Met4Cast

February 21

Wind reversal. Westerly winds are inherently more stable than easterly winds. Descending easterlies disrupts the usual westerly flow in the troposphere which can lead to blocking.

February 21

2 hours ago, keithlucky said:

Big reversal coming,winter has a sting in it's tale.Expect rapid cooling as wind swing to easterly.

No real sign of that currently & SSW’s do not always = cold weather of course.

Given the timeframes there’s a lot of uncertainty but current modelling doesn’t indicate a quick downwelling & tropospheric response to the early March SSW & the MJO won’t be particularly favourable for blocking (it may become so later).

I wouldn’t expect much impact until mid March at the earliest.

February 21

On 14/02/2024 at 13:23, Met4Cast said:

There's no real sign in this fluxing switching to poleward fluxing and as a result there isn't much forcing to amplify Atlantic ridging into high latitude blocking later this month, hence the forecasts (and this is well represented within NWP modelling) generally favouring an amplified Atlantic ridge sending energy SEwards, for the UK this will likely bring colder weather than the anomalous warmth we've been seeing/will see in the coming days but probably wont be enough for a notable cold spell or widespread snow risks.

Just over a week on & indeed transient Atlantic ridging seems the most likely weather pattern through this month, certainly a departure away from the recent anomalous warmth across the UK (and indeed Europe) but nothing overly sustained or notable as largely expected. Any snowfall in the coming week or two will be mostly restricted to higher ground in the north.

Going forward I'd not be surprised to see this pattern repeat a few times, i.e Atlantic ridging before collapsing. The MJO signal has now decayed into the Circle of Death (COD) and is not currently driving global weather patterns, a result of this is an abrupt loss in westerly inertia triggered by a strong -MT event, led particularly by very strong -EAMT.

There is now evidence of poleward propagating +AAM anomalies which should help to generate Atlantic ridging and amplify it away from Iberia (the opposite to the recent equatorward fluxing helping to strengthen the Iberian high). The forecast going forward is quite uncertain, the MJO seems likely to emerge into phase 4/5/6 but the direction of travel & amplitude is very hard to ascertain at the moment. A major and sustained (perhaps final warming) SSW looks to take place during early March helping to destabalise the polar field but there's currently little sign of these -ve anomalies downwelling to impact tropospheric patterns before the middle of March, again though this is a large uncertainty within the forecast period.

I'm still of the opinion that early-mid March will feature an increasingly -NAO regime albeit probably not notably cold across the UK with the high never really gaining much in the way of amplitude due to the lack of tropical (MJO) forcing.

Looking beyond the early March period is difficult, much will depend on the outcome of the major SSW (and any downwelling impacts) & the trajectory of the MJO, again currently uncertain.

February 21

2 hours ago, Methuselah said:

But, haven't all this year's SSWs amounted to nothing; just like 2019's SSW amounted to nothing? IMO, something (AGW?) is outcompeting the usual teleconnective drivers. . .

Yes, I think too much emphasis is being placed on this likely SSW in early March. Firstly because the timings involved here are worth mentioning, we've seen how poor modelling has been with SSW's this winter, why should we assume this one will be any differently? Secondly, an SSW in early March, assuming the usual 2 week lag we're looking at the 2nd half of March for tropospheric impact. Too little, too late imo.

That said - As stated a day or two ago, I do think a -NAO regime is more likely than not going into March regardless of a potential SSW or not. AAM tendency has fallen through the floor as the MJO now sits inside the COD, this removal of tropical forcing has caused the previously sustained +MT to collapse into strongly negative territory, as a result westerly inertia is now being scrubbed from the atmosphere.

Another way of visualising this is with the 850hPa anomaly hovmoller, a rather sharp increase in easterlies just west of the dateline following the collapse of the MJO.

There are signs the MJO will return into phase 5/6 but the forecast on this is rather uncertain. The important part though is the sharp fall in momentum which is driving the pattern change currently advertised within NWP outputs. Atlantic ridging/Scandi trough scenario currently the favoured option but huge uncertainty over Scandinavia with regards to possible height rises around the Urals, the precursor to the early May SSW. I suspect any high pressure that does build to the NE will likely end up too far NE to provide much hope to those still chasing colder outcomes.

In terms of the UK - Chillier than of late but little currently to suggest widespread cold/snowfall, as we go through March we require more spectacular synoptics to produce the goods.

So to summarise, -NAO regimes are likely, UK will be colder than of late as a result but in terms of UK specific cold/snow likelihoods I'd still rate these as being rather low.

