has been incipient in initiating stratospheric diminution, and it could be that falling momentum heading into late winter & early Spring wavelengths augments  -ve AO/NAO synoptic responses.

The same was said in January of course & the nature of the briefest of reversals underwent a reflective u-turn which allowed the polar jet to return unencumbered as angular momentum fell back The circumstances this time are quite different & the sheer scale of the stratospheric diminution heading towards the start of Spring puts at least a preliminary question mark over the return of the vortex as a final warming event.

In turn it suggests a relatively lengthy period of fractured jet flow betwixt & between blocking structures

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034002

[Weather-history]

 WeatherArc

 

Not far off what happened at the end of Feb 1960, 15.6°C was recorded at 7am in London 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034344

[bluearmy]

 This won’t be exactly how it plays out 

The 12z runs have begun to sniff out the way forward but they can’t have stumbled across the correct route first time.  At least it makes the next few runs worth watching …..

note that the mid/upper strat was more split on the 00z run at day 10 than the 12z. Another run which doesn’t stack up as you might have expected 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034508

[Singularity]

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

[bluearmy]

 Singularity. It is open to interpretation.  This winter is quite strange high up and perhaps difficult to make judgments upon.  If you look at the 00z gfs op, you will see a neg AO in the trop which is nothing to do with the reversal high up that occurs at the same time. The positive flow sitting between in the mid to lower strat  

 

in the past, we have seen reversals take place with a positive AO in place and within a few days a big ridge is thrown up on the pacific side which punches across the Arctic and affects our side of the NH.  That would be visible as the blue negative flow in the cross sectional chart working down top to bottom. That’s what a QTR is.  This winter with the trop and strat generally in tandem it’s probably not so relevant. It’s been tough to see whether the impetus for neg flow has come from a downwelling wave or is instigated from the trop and then quickly upwelled. Mike has posted on this a fair bit. 

fwiw, next weeks reversal is currently uncertain in how it plays out. I think that until we get a handle on that we can’t set much store in the modelling for week 2. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034797

[bluearmy]

I’ve been watching gfs ops playing later week 2 with the strat pattern and we begin to see some consistency now 

if we take current modelling on gefs, eps and the ec and gfs op at face value, there will be an ssw just beyond mid month 

this will be caused by an Asian ridge moving across the Alaska and then pushing into the pole and stretching the spv over our side of the pole 

this at 10hpa. As we go down through the strat, the vortex is more stretched and infact has begun to split 

a couple days later we have a n russian master and Icelandic daughter split 

At 50hpa this split is more evident with the daughter almost making it to Newfoundland. 

so by day 10 we have a scenario where the split in the atmosphere is more defined lower down with clarity in the trop ref a split 

 

so you could argue that the stretching of the spv is fairly instantaneous through all levels of the strat but it seems lower down the split is clearer 

the concern I have had over the past couple days is illustrated later in this run 

A day later and at 10hpa, the split is no longer evident with the Icelandic daughter absorbed back into the Asian master as a trough extension across the Atlantic sector 

 

meanwhile, lower down in the mid strat, things look pretty good at the same timescale as far as split sustainability is concerned 

 

But this is where it gets ‘ugly’ (and has often done so for the past few runs ). Initially the mid strat split tries to widen further but quickly begins to return east to join back up with the master segment which is itself drifting west into n Scandinavia and extending further west.  So we end up with 10hpa , 30hpa and 50 hpa like this at day 16

 

this is mirrored as we head down through the strat 

so to begin with, we have the whole strat moving very much in tandem as it splits but high up, that split is brief and like an elastic band, it begins to pull the vortex segments further down back together over the next few days. 

this is an explanation as to why we are likely to see the Atlantic jet firing up as we go through the latter part of week 2.  It may not be so different to what we saw in January where a decent looking trop profile was blasted away quite quickly. 

we would need a wedge or weak ridge to deflect the jet onto a southern trajectory such that we stay on the cold side when it arrives nw europe.
 

it’s not a done deal by any means but I think this is where the modelling is getting the strengthening atlanttic profile from. it may not be a bad thing if we get our ducks in a row when it happens. The gfs 06z op looks like it re amplifies behind the system heading east. 
 

just thought it interesting to see how the waves seem to head down/up/ down through the atmosphere 

 

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034946

[Cambrian]

This has been the feature of the winter for me - the recurring Scandinavian trough. It hasn’t delivered wintry weather for all parts of the UK and Ireland equally, far from it, but it alone has been conspicuous in consistently delivering the right kind of air mass to our shores.

