Singularity

ECM 12z edged into the south that rarest of phenomena for our part of the world: a ‘heat dome’. Characterised by a stagnant hot airmass, no plume setup involved. Can be hard to shift once setup right over an area - something almost unheard of in the UK, Aug 2003 perhaps one of very few examples. The ECM 12z didn’t take it that far; S UK is only on the fringe, not right under it. Rarely makes it beyond France and you can get a sense of why with the UKM 12z which is very similar on the broad scale yet doesn’t look likely to advance the heat dome far enough to reach S UK, due to having a little less of a ridge in the vicinity of Belgium next Mon.

It’s been fascinating to see the models accelerating the whole life cycle of the trough later this week; it now deepens quicker, pushes a front across the UK sooner, then fills more quickly giving more room for a ridge to build across the UK this weekend (thanks to the trough having stalled so far northwest). Could well be that the low Atlantic westerly momentum phase of the response to the ongoing AAM cycling was originally being brought on too soon. For a while the modelling had a very weak Atlantic jet throughout this week. Next week, some lagged response is likely to a trade wind burst across the Pacific with a strengthening jet attempting to flatten out our pattern and introduce changeable westerlies to the UK. However, signs are that burst may be short-lived with the Nino base state fighting back before long. So there’s a chance that the forcing effects of the burst prove too weak or fleeting to move the pattern on from a Sceuro ridge setup. Hence the clusters we see in the ensemble modelling. Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4905398

It’s been fascinating to see the models accelerating the whole life cycle of the trough later this week; it now deepens quicker, pushes a front across the UK sooner, then fills more quickly giving more room for a ridge to build across the UK this weekend (thanks to the trough having stalled so far northwest). Could well be that the low Atlantic westerly momentum phase of the response to the ongoing AAM cycling was originally being brought on too soon. For a while the modelling had a very weak Atlantic jet throughout this week. Next week, some lagged response is likely to a trade wind burst across the Pacific with a strengthening jet attempting to flatten out our pattern and introduce changeable westerlies to the UK. However, signs are that burst may be short-lived with the Nino base state fighting back before long. So there’s a chance that the forcing effects of the burst prove too weak or fleeting to move the pattern on from a Sceuro ridge setup. Hence the clusters we see in the ensemble modelling.

Accounting for typical bias, a lot of model runs for next Thu or Fri onwards now being either a dry E to NE flow with mid-high 20s widely, locally low 30s in some cases, or a hot & very humid S to SE flow with temps high 20s to low 30s, locally mid-30s. With the humid scenario it’s worth bearing in mind that high atmospheric moisture content can reduce the lapse rate compared to the typical value, as not only does water vapour scatter longwave radiation back down, much as CO2 does, but rising air parcels cool more slowly (water vapour has higher specific heat capacity than dry air).

Bit of a trend afoot to hold the main Atlantic low further west toward D10 which allows part of the major blocking high to drift back toward Iceland. Perhaps via interplay between a new round of lagged MJO phase 7 forcing (encourages high pressure over N Europe) and downwelling anomalies from the stratosphere that keep the AO negative and allow high pressure to locate further northwest than would usually be the case. Given I’ve long been expecting main SSW impacts by the start of 3rd week March, this is concerning for those who’d prefer springlike weather - there could be a very long spell of well below normal temps (overall day & night combined). A chance of light winds and sunshine providing respite on some afternoons, though. Original post: https://community.netweather.tv/topic/98450-model-output-discussion-a-cold-start-to-spring/?do=findComment&comment=4814923

Bit of a trend afoot to hold the main Atlantic low further west toward D10 which allows part of the major blocking high to drift back toward Iceland. Perhaps via interplay between a new round of lagged MJO phase 7 forcing (encourages high pressure over N Europe) and downwelling anomalies from the stratosphere that keep the AO negative and allow high pressure to locate further northwest than would usually be the case. Given I’ve long been expecting main SSW impacts by the start of 3rd week March, this is concerning for those who’d prefer springlike weather - there could be a very long spell of well below normal temps (overall day & night combined). A chance of light winds and sunshine providing respite on some afternoons, though.