February 19

13 minutes ago, Catacol said:

Meanwhile summer approaches and I am dreading how much heat may come with it. Perhaps something significant can shift next winter so that the patterns and the accompanying temperatures can give snowball players more of a chance.

Agreed re: Cold/snow likelihoods, I think we will see a -NAO return but I’m not confident on that translating to a winter wonderland, like you it will probably be more chilly rain.

On the quoted part though - I agree re: Summer, this perpetuating Iberian high pattern going into summer wouldn’t be good, Spain is already drought stricken & soil moisture is rapidly drying up now, I can definitely see scope for yet more record breaking temperatures across southern Europe this summer, especially when you factor in the declining El Niño & associated lag with the AAM as we transition into La Niña (probably sometime around mid summer).

Early indications of summer patterns don’t bode well.

February 19

The idea of blocking during March certainly isn't without merit and for those chasing some early spring warmth (like me) the direction of travel is rather poor.

The MJO looks likely to return to phase 5/6 before the end of the month if we look at the VP200 anomalies (RMM plots are all over the place currently, some support this idea, some don't). I wonder if this is SSW related?

An uptick in +EAMT is expected thanks to a strengthening East Asia jet stream and we're currently seeing the highest amount of +ve westerly momentum since early December, this shown by the GWO plot being the highest of the season so far.

With the current SSW helping to destabalise the polar field combined with another likely weakening/possible reversal (maybe final warming?) early-mid March the idea of an increasingly -NAO regime seems to be the best bet currently.

February 17

On 14/02/2024 at 13:23, Met4Cast said:

There's no real sign in this fluxing switching to poleward fluxing and as a result there isn't much forcing to amplify Atlantic ridging into high latitude blocking later this month, hence the forecasts (and this is well represented within NWP modelling) generally favouring an amplified Atlantic ridge sending energy SEwards, for the UK this will likely bring colder weather than the anomalous warmth we've been seeing/will see in the coming days but probably wont be enough for a notable cold spell or widespread snow risks.

I think a good way to see how thing are progressing is to check previous posts so I've quoted this over from the teleconnections thread. Broadly this is how things are heading.

Certainly turning colder as next week progresses but for most not cold enough to produce snowfall to lower levels. Further ahead into March is rather complicated, the RMM MJO plots are proving rather useless currently but viewing the VP200 anomalies seems to suggest a decaying MJO signal into the COD through phase 8 before a resurgence into phase 5/6.

Broader westerly momentum continues to rise as yet another +EAMT begins to take hold which perhaps gives some confidence to the current modelling re: another stratosphere u-wind reversal early - mid March, an early final warming perhaps? Too early to say.

Given this, I'd not be surprised to see a -AO and perhaps trending -NAO towards the middle of March. Blocking patterns potentially finally taking hold, once again, too late?

February 17

4 hours ago, TEITS said:

Whilst I shall continue to look at the teleconnection predictions and seasonal models. I shall look and see what the teleconnections suggest for the whole of Europe rather than the UK. I do not believe you can use these forecasting methods just for a small island like the UK.

Thank you for the mention and I 100000% agree with this.

Teleconnections cannot be used to forecast for the UK specifically but can be used for the broadscale direction of travel, i.e blocking regimes might be more likely or even develop but what that translates to UK specific weather is impossible to say! That includes the MJO & use of it’s composites etc.

February 16

8 minutes ago, Frigid said:

Mean going below 0 again in March.. laughable really. The strat has been trolling us all winter, only about time it delivers the goods

In fairness that’s not too unusual, will likely be the final warming.

February 16

Blessed Weather

Great post! I had actually been planning to create a post on this at some point in the coming days so good timing too. I think your conclusion on it being a strong reflection is probably correct, there were several things going on which likely led to the results we saw. It's quite difficult to "untangle" it all with limited vertical direction of travel on the charts, but I'll try & explain what I think happened using this strat observe chart.

1: Minor warming occurred in early January and as highlighted in your post, Amy Butler & Simon Lee both talking about this leading to the -NAM state & Greenland high, I think this is probably right but we also saw AAM anomalies propagating poleward with support from the MJO meandering in phase 3, this minor warming likely added some extra juice to what was already becoming a favourable tropospheric led pattern for Greenland blocking/HLB.

2: This -NAM then worked upwards into the stratosphere at a time we saw yet another SSW (whether this caused it or merely helped it? I'm not sure), I suspect this -NAM is the reason for the technical reversal we saw with this secondary strat event, the upwards propagation of -NAM however probably served to "cancel out" any downwelling from the major SSW in mid-Jan which then ultimately led to a breakdown in blocking and return to +ve zonal winds in the troposphere, as shown on the strat chart above due to the Canadian vortex shifting eastwards into the Atlantic sector.