It’s been a winter where the modelling of heights into Scandinavia / easterly tease, though not transpiring, has proven to be a frequent prelude to the reassertion of the trough from the PV as it shuffles each time over from Canada to settle for lengthy periods on the Siberian side of the Arctic. 

If we look at the NH profile, we can see that it’s at it again! The signs were there last night on the GEFS, but now indeed, its arrival is being smartly expedited in the 0z ECM models. Yesterday’s 12z mean at 252h versus today’s 0z at 240h. 

 

All hail the arty rabbit looking thing - may your front legs grow long and maintain that southwestward orientation trend. 

Atlantic ridging and western Atlantic trough also a good deal further west, that push of heights into Greenland is outstanding for a day 10 mean, driving amplification for the trough but also serving to push the southwesterly flow well away and opening us up to the north. This is reflected in the mean T850’s too.

 

A big change in the models over just 12 hours.

There’s evidently still plenty of time for a meaningful cold spell. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5034848

[Nick F]

 Derecho I've lost faith in using the MJO passage through the phases as precursor to changes in weather patterns towards cold or mild. Following 'warm phases' of 3-4 in early Jan we saw a 2-week cold spell, we've seen the MJO recently pass through 6 and now loitering and weakening through 7, but very little amplification in the upper flow at mid-to-high latitudes to show for it in the models. What high lat blocking that has been showing and will likely continue to be shown down to changes in the strat, i.e. downwelling of weakening winds.

And I don't get this obsession with using AAM as tool on its own, it seems that when ever it rises all falls we are told to look out for changes but it seems, like when the MJO moves through the historically 'colder' and 'milder' phases, many other drivers are at play, some not so obvious, drowning it out much influence from it over the N Atlantic sector / Europe. Though the biggest interference in what we would expect in lag to certain MJO phases being climate change. 

The state of the stratosphere is one driver, when it couples with the troposphere, still one to keep an eye on though. 

Be glad not hear about the MJO for several months, until the next cold season. I've given up talking about how it may influence patterns, as it makes me feel like a Snake Oil Salesman!

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035341

[Met4Cast]

18 minutes ago, Nick F said:

I've lost faith in using the MJO passage through the phases as precursor to changes in weather patterns towards cold or mild. Following 'warm phases' of 3-4 in early Jan we saw a 2-week cold spell, we've seen the MJO recently pass through 6 and now loitering and weakening through 7, but very little amplification in the upper flow at mid-to-high latitudes to show for it in the models. What high lat blocking that has been showing and will likely continue to be shown down to changes in the strat, i.e. downwelling of weakening winds.

And I don't get this obsession with using AAM as tool on its own, it seems that when ever it rises all falls we are told to look out for changes but it seems, like when the MJO moves through the historically 'colder' and 'milder' phases, many other drivers are at play, some not so obvious, drowning it out much influence from it over the N Atlantic sector / Europe. Though the biggest interference in what we would expect in lag to certain MJO phases being climate change. 

The state of the stratosphere is one driver, when it couples with the troposphere, still one to keep an eye on though. 

Be glad not hear about the MJO for several months, until the next cold season. I've given up talking about how it may influence patterns, as it makes me feel like a Snake Oil Salesman!

You're talking about the MJO & AAM as if they are two separate things, but they're not. 

AAM is simply a measurement of how much westerly momentum is in the atmosphere, the MJO influences this via frictional & mountain torques, i.e driving positive MT increases westerly momentum. More westerly momemum tends to = a more perturbed jet stream and thus, a higher likelihood of high latitude blocking but it's equally important to see which direction those anomalies are travelling. The MJO is taken in isolation too much in this forum & more generally, it's just 1 variable of the global windflow budget so it's no real surprise that looking at the MJO, checking the composite & expecting the atmosphere to respond and look like the composite isn't really working, the whole point is to view the MJO as part of the windflow budget, the MJO is incorporated into the GWO. 

The AAM, or GSDM isn't supposed to be used to determine regional specific weather but to help diagnose global weather patterns, it's essentially the best tool we have for medium-extended range forecasting. 

GSDM (which incorporates processes like the MJO) > Global NWP modelling > High res modelling

The GSDM provides context to NWP modelling in the same way NWP modelling provides context to more localised high resolution modelling. You're right, the MJO is currently in a phase which if you view in isolation and look at the composites, does favour high latitude blocking, but that's not the entire story.

AAM is positive at the moment indicating a good link between the +ve ENSO state & the atmosphere, however as discussed here in recent days fluxing of positive (and negative) anomalies are and have been equatorward rather than poleward, this serves to inflate the Iberian/S Euro high, whereas poleward fluxing (see early Jan) helps to amplify high pressure into the Arctic regions. 