Given the very earliest probable timeframe for any SSW impacts is 1st week March, the outlook is interesting with its continued dominance of high pressure in the UK vicinity. Historically this being so right after a SSW has some linkage to a cold pattern during the impact period, likely because a settled UK often occurs as the MJO transits phases 5-6-7 at high amplitude, which itself is a precursor to cold patterns (chiefly -NAO) a week or so later on. Any notably cold weather prior to any SSW impacts will be driven from within the troposphere rather than above. The big uncertainty has been how strongly the tropospheric/lower stratospheric polar vortex will keep the pattern flat despite that MJO forcing. Across the past 4 days the pendulum has swung away from the MJO being dominant then back toward it. Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4805659

Given the very earliest probable timeframe for any SSW impacts is 1st week March, the outlook is interesting with its continued dominance of high pressure in the UK vicinity. Historically this being so right after a SSW has some linkage to a cold pattern during the impact period, likely because a settled UK often occurs as the MJO transits phases 5-6-7 at high amplitude, which itself is a precursor to cold patterns (chiefly -NAO) a week or so later on. Any notably cold weather prior to any SSW impacts will be driven from within the troposphere rather than above. The big uncertainty has been how strongly the tropospheric/lower stratospheric polar vortex will keep the pattern flat despite that MJO forcing. Across the past 4 days the pendulum has swung away from the MJO being dominant then back toward it.

It would be highly unusual to see SSW impacts so soon after a displacement event. However, GFS has been subsequently splitting the vortex in two, so it's possible that it predicts a quick response from that, whether rightly or wrongly... such a quick downwell isn't usually a feature of post-displacement splits but the one GFS keeps coming up with looks atypical of those with a more distinct, strong secondary round of warming doing the job. So who knows, something peculiar might be on the cards. At this stage we can only wonder!

Wouldn't that be ironic, to be left high & dry when usually with displacements we're too far east with any -NAO tending to be west based. Without the MJO helping out I think the negative zonal wind anomalies would struggle to get through the mid-stratosphere. As it is, the SSW itself makes a strong MJO more likely via cooling the tropical stratosphere, and there's good model consensus now for it to be crossing the Pacific as we transition from Feb into Mar. Interesting to see EPS already exploring some -NAO ideas for the final 4-5 days of Feb. Even with an accelerating trend to the MJO projections, that's a little sooner than I expected but I can see how it could happen if the SSW and MJO both help each other out, so to speak, making it possible for high pressure to establish in the Greenland vicinity sooner than usual relative to the MJO cycle (especially during a La Nina base state when the -NAO response is usually more from phases 7-8 as opposed to 6-7). Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4803891

Wouldn't that be ironic, to be left high & dry when usually with displacements we're too far east with any -NAO tending to be west based. Without the MJO helping out I think the negative zonal wind anomalies would struggle to get through the mid-stratosphere. As it is, the SSW itself makes a strong MJO more likely via cooling the tropical stratosphere, and there's good model consensus now for it to be crossing the Pacific as we transition from Feb into Mar. Interesting to see EPS already exploring some -NAO ideas for the final 4-5 days of Feb. Even with an accelerating trend to the MJO projections, that's a little sooner than I expected but I can see how it could happen if the SSW and MJO both help each other out, so to speak, making it possible for high pressure to establish in the Greenland vicinity sooner than usual relative to the MJO cycle (especially during a La Nina base state when the -NAO response is usually more from phases 7-8 as opposed to 6-7).

The imminent now almost certain SSW has really caught my attention now because of two things: - The vortex looks to be displaced to Scandinavia which is at the eastern end of historical precedent. This doesn’t look to be the usual orientation of displacement which places the vortex above the North Atlantic & encourages strong troughs there. Instead, one more supportive of a -NAO pattern as the anticyclone locates above Greenland or close to there. Provided, that is, the zonal wind reversal propagates down, which brings me to the second thing: - Active MJO projected to cross Pacific in 2-3 weeks time. This aligns with the lagged displacement type SSW impact window of 2-4 weeks after the event. It means the MJO will be forcing the troposphere toward a -AO while the reversal is looking to ‘connect’ with it from above. So, the troposphere will be receptive to it rather than resistant. I believe this is a key factor behind some displacement type SSWs having major weather pattern impacts and others hardly any. So overall, I think this SSW, provided it occurs, has a higher chance of forcing a -AO & -NAO episode sometime late Feb through mid-March than past precedent for displacement type events as a whole suggests. No guarantees of course but plenty to keep an eye on in the modelling during the next 2+ weeks. Original post: https://community.netweather.tv/topic/98326-model-output-discussion-into-february/?do=findComment&comment=4803777

The imminent now almost certain SSW has really caught my attention now because of two things: - The vortex looks to be displaced to Scandinavia which is at the eastern end of historical precedent. This doesn’t look to be the usual orientation of displacement which places the vortex above the North Atlantic & encourages strong troughs there. Instead, one more supportive of a -NAO pattern as the anticyclone locates above Greenland or close to there. Provided, that is, the zonal wind reversal propagates down, which brings me to the second thing: - Active MJO projected to cross Pacific in 2-3 weeks time. This aligns with the lagged displacement type SSW impact window of 2-4 weeks after the event. It means the MJO will be forcing the troposphere toward a -AO while the reversal is looking to ‘connect’ with it from above. So, the troposphere will be receptive to it rather than resistant. I believe this is a key factor behind some displacement type SSWs having major weather pattern impacts and others hardly any. So overall, I think this SSW, provided it occurs, has a higher chance of forcing a -AO & -NAO episode sometime late Feb through mid-March than past precedent for displacement type events as a whole suggests. No guarantees of course but plenty to keep an eye on in the modelling during the next 2+ weeks.