Quite a complicated entanglement of different forces/drivers coming together, one that would absolutely not have been possible to predict at longer ranges and something that seemingly caught even the experts out, let alone us mere mortals! From what I've read since, these reflection events are rather rare.

So.. typical UK winter luck at play?

February 15

8 hours ago, Weather-history said:

Has this been a bad winter for the teleconnections pundits and medium/long models?

In a word, no, as my post 1 page back attempted to diagnose & show.

2 hours ago, Penrith Snow said:

The fact that other 'signals' got it so badly wrong goes to prove how unreliable they are as a forecasting tool, yet we still hear evangelist going on about MJO phases, AAM and the dreaded mountain torque, give it a rest guys or at least explain why it went so wrong while good old fashion pattern matching had February spot on

Perhaps if you took the time to read my post 1 page back on this thread you’d see that i’ve done exactly that. You’d also see that broadly teleconnections have gone largely as anticipated throughout this winter but shorter term changes within the stratosphere (that cannot be predicted) have caused a lot of issues.

February 15

13 hours ago, stodge said:

As to how it will behave in a warming world, I don't know either. A warmer atmosphere suggests more energy and more volatility so you'd expect more warmings but would it be weaker longer term?

Very likely the opposite based on a few different studies.

The Stratosphere is actually cooling with climate change rather than warming, increasing amounts of CO2 trapped in the atmosphere means less heat radiating out & as such heat gets trapped primarily in the troposphere allowing the stratosphere to cool, a cooler stratosphere = larger temperature gradient between the Arctic and Equator which would lead to a stronger polar vortex rather than a weaker one.

You’re right though, more energy in the troposphere would suggest an increased likelihood of stronger “attacks” up into the strat so it’s quite a difficult one.

February 14

1 hour ago, Metwatch said:

Thanks, that is helpful, but specifically looking for charts like this below which Met4cast posted. I've seen them for years going all way back to the 80s.

They’re available here;

http://www.bom.gov.au/climate/enso/#tabs=Tropics&tropics=History-to-now

February 14

1 minute ago, stodge said:

I don't think a "major" SSW was ever on the cards.

Just 2 runs ago 100% of EPS members had a technical reversal (Major SSW) The GFS was less keen but still had a significant number of members reversing u-winds.

So in fairness expectations of a major SSW were high.

February 14

1 hour ago, Bricriu said:

The background drivers didn't fail, they are just a reminder that they do not guarantee cold for us.

Unfortunately as highlighted by myself & others in recent days, “background drivers” have been against the idea of a significant high latitude blocking episode.

It seems a major SSW is now unlikely too with a relatively quick sPV recovery compared to recent forecasts.

Very odd winter in terms of the stratosphere.

February 14

Yuck..

Absolutely no sign of anything of interest on the ECM det (or the GFS, really)

February 14

Given the recent discussions around teleconnections/AAM & the broader GSDM framework I thought I would begin posting in this thread, hopefully we can build up something of a more technical discussion here & separate from the usual chaos and ups and downs of the model output thread. Winter is coming to an end now so I suspect the forum will be quietening down so it's perhaps a good idea to start and see where we go through summer.

I've been delving into the GSDM and broader teleconnections for just over a year now after DMing Tamara & asking for some guidance. I'd still consider myself a novice but I do think this is the way forward in terms of mid-extended range forecasting despite recent comments from some. I wonder if @Paulor another mod could move this thread to the model output section? It seems a better fit vs here. My main goal is to try and get broader teleconnections discussed more by those who have an interest in the science & in forecasting vs just wanting a particular weather type.

Late February

I've posted elsewhere re: late February prospects but figured I would discuss the current situation here too. The MJO is currently dwindling through phase 7 and into phase 8 which if taken into isolation correlates to a mid Atlantic ridge/amplified Azores high.

The up-coming stratospheric warming can be linked back to the MJO passing through the Maritimes and into the Pacific Ocean (Phases 4 > 5 > 6) through mid-late January, this created +ve Frictional Torque which in turn creates Rossby waves that amplify northwards, the impact of this was a very strong +ve mountain torque event in late January leading to a Pacific jet extension, this created further Rossby wave packets that have led to anti-cylonic wave breaking into southern Europe helping to strengthen the Iberian/S European high over the past couple of weeks. You can see these Rossby wave packets on the ECM time longitude plots.