The current state of AAM fluxing helps explain the current patterns we've been seeing despite, on the face of it, a favourable MJO. I do wonder if the very warm SErn Atlantic waters has perhaps helped to extend the Hadley cell this winter and thus inflate the Iberian high regardless of other forcings such as the AAM, it seems very much to have been the default, base state through much of this winter despite seemingly (and often) favourable teleconnective forcing. Climate change is without doubt changing the dynamics.

I don't think we should just ditch the MJO, GSDM etc just because it hasn't performed with pre-determined outcomes. If anything, this winter has been a fantastic learning point on this, but it is important not to take them in isolation or out of context.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035355

[Nick F]

 Met4Cast i am aware of how AAM and mountain torques are linked to the MJO. But I'm not convinced that the fluxes poleward or equatorward can specifically be linked to anomolous pressure tendencies in a particular locale, such as 'inflating Iberian / S Europe high pressure cells. What literature did you get that from? I would have thought Atlantic SSTs, TPV extending south from Greenland premoting WAA and expansion of Hadley cell north in winter would be the main drivers. 

Does comes across to me like GDSM forecasting is an exclusive club who's members have secret access to AAM charts. Where do the rest of us find them?

The joy of this thread is everyone has access to a huge range of NWP charts now, much more than 2004 when I joined Netweather, so we can discuss, debate and challenge each other and those with less understanding can ask questions of the more knowledgeable. But with the GDSM posts by a few on here who have the special access generally go unchallenged and so readers just take your word for it. 

I'm not saying I don't appreciate the time you guys put in to explaining how you interpret the AAM charts and come to your conclusions. But most of us don't know how these charts change with each update so can't discuss them and perhaps challenge why a previous forecast suggested x may happen but in the end the weather had other ideas.

When I started learning meteorology in depth in the early 00s no meteoroloy books or journals I remember refered to GDSM. But I have read some of the papers Blessed Weather has kindly put on Netweather. But still yet to be convinced it is any better than other tools for longer range forecasting.

Browsing wx forums in the USA, notably Americanwx, there is seldom reference of AAM by ProMets, only really on here gets a mention. Yet to be convinced myself, but you never know, I may be swayed more one day.

 

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035396

[MattH]

 

Let's start with the AAM - If the number of posts over the years from Tamara et al haven't highlighted the importance and usefulness of the GSDM and the AAM, then I don't know what more can be done, so for those who feel that way, then, yeah, move on, but to take a quick look at the forum and see phrases like 'Snake Oil Sales' is just pathetic.

In addition to the AAM - The charts that a set few posts are from a source that is run by the IBM Chief Met in the USA, David Gold. Nearly a year ago now the login for these was leaked and, as a result, his server was overloaded and it went down, for months. Those that use these and despite what some have recently said, even in the USA, where they are used, individuals were heavily disappointed they were down. Therefore, anyone after access to these needs to get in touch with him directly, as another burst of logins would cause further problems once again. There is no 'secret club' or anything like that, just a ridiculous statement, there is a reason, it's not a widely known source and I certainly won't be sharing login details when told not to.

I've been using and studying the GSDM for the best part of 4 or 5 years now and within that time the amount of examples where it has helped to pre-empt NWP and also help to reinforce it are numerous. This area of meteorology has crept into forecasting more and without providing details is a foundation for some of my long-range forecasts for work and clients, especially in the energy sector. Meteorology and weather forecasting is a fluid process, things change, adapt and have to be modified. When an outcome doesn't happen as some expected then it is the comments, previously, that make an appearance, but shouldn't. As Tamara has stated many a time it isn't A+B=C and that includes the teleconnections and the GSDM, but it is how it is used and adapted that is key.

I can't add much more, to be honest. We all have our opinions, but despite months and years of some trying to educate and discuss the likes of the GSDM to then swiftly knock it all back, including teleconnections too is ridiculous and I apologise for the off-topic post, but this needed to be said.

Regards, Matt.

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035436

[Cambrian]

Difficult to pretend that the models are playing ball in terms of anything decisive. But what is going on is definitely fascinating.

I’m taking the 0z GEFS because it’s been great value this winter, and using the mean MSLP charts because we don’t show them enough. We’ve well and truly learned how to practice the virtue of patience, so let’s look at the next couple of weeks. 

Day 7 shows a long fetch southwesterly from the Azores, riding along the northern flank of the high pressure to the south, likely mild and very moist, the jet stream continues to water the meadows. Let’s move on.

But before we do - ironically, it’s that high pressure, another wave of Iberian heights, that begins the transition to another pattern. 