From what I’ve read in the past week, the events of Jan have primed the polar vortex for absorbing vertical wave activity flux and taking a big hit. The trouble for those seeking cold UK weather as a result this side of end Feb / early March is that any resulting SSW looks to be a displacement type. Even if the majority vote outcome for where it displaces to - Scandinavia - comes about, the typical time taken for displacement-driven anomalies to propagate down to the troposphere is 2-4 weeks. By then we should be seeing AAM reaching positive territory with GWO in phases 5-6. That’s usually conducive to strong high pressure slowly retrograding from Europe to the Atlantic at the mid-latitudes, but *if* there has been a zonal wind reversal and that propagates down as the MJO reaches phase 6, then we could see more of a high latitude block take shape. This, however, would mean an extended winter, delaying spring. Some would enjoy that but a larger number wouldn’t when compared to such events playing out in late Jan or early Feb. Im not convinced by recent ECM runs suppressing and retrograding the subtropical high so much. Where does the forcing come from to drive such alignment with the stratospheric pattern so soon?

Could be one heck of a battle between the polar vortex and the MJO-enhanced Azores High looking to vacate over Scandinavia. Result, highest chance of settled weather in far south of UK, as is the highest chance of a cold continental flow establishing sometime in 2nd week Feb. A stalemate with the south instead ‘least mild’ in a slack flow is arguably the safest bet. I remain sceptical of runs that bring unsettled conditions all the way south within the next fortnight, as opposed to just occasional mainly weak fronts. That would require an unexpected capitulation of the MJO within the next week.

Quite puzzling that while the GEFS 12z shifted substantially toward recent GFS runs, the EPS 12z hardly budged, despite the det. run doing so and the control run being along similar lines as of day ten. Something's off somewhere. Either EPS are seriously underplaying the MJO influence relative to stratospheric, or the other modelling is overplaying it big time.

Nail on the head here IMO, I’ve been Tweeting along such lines for a few days. With the usual cautionary notes of course. What we’re seeing is models increasingly resolving a tropically forced pattern that will do battle with the displaced stratospheric polar vortex. If the former dominates then we likely see either a ‘Sceuro’ high (usually either mild or chilly in UK) or a Scandi high (usually chilly or cold). If the latter does instead then something more akin to recent ensemble means is more likely although probably not as unsettled in southern UK. Signals lead models and its in these situations that organisations like the Met Office have more of a propensity to slip up due to high reliance on what the models show as opposed to what the signals suggest they should be showing. Original post: https://community.netweather.tv/topic/98196-model-output-discussion-new-year-and-beyond/?do=findComment&comment=4797727

Nail on the head here IMO, I’ve been Tweeting along such lines for a few days. With the usual cautionary notes of course. What we’re seeing is models increasingly resolving a tropically forced pattern that will do battle with the displaced stratospheric polar vortex. If the former dominates then we likely see either a ‘Sceuro’ high (usually either mild or chilly in UK) or a Scandi high (usually chilly or cold). If the latter does instead then something more akin to recent ensemble means is more likely although probably not as unsettled in southern UK. Signals lead models and its in these situations that organisations like the Met Office have more of a propensity to slip up due to high reliance on what the models show as opposed to what the signals suggest they should be showing.

Frictional torques have just started responding to the highly active MJO crawling through phase 3 on route to 4. So begins the lengthy process that will lead to the next *chance* (can't be stressed enough) of a high latitude blocking episode, roughly around mid-Feb. Odds of that will be increased if the stratospheric polar vortex is near or below normal strength as per current extended model means. Otherwise blocking highs may be held more toward mid-latitudes, resulting in another spell similar to what we have now. In the meantime, a more zonal trend is indeed likely in early Feb but I'm not convinced it will be all that unsettled. A high amplitude MJO phase 3-4 on top of a La Niña background should inflate the Azores High to an impressive extent, feasibly enough to maintain an at least transitory influence over much of N. Europe including S UK. There is however a risk that the displaced polar vortex influences the troposphere enough to fire up some very deep Atlantic lows & throw them up against the strong ridging, resulting in extremely steep pressure gradients i.e. a threat of really strong wind events. Original post: https://community.netweather.tv/topic/98196-model-output-discussion-new-year-and-beyond/?do=findComment&comment=4796808