Back to the SSW (technical or not), the strong +ve East Asian Mountain Torque (EAMT) essentially deflects waves from the troposphere up into the stratosphere, this has been sustained more or less since late January with wave activity impacting the stratospheric vortex & now resulting in it's collapse. You can see this heatflux on the stratosphere plot from stratobserve, it's no coincidence that this is showing strongly downstream of the Himalayas (this is the EAMT at work)

North American mountain ranges have also helped to generate heatflux (namely the Rocky mountain range) albeit to a lesser extent and now we're in a position where we're expecting significant deceleration of u-wind speeds in the stratosphere.

AAM anomalies and indeed flux direction plays a part in the Iberian high mentioned above & subsequent anticylonic breaking. I wont go into too much detail on this in this post as there's always a lot of information here, but in summary;

+ve anomalies have been broadly equatorward since late January (between 30S and 0) with little momentum propagating poleward, this (from my understanding, happy to be corrected) tends to drive energy into sub tropical ridges (Hadley cell inc. the Azores high) and this broadly has been the dominating pattern through February so far.

There's no real sign in this fluxing switching to poleward fluxing and as a result there isn't much forcing to amplify Atlantic ridging into high latitude blocking later this month, hence the forecasts (and this is well represented within NWP modelling) generally favouring an amplified Atlantic ridge sending energy SEwards, for the UK this will likely bring colder weather than the anomalous warmth we've been seeing/will see in the coming days but probably wont be enough for a notable cold spell or widespread snow risks.

Any questions please feel free to ask, the idea of re-igniting this thread is to get more people interest in the science & forecasting aspect this can provide, the more people joining in and being open to background teleconnective forcing the better in my opinion. It is a shame the GSDM data is locked away and it does somewhat limit potential learning on the topic but I think we can get around that via discussions/sharing of data where possible on this forum.

February 14

4 minutes ago, WINTRY WALES said:

But if she made the move for a warmer climate then that would indicate Tamara does have a bias towards what the weather could be.

I think the point being made is that Tamara doesn't live in the UK and therefore the weather the UK sees has absolutely no impact on her personally regardless of personal desires for particular weather types, given she's not in the country, that alone removes any bias and allows her to be completely objective & scientific when it comes to UK specific weather.

February 14

13 minutes ago, Chesil View said:

As you say Tamara's unbiased posts that are made with out a required emotional outcome are valuable in keeping ones feet on the ground although even here Tamara's suggested shuffling hieghts scenario has not really come to fruition either and I say that with greatest of respect because it simply goes to show how ultra sensitive our uk based outcomes are to small changes in the background drivers.

Indeed, unfortunately one of those situations where short-term changes completely reshuffle the entire pattern. Covered this in my post yesterday in the Post Mortem thread but broadly the SSW reflected back off the trop allowing the Canadian vortex to shift east into the N Atlantic, those "shuffling heights" were then blown away and the cold spell came to an abrupt end.

Quite a rare thing to happen but something that's just not possible to forecast at longer ranges despite the best intentions/attempts, frustrating but is what it is!

Ironic really.. all the winters we've chased SSW's in the hope of cold, one comes along and then ruins what was otherwise a very, very favourable tropospheric pattern

February 14

Blessed Weather

On the other hand, this will be the third SSW of the winter that has utterly failed to produce. A lot of hope is placed upon SSW's to produce but the truth is roughly 70% of split events produce colder weather across NW europe, that % is significantly lower for displacement events (as you know, I'm sure!)

SSW's can produce holy grail synoptics, 2018 for example, but that was one of the most extreme synoptic responses to a SSW on record, the reality is usually (and unfortunately) more muted.

February 14

On 09/02/2024 at 19:25, Met4Cast said:

Teleconnections aren’t providing much in the way of clarity either, equatorward fluxing of AAM anomalies are I suspect, helping to strengthen/maintain heights to the south of the UK whilst MJO through phases 7 & likely into weak amplitude 8 suggests blocking of the higher latitude type, two evidently opposing signals & an occasion when the tropics & sub tropics are not aligned towards the same potential outcomes, i.e opposing signals playing havoc with both NWP modelling & our ability to forecast in the medium-extended range with any clarity or confidence.

A potential SSW in the mix too only further complicates the picture. I think getting cold into the UK is going to be difficult, particularly further south and particularly during February.

I just want to go back to this post last Friday & indeed much of this is now coming home to roost within NWP modelling. If you're on a northern hill there's a good chance you'll see some snowfall next week as a deep trough slides SEwards with a little amplification of the Azores high following behind, not much luck for those further south though.