 

By day 10, it’s working north towards the UK and Ireland. Like over recent days, the highs modelled to be aligning during this period from Western Europe through the pole to the Bering Sea. 

It’s that “super-ridge” of high pressure that both holds the Atlantic at bay and as it moves west, makes room for the developing Scandinavian trough, and the establishment of a cold northerly sourced pattern by day 14. 

 

Fascinating pattern towards the end of the month, low pressure anomalies to the south, high pressure anomalies to the north, consistent with a mollified jet stream, weak and running far to the south, heading into North Africa, Western Saraha, Tropic of Cancer, around 23 degrees North. 

No great heights to the south at that stage and the entire pattern north of the jet stream is profoundly slack, it looks particularly challenging to model, and is very open to a much higher potential for surprise than normal. It’s at that stage that any modest bit of circulation out of the Arctic, that won’t even have started to develop yet, may well turn out to be more decisive in terms of the weather for northwest Europe. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035424

[bluearmy]

 Froze were the Days  Gowon

We should wait until closer to the reversal to see if the models begin to show better grouping around a different way forwards and certainly until afterwards for signs that they didn’t work it out 

Re timing -  whilst there will be a degree of variance on dates for reversal at 60N 10hpa between the 16th and 20th, the action up top  starts 15/16th and thats the date to use to check continuity. 

We are seeing inconsistencies run to run on gfs ops ref the reversal and what strat pattern follows 

what seems fairly solid for now is that a split spv is more likely lower down than high up. That throws in a degree of uncertainty. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035593

[Jordan S]

Here’s the increased snow chance I highlighted for this upcoming week

Ah well, sometimes forecasts don’t go to plan on a more widespread scale but that’s the nature of weather of course.  

Anyway, going into more detail of what may happen for end of next week and beyond again but relatively quickly..

Not much more to add from my previous post on the outlook for next week other than emphasising the brief drier conditions that may develop more widely at the end of next week, over northern areas Thursday/Friday especially Scotland with wet conditions likely continuing for some southern and western regions at this time, then those drier conditions likely further south by Saturday with winds switching from the southwest/west to possibly then from the east/southeast with that higher pressure mentioned previously that moves over to our east/northeast along with that slightly colder air perhaps into eastern parts. 

Sunday 18th/ Monday 19th more unsettled conditions are more likely to move into Ireland and Northern Ireland, heralding a possible slow northeastwards progression of wet weather for some as the week beginning 19th February goes on, but most or all of the unsettled weather at the moment looking like being mainly in the west and south west through the first half of that week 19th/20th/21st with higher pressure again to our northeast perhaps keeping things drier further east and north for a time.

With regards to what I’ve mentioned in a few posts with a likelihood of high pressure to our north/northwest as well through this period this still looks true to an extent but the emphasis on any possible snowfall along boundaries of weather fronts looks have shifted to later in the time period and to be for hills,  mainly for Scotland if any weather fronts work that far north.

Winds may blow from the northwest more widely later in the following week  onwards towards the end of February though as heights lower to our immediate north and northwest (though staying relatively high close to Iceland/Greenland at times) and pressure more likely than not increasing and moving closer to our southwest around the 21st-26th February, this would increase the chance of more widespread wintry showers into northwestern areas. especially over high ground but drier conditions and slightly higher temperatures in the southwest, south and east of the country, though not particularly mild.

Details may of course change somewhat as the time period moves closer, I expect that to be more likely to happen compared with normal, so expect adjustments to this as my updates go along, but I hope to have a steady forecast consistency in my posts once again soon. 

I will have another update posted early next week for anyone interested probably Tuesday time.

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035929

[Cambrian]

Variations on a theme from the last couple of runs from the ECM op between days 6 and 8, yesterday’s 12z on the right, with a welcome high pressure building in the vicinity of the UK and Ireland.  

  

The variation is toward the north of that in how cleanly the linkup with the polar high separates the Canadian and Eurasian lobes of the PV. 

It’s supported by the 0z ECM ensemble mean as a stepping stone between where we are over the next few days and the setup for the end of the month, day 4, 8 and 14. It’s a nice evolution.

  

Something like this happening is clearly on the cards, to get from here at day 4 on the 0z GEFS to where we are at day 12 (a couple of days quicker than ECM).

The extent and nature of the split of the tPV is clearly still to be pinned down, but strong indications that its machinations will switch the Northern Hemisphere profile from lateral to longitudinal. 