Frictional torques have just started responding to the highly active MJO crawling through phase 3 on route to 4. So begins the lengthy process that will lead to the next *chance* (can't be stressed enough) of a high latitude blocking episode, roughly around mid-Feb. Odds of that will be increased if the stratospheric polar vortex is near or below normal strength as per current extended model means. Otherwise blocking highs may be held more toward mid-latitudes, resulting in another spell similar to what we have now. In the meantime, a more zonal trend is indeed likely in early Feb but I'm not convinced it will be all that unsettled. A high amplitude MJO phase 3-4 on top of a La Niña background should inflate the Azores High to an impressive extent, feasibly enough to maintain an at least transitory influence over much of N. Europe including S UK. There is however a risk that the displaced polar vortex influences the troposphere enough to fire up some very deep Atlantic lows & throw them up against the strong ridging, resulting in extremely steep pressure gradients i.e. a threat of really strong wind events.

Interestingly it gets there via a deep cyclone running from NE of Japan to far-NE Russia, where it peaks & drives an anticyclonic wave break across the Pacific side of the Arctic. More interestingly still, the ECM 12z has the same sequence of events. Being beneath the stratospheric anticyclone, such a high would likely have some staying power, plus sufficient strength to have far-reaching consequences on mid-latitude weather patterns. A wildcard, sure, but with origins at the 4-6 day lead times which is near enough to make it worth keeping an eye on.

The wonders of Twitter! Pretty much yes, in fact when the response is highly amplified the result can be exceptionally warm yet dry as well, late Feb 2019 being a prominent example in the wake of a Jan displacement, with the widespread record highs. I expect such is also possible after a split type if factors such as the MJO are strongly opposed to high-latitude blocking high positions. If only I had the time to study what happened after each and every historical event! Perhaps I will find it at some point.

I subsequently improved my SSW analysis with better targeted 'impact periods' for the historical events. That reduced the difference between displacement and split type events with respect to temperatures across NW Europe , though the latter remain the more conducive to cold conditions. From reading around over the years, my impression is that split type events tend to produce longer-lasting, less marginal cold spells compared to displacements. Enough so that the cases where a cold spell stops short of the UK are outweighed in the mean for split type events but not displacement type ones where it balances out. Note the different story in the eastern US where displacements tend to hit harder. I believe this is due to a propensity for such events to place a stratospheric high near or over Canada, leading to an Arctic High regime over N. America. Unlike here in the UK, though, both type of events tend to result in notable cold spells (but over different timeframes - often it turns cold in the US at around the time that it becomes less cold in Europe).

With a trend to a stronger MJO presence across the Indian Ocean, it's no surprise to see a high over Scandinavia attempting to head toward Greenland. Some excellent posts on here recently linking this back to the TNH (tropical northern hemisphere) setup and how that has been the catalyst for increased disruptive interference to the La Nina event (usually, the MJO is very quiet during a La Nina). By Feb, the phase 3 response does become more (but not full-on) zonal, though not necessarily unsettled for the UK. Phase 4, on the other hand, is very much classical La Nina territory, so by then it will really depend on what the stratosphere is up to, in particular what pattern is propagating down from there, if anything at all. I concur with Eric Webb regarding SSW chance & models underestimating that - there's so much going on that fit into the 'SSW precursor' bucket. Not that I'd dare give it more than a 50% chance at this stage, though! Original post: https://community.netweather.tv/topic/98196-model-output-discussion-new-year-and-beyond/?do=findComment&comment=4792274

With a trend to a stronger MJO presence across the Indian Ocean, it's no surprise to see a high over Scandinavia attempting to head toward Greenland. Some excellent posts on here recently linking this back to the TNH (tropical northern hemisphere) setup and how that has been the catalyst for increased disruptive interference to the La Nina event (usually, the MJO is very quiet during a La Nina). By Feb, the phase 3 response does become more (but not full-on) zonal, though not necessarily unsettled for the UK. Phase 4, on the other hand, is very much classical La Nina territory, so by then it will really depend on what the stratosphere is up to, in particular what pattern is propagating down from there, if anything at all. I concur with Eric Webb regarding SSW chance & models underestimating that - there's so much going on that fit into the 'SSW precursor' bucket. Not that I'd dare give it more than a 50% chance at this stage, though!