The SSW has trended more towards a displacement event vs what had previously looked like a split event and as such, I'm not sure how much hope we can have on this being able to overcome other drivers (discussed in the quoted extract above)

A perfect example, if ever, that taking the MJO in isolation & not in the context of broader windflow budgets often doesn't produce the desired outcome.

February 14

This up-coming SSW does appear to be strongly linked to the MJO & associated +MT events, particularly the East Asian Mountain Torque. A paper from, Balwin & Dunkerton 1992 titled "Stratospheric variability has important implications for surface climate" seems to suggest greater forecast accuracy of SSW events following MJO orbits through phases 6/7.

We've been in a slow meandering phase 7 for a little while now but the previous eastwards progression into the Pacific triggered a (albeit quite weak) +FT event, the slow phase 6/7 has helped to amplify MT quite significantly & more prolonged than the initial +FT would have led you to believe.

This strong & sustained +EAMT has allowed for continued heatflux to deflect upwards into the stratosphere, unsurprisingly originating from the Himalayas (and to some extent N American mountain ranges) as per the first strat observe chart attached below.

What comes next in terms of predicting future weather is quite difficult. AAM anomalies continue to flux equatorward so despite a likely SSW & continued forcing from the MJO on the face of it suggesting high latitude blocking I'm still not convinced there'll be enough "oomph" to transition from an Atlantic ridge to a Greenland ridge.

I'm of the opinion that the late February cold weather will be more transient in nature, snow risks across N hills, perhaps along the N boundary of low pressure systems but by and large nothing out of the ordinary or exceptional, southern England once again probably struggling but determining the cold/mild boundary at this range is virtually impossible. Deep Scandi trough > Semi amplified Atlantic ridge seems the best bet currently. Colder than of late certainly, but widespread snowfall? Low likelihood from me currently.

February 13

Lets break this winter down into something a little more digestible in terms of the GSDM/teleconnections and some of the predictions that have been made based on them. I'll be mainly quoting my own posts through the winter but will add in some of Tamaras too if/where appropriate, compare that to the observed state of the AAM/MJO and the corresponding impacts on the weather patterns. I'll discuss the December & in particular the January cold spells too and try and diagnose where things went "off the rails" vs the expectations, particularly with the January cold spell.

Any bold text within the quoted sections is something I've added to explain certain variables/add context to certain things that otherwise is not explained within the quoted text itself.

Hopefully this post will provide some context on what was expected & why vs the outcomes. Perhaps those that claim it's a load of "rubbish" will gain some much needed context.

Early December:

The opening few days were cold across the UK with blocking extending up through the N Atlantic and into Greenland.

Blocking broke down rather quickly. When did forecasts first start to pick out this colder spell of weather? (I wont be focussing on NWP modelling, mostly teleconnective) here's one of my posts from November 22nd:

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What has happened up until now? A strong westerly wind burst event occurred in the Pacific just west of the dateline, this injected westerly momentum into the overall wind flow budget which sent AAM tendency surging upwards, the prolonged nature of the WWB event and transition east helped to keep AAM tendency positive for a while, this triggered a rise in both Frictional Torque (+FT) and Mountain Torque (+MT) pushing global atmospheric angular momentum (AAM) into positive territory, for a time we had a ocean/atmospheric coupling with the ENSO base state (El Nino).

AAM charts:

If you look at the right AAM chart above, we can see that some of these positive AAM anomalies did transfer northwards towards the pole, albeit not with any particular 'oomph' or real amplification (Atlantic ridge signal). Meanwhile, the MJO was slowly transitioning through a very low amplitude phase 8 which correlates broadly to a, you guessed it, mid Atlantic ridge. Note how broadly similar this MJO phase 8 composite for November is to this weekends pattern;

That is the signal for this weekends Atlantic ridge which models initially struggled with, the tropics & subtropics working in tandem. GFS initially being too amplified and ECM being too flat just to keep it broad. Going into this weekend we will see HP amplify and colder air will sink southwards across the UK. This is now well signposted within broader model outputs and accepted as the evolution going forward.

The polar jet hitting a roadblock (HP) did amplify the high into Greenland in the end, something that I hadn't expected, this was my own failing of understanding however.