Reversal of the Barents / Kara ridge to a trough, ridging up through Greenland all the way up through the pole so that the Arctic cold is split, cold air sweeping south over both North America and northwest Europe…

  

…plenty of interesting variations on this theme likely to become available as the season has finally managed a 3 lobe PV pattern with a favourable 2-way split of the cold air, with the strong Greenland ridge, and the East Asian and North American lobes showing signs of linking up.

The interest for me is not all about hanging on in there for at least some prospect of seeing some wintry weather to round off the winter.

The end of February, beginning of March is often cold, even in years with a strongly coupled PV, so I’m really interested in how the troposphere plays out the final throes of this season characterised by a phenomenally weak stratospheric PV. 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5036584

[WYorksWeather]

Worth a separate post this, because I've seen a lot of people asking two things lately - how do weather models actually work, and where do errors come from?

How models work

First off, none of what I'm about to say relates to any AI-based models.

Essentially weather models should be viewed as an enormously complex physics simulation. You take in some data at a particular time, call that T+0, and then you have a series of very complex equations that relate the state of the simulation at a particular time, to the state one hour later, for example. Run this repeatedly, and you produce a forecast for the next few days up to a couple of weeks, depending on the model.

Also a good point to clear up some common misconceptions. The models don't have a built in knowledge of previous data, so for example they are able to simulate unprecedented situations that break records. They also don't generally aim to simulate certain large-scale features - they emerge from the model. For example, the MJO, weather regimes, jet stream, polar vortex are allowed to evolve freely according to the physics. The models don't have built in parameters that do things like 'promote blocking after an SSW' or 'ensure weather patterns stay close to the MJO composites'.

So, next question is where do model errors come from?

Source 1 - discretisation

The world is analogue. Ideally, with unlimited computation, you would simulate every single atom in the atmosphere and calculate its evolution. You would also like to use an extremely small time step for evolution of the model, in other words simulate from T+0 to T+0 plus one millisecond, then T+0 plus two milliseconds. Obviously that's impossible.

The way the models avoid this is to reduce the complexity by dividing Earth into a two-dimensional surface grid (typically ranging from several kilometres to several dozen kilometres on a side), dividing the atmosphere into vertical levels, and dividing time into more manageable steps (typically minutes or hours). This makes computation possible in a reasonable amount of time.

Unfortunately, it also introduces errors. Very small features, particularly those involving convection, or terrain details, are hard to simulate, because the model is too coarse to identify them. Very brief weather events may also be poorly simulated.

Source 2 - data inputs

The second type of error comes from data inputs. Infamously, this was the largest source of error in the incorrect prediction of the Great Storm in 1987. This was because data inputs were more reliant on ships, and of course with a big storm on the way, many ships were avoiding it. That limited accuracy of model projections

Of course that specific limitation is no longer present, but data inputs can still present problems. Sensors have finite resolution, can't be everywhere, and there may be delays in receiving and integrating the data into the model. This means that the model at T+0 is only an approximate view of the current state of the atmosphere, though hopefully a very accurate one. This is why ensemble forecasts are used, to make subtle alterations to the model initialisation at T+0, and see what the resulting effects are.

Source 3 - physics models

Of course, the actual physics simulations are also not perfect. Some of these are known issues. As an example from the known issues with the ECMWF forecasting model, the ECMWF does not correctly simulate the landing point of precipitation. So in cases where rain and/or snow is moving from sea to shore, the extent of inland rainfall / snow is often underestimated, as where winds are strong, the landing point of precipitation may be a significant distance from where it begins to fall at cloud level.

Another example is that the GFS has a notable upward temperature bias in certain situations. In summer, following exceptionally dry periods, the GFS can sometimes overestimate temperatures. As an example, it has at times predicted infeasibly high temperatures in the high 50s Celsius for California's Death Valley, and during the July 2022 heatwave, it predicted infeasibly high temperatures in the high 40s Celsius for parts of France.

Source 4 - chaos theory

Unfortunately, even if we achieve massive improvements in 1, 2, and 3 (and we already have, compared to decades ago), this source of error remains. Even the most trivial defects in the model tend to accumulate over time, and hence at a long lead time the atmosphere is fundamentally chaotic. Therefore, the skill of the longer range forecasts, for example the ECM extended, or the CFSv2 weeklies, is always going to be limited.

This is a fundamental limitation. It is therefore very unlikely we will ever see, for example, the ability to accurately forecast the weather in anything more than general tendencies relative to average beyond the period of 10-14 days in the UK.

Summary

Hopefully I've explained this fairly clearly. Happy to be corrected if any of the veterans here think I've got something clearly wrong or missed something out. In any case, I hope this post proves useful to someone, in which case it was worth writing.

 

 

Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5036913