Met4Cast

Rain All Night 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.

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.

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.

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.

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.

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.

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?

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.

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

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?

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

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.

They’re available here; http://www.bom.gov.au/climate/enso/#tabs=Tropics&tropics=History-to-now

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.

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.

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

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.

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.

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

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.

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.

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.

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

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.

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.