That post from November 22nd went onto discuss what comes after the surge in momentum and the MJO progression continued to cycle on;

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Now.. we're seeing a reversal. Referencing the AAM and Hovmöller charts above, the WWB event has waned and the atmosphere in an attempt to rebalance momentum reverses to easterlies, this tanks AAM tendency and we're beginning to see a fall in overall GLAAM as a result. Meanwhile, the MJO will be transitioning through a relatively high (compared to recent) amplitude phase 1-2 which correlates to HP moving eastwards towards Russia and low pressure from the Atlantic taking over, broadly a typical "zonal" signal, i.e high pressure to the south, low pressure to the north, less south shifted than we've seen recently.

With all of this in mind, the GFS doesn't seem particularly plausible (GFS at the time was suggesting prolonged blocking), but the ECM solution does. (ECM at this time showed a collapse in blocking/return to Atlantic weather) I doubt the ECM has it exactly right re: details but the signal from the EPS is very much in line with everything I've discussed here. Cold next week, yes. Wintry? Perhaps in places, but a prolonged spell of high latitude blocking looks very unlikely and I suspect the GFS will end up with some egg on it's face. It's not too dissimilar to the ECM it's just probably over-doing the amplification.

(Blocking as expected, was not a prolonged event and weakened quite rapidly into early December as above re-analysis charts show)

The signal going forward through early December at the moment is overwhelmingly to one of Atlantic driven weather with the worst of the unsettled weather in the north. I suspect we will eventually see NWP outputs beginning to reflect this once the reversal to easterlies in the Pacific starts to feed back into NWP initialisation data in real time.

So.. the signal on November 22nd was for colder weather across the UK late November/early December but teleconnections, in this case the MJO & AAM were towards a return of the Atlantic. By the end of week 1 of December blocking had gone and Atlantic weather systems were across the UK. A prediction that proved right 2 weeks in advance using the teleconnections. I'd class this as a win.

In that same post from November 22nd I went onto discuss the expected progression through December in terms of the background state flagging late December/early January as a potential time period where blocking/cold weather could manifest, an attempted prediction more than a month ahead.

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Beyond that? Well, now it starts to get exciting. On the Hovmöller above we can see the next MJO wave expected for December beginning to appear in the Indian ocean. This could be more amplified, trigger another more substantial rise in AAM and cycle round into phases which support proper blocking in a hopefully a more amplified state. Late December/early Jan holds a lot more potential.

Did this prediction hold true? In terms of the AAM, yes. Through December we did indeed see a substantial rise in total AAM thanks to the MJO progressing through the Indian ocean and into the Maritimes. Unfortunately, as we know the MJO began to collapse as it passed through the Maritimes, this is something that happens fairly frequently during El Nino winters and particularly in the context of a strongly +ve IOD which, despite weakening, was in place during December.

FIRST FAIL:

The expectation of a colder late December was a fail, the MJO cycle did not progress as anticipated and due to collapsing this left little "oomph" to generate the blocking that was anticipated/mused upon. I wouldn't say this was a fail in teleconnections but a failure in forecast modelling, indeed models had been overly enthusiastic about a more amplified cycle (albeit GFS/ECM were less enthusiastic than the ridiculous output from the BOM model at the time)

Here is the complete MJO cycle, the blue line represents Decembers progression & then subsequent collapse.

This meant, late December was mild with little in the way of blocking, despite the AAM/GWO responding through December as expected thanks to the very same MJO progression. +Frictional Torque led to +Mountain torque adding westerly momentum into the atmosphere and driving total AAM upwards, as depicted here;

MJO moves eastwards creating +FT, this, via rossby waves transfers northwards increasing +MT and we see the substantial total AAM rise following these two events. The collapse in MJO however led to this late December pattern;

Interestingly, the +EAMT during December helped to trigger the minor SSW in late December. +EAMT increases heatflux into the stratosphere. So, prediction of the AAM proved correct but collapsing MJO failed in the late December expectation & indeed we missed the boat for early January, JUST!

Indeed, contrary to the belief in this thread that "People who use the GSDM never explain when it fails", I did quite an in-depth post discussing why anticipated late December blocking had perhaps failed and indeed discussed my own personal failings in understanding, you can read that here;

Early - Mid January

Through early to-mid January we saw what was technically a 2 week cold spell, the first week was largely dominated by high pressure (with a brief less cold spell in the middle) with the 2nd week of the cold spell being much colder. Here are a few re-analysis charts to show what happened;

This cold spell was the subject of much debate on teleconnections at the time which had seemingly favoured a continuation of cold beyond the 20th (when the breakdown ultimately came) with expectations very much being that wedges of high pressure could continue to deflect the jet stream southwards & keep the cold in place, despite the waning of high latitude blocking, naturally, this didn't come to pass and instead we saw a quick return to mild & indeed stormy weather with 2/3 (I forgot) named storm systems.

My post on Christmas Eve below, highlighting some excitement re: potential for cold patterns through January.

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MERRY CHRISTMAS!!

AAM tendency has gone through the roof, we're seeing a massive uptick in +ve MT, notably (and finally) a large +EAMT event which can be a strong precursor/trigger to SSW. This was all largely expected but has been delayed somewhat due to the weakening of the MJO cycle so we've missed the boat on Christmas but things are now unfolding very nicely for the new year.

AAM tendency spiking with +ve momentum now shifting polewards, total AAM is now at the highest level since July/August and will likely continue to rise on the back of the current very strong MT event.

With the MJO moving through phase 1 seemingly rather slowly, all this feeds back into an increasing likelihood of high latitude blocking developing early-mid January, taking a lag of 14-20 days into account. On top of this we have a potential SSW unfolding which could have further implications later into January depending on downwelling etc.

All in all.. despite the "delay", things for this winter are shaping up very, very nicely.

Now, some IMPORTANT CONTEXT: This was around the time a SSW was unfolding, originally modelling had predicted it to be a major SSW (technical reversal) but modelling backed away from this idea. My post on the 27th December highlighted this;

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The GFS has been gradually backing away from a SSW in recent runs, particularly last nights 00z with a fairy substantial shift away, following yesterdays 12z EPS suite.

Tonights EC46 will certainly make for interesting viewing, I'd not be surprised to see this trend away from the idea too given more recent modelling.

Ironically on the same day (27th December) I had urged caution re: a SSW disrupting what at the time teleconnectively was a favourable tropospheric pattern for cold weather patterns.

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Personally I'd prefer a SSW doesn't happen (though I would 100% take a very, very weak SPV)

We've got some very, very favourable trop led drivers coming into play during January without the help of a SSW, whilst a SSW could potentially amplify the cold signal/prolong any cold/blocking we end up getting, it could also "reshuffle" things into something unfavourable for us.

SSW's are always risky.. exciting when nothing else is going our way but always a concern in situations like the one we're looking at going into January. An extraordinary amount of luck and January could be very, very special..

So.. these two posts 11 days before the first day of the January cold spell (beginning on the 8th January).

Tropospheric & teleconnective drivers pointed towards blocking but I was cautious re: the SSW potentially making things unfavourable instead. So, yet another example of teleconnections leading the way in terms of broadscale predictions. In actual fact, during early January the mod thread was awash with comments about sinking highs and the likelihood of cold collapsing, those of us subscribing to and applying the GSDM knew this wasn't going to happen, here are a couple of posts from the 3rd January where I tried to reassure that this wasn't what was expected.

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Back to reality..

The GEFS show the broad evolution, much colder next week before slightly less cold air (at least aloft, the surface will remain cold throughout) filters in with the high re/shuffling before a second bite of the cherry later in the run as the high retrogresses.

Still a potential for wintriness in the E/SE from the initial UK based high, how much amplification still needs ironing out.

And;

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It was never “near 0” and the high isn’t “sinking”. A sinking high is a high that goes into Europe and allows the Atlantic in, that’s not what is being modelled here.

A UK based high has been favourite for days now. What is up for grabs is how much amplification can be squeezed out of it.

Indeed, as the re-analysis charts above show, we did NOT see a sinking high & we saw retrogression week 2 of the cold spell. Another win for teleconnections vs modelling.

Now, lets fast forward a little bit. We know how the cold spell evolved & the thinking that was being put forward at the time. I.E talks of 1979 and 1947, teleconnectively this was being advertised & indeed we saw some absolutely stunning modelling output in the extended at the time,remember that SSW I just talked about? That's about to bring us resoundingly back into reality. The impact of this SSW could not have been forecasted beyond a week & unfortunately, the late December SSW came back to haunt us.

Here's a post I made on January 9th.

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FWITW the 00z ECM is highlighting where I broadly expect things to head as outlined in the quoted post from yesterday.

Cold air across the UK vs incoming disruptive lows = high impact snowfall events.

I do think the ECM is being a little too progressive with the lows though, I suspect the first couple will "miss" and pass to the south, again still looking towards the 20th - 20th as a timeframe, although a day or two before can't be discounted given current modelling.

J

Just for context, here's what actually happened on the 16th & 17th:

The low missed southern England by just a couple of miles and our friend the southern Euro high was further north than previous modelling had anticipated. The conversation within outputs between the 10th - 15th was largely focussed around the tracking of that low pressure system. Zooming back out to teleconnections though and the previously mentioned SSW, when precisely did the wheels fall off? Remember, the cold spell came to an abrupt end on the 20th January, on the 9th things were ticking along nicely.

On January 12th, @Tamara posted this;

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The suspicion I continue to have is that these poleward eddies that the models propose to increase polar jet momentum, will get sheared off and this whilst this still leaves a flat jet stream coming from upstream, the shearing off process will cut off the supply to the polar jet in mid stream and force it to backtrack into the sub tropical jet. Much the theme of discussion of late.

That process, leaving some of the residual ridging between Greenland & Iceland intact and/or re-programming re-emergence of wedge type ridging and subsequent disruption of Canadian lobe energy in mid Atlantic. This leaves southerly jet kayaks to resume progress and changes the extended ensemble means that propose a flat westerly pattern across NW Europe & inflating Azores High.

It will be very interesting (as I see it) to watch and monitor if this does actually transpire.

And my subsequent response;

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Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards.

What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling.

I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens.

No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way.

Expectation on the 12th January, 8 days before the cold spell ended was for residual heights to remain around Iceland/N of the UK, it was clear the main blocking would weaken and indeed less cold air was being advertised but as Tamara explained, a southerly tracking jet was still a possibility underneath wedges.

By the 13th, it was largely game over for a continuation of cold, here's what I posted;

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I think the hope of wedges remaining to the north with colder air across the UK beyond the 20th can be discounted now, this is a very strong signal from the EPS for a return to milder, wetter weather.

The Canadian PV lobe being pulled towards Siberia (likely thanks to the recent warming) is the dominating signal and will override any potential blocking.

Late January into early February however does still hold potential for a return to more blocked conditions with this next momentum surge.

Before all of that though - A week of cold weather across the whole of the UK with the potential for snowfall just about anywhere.

I theorised at the time that the recent warming in December had perhaps reshuffled things out of favour with blocking & indeed subsequent analysis (especially when factoring in what would be a technical SSW on January 16th) would show this to be the case. The warming shifted the Canadian PV eastwards into the Atlantic sector which caused the entire pattern to flatten and pulled the jet northwards, any hopes of residual highs to the north were gone.

This is NOT something teleconnective forecasting can predict, this is not something that was foreseen at long lead times but was actually a very short term change which changed the entire outlook going forward and we all know what happened beyond the 20th. On the 18th, I posted this.

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Nothing is ever definite in weather, the atmosphere is chaotic, complicated and unless you can model every single molecule of air you'll never have 100% accuracy. However; as described above teleconnections have led the way in terms of the broader forecasts but of course that's never 100%. Unexpected things crop up (such as the SSW impacts) that you just can't account for.

Long-range forecasting is essentially a "best guess", but I don't think teleconnections/background signals are too over stated or relied upon, you just can't expect 1+2 to always = 3, sometimes you end up with 2+2 unexpectedly.

I stand by that. Hopefully I have shown in this (admittedly very long post) that teleconnective forecasting has done well throughout this winter and has signposted the broader changes within northern hemisphere patterns. What is hasn't done is tell you that it will snow in your location on a specific day, or promised snow to anyone.

Rubbish the science as you wish, but it has proved a valuable tool throughout this winter, short term changes within the configuration of the polar field cannot be predicted at longer ranges but can have a big impact on the weather/broader patterns regardless of teleconnective forcing, this, as stated, cannot be predicted or incorporated particularly well within the GSDM.

Once February is over I will do another post analysing what happened teleconnectively through this month, expectations for a cold February were high during January, evidently that hasn't come to pass.

February 13

feb1991blizzard Indeed, does make me wonder how much impact this SSW will have, current EP flux forecasts (GFS) suggest mostly EQ-Flux but these are based on the GFS det so change as frequently as that does.

I really wish we had more data available in this area by other models.

February 13

feb1991blizzard

Yes much better. As stated in previous posts I think late Feb/early March holds the best potential.

Slow moving MJO phase 7 through late Feb > early March suggests Atlantic ridging retrogressing to Greenland & eventually a west based -NAO (GFS dets currently hinting at this broader evolution in the above timeframe)

In terms of the AAM we still have a lack of poleward fluxing but with a major SSW & split there is a window of opportunity for blocking beginning to appear within the context of a very unstable polar profile.

Not convinced on anything *prolonged* given the above but certainly potential for a cold spell at the end of the month. Blocking however might struggle to become sustained in the right places for long periods.

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