Mike Poole

Scorcher Yes, high hangs around in some shape or form until the end of the run on the GFS, although drifts slightly northwest later. GEM 12z run is a cracker through to T240, things starting to properly firm up now:

After a pretty torrid couple of months, the pieces are starting to move towards something much more palatable. It is no surprise that this is happening now, as the effects of the significant zonal wind reversal in the strat at the end of February start to wane. The NAM index over the last couple of months looks telling: This is effectively the Arctic Oscillation (AO) but at all layers of the atmosphere, and you can see from the plot how the negative values from the SSW have continued to just sit above the troposphere and impinge our weather ever since. That doesn’t mean we’ve had constant easterlies here - not all SSWs result in this anyway for the UK locally (and I think the ones that do are becoming rarer, I’ll come back to this), but the effect on the northern hemisphere more generally has been marked. The plot also shows that into the forecast period, that reign of terror from the strat is now over it has fully downwelled - it is benign from now on, so normal spring patterns have a belated chance to take hold. A final comment re the strat, and how things may be changing with global warming. With only 6 SSWs historically per decade, it is perhaps too early to tell, but my intuition is, well, two things, a) that SSWs are becoming more frequent, and b) that, when they occur, they are becoming less likely (than the 66% rule of thumb) to deliver cold scenarios for the UK in winter. A much more settled outlook is starting to become apparent in the 8-15 day modelling, here’s the ECM clusters T192-T240: Clusters 1 and 2 (combined with 44/51 of the runs) showing high pressure in charge over the UK, how welcome is that! T264+: Clusters 2,3,4 show a continuation of the high pressure local to the UK, but the main theme from all the clusters is the lack of anything from the Atlantic, finally, with the other clusters generally having a ridge to the west of the UK, which would be a cooler scenario. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-spring-has-sprung/?do=findComment&comment=5057333

After a pretty torrid couple of months, the pieces are starting to move towards something much more palatable. It is no surprise that this is happening now, as the effects of the significant zonal wind reversal in the strat at the end of February start to wane. The NAM index over the last couple of months looks telling: This is effectively the Arctic Oscillation (AO) but at all layers of the atmosphere, and you can see from the plot how the negative values from the SSW have continued to just sit above the troposphere and impinge our weather ever since. That doesn’t mean we’ve had constant easterlies here - not all SSWs result in this anyway for the UK locally (and I think the ones that do are becoming rarer, I’ll come back to this), but the effect on the northern hemisphere more generally has been marked. The plot also shows that into the forecast period, that reign of terror from the strat is now over it has fully downwelled - it is benign from now on, so normal spring patterns have a belated chance to take hold. A final comment re the strat, and how things may be changing with global warming. With only 6 SSWs historically per decade, it is perhaps too early to tell, but my intuition is, well, two things, a) that SSWs are becoming more frequent, and b) that, when they occur, they are becoming less likely (than the 66% rule of thumb) to deliver cold scenarios for the UK in winter. A much more settled outlook is starting to become apparent in the 8-15 day modelling, here’s the ECM clusters T192-T240: Clusters 1 and 2 (combined with 44/51 of the runs) showing high pressure in charge over the UK, how welcome is that! T264+: Clusters 2,3,4 show a continuation of the high pressure local to the UK, but the main theme from all the clusters is the lack of anything from the Atlantic, finally, with the other clusters generally having a ridge to the west of the UK, which would be a cooler scenario.

But surely, this heat in southern Europe shown on GFS 0z FI has to be a necessary first step, from where we are now, before there is any reasonable chance of seeing some sustained heat here? By the way, I’m still not expecting much to write home about in the UK before the last third of April.

A good while since I posted in this thread, and the relentless dross continues… I seem to remember saying, what with the late strat reversal, that it would be the final third of April before anything settled and warm/hot took hold. It is really difficult to see any improvement before that. GEFS 12z mean at T300: Everything is continuing to go the wrong way, with the jet on a NW/SE track (black arrow), nowhere near what we need to see to set us on a summer trajectory (red arrow). I remain hopeful that the pattern (with at least some of it related to the stratosphere) will blow itself out by the end of April, two scenarios for you at that point, 1) sudden flip to warm/hot, 2) a slower burner, gradual improvement, which still means most of spring will have been unremittingly rubbish. My money is still on 1) but there’s no sign of it in the models yet…maybe see it in the outer reaches after another week?

There’s some wintry precipitation on the latest HARMONIE for the south west right into tomorrow: Looks like there’s some quite heavy precipitation in places with that, so throw some evaporative cooling into the mix, and I wonder if a few more areas might get a snow surprise overnight.

Good post. The fact that the models perform better in winter than summer in the northern hemisphere is well known and goes back ages, here for heights: There are a couple of things of interest here. First, the same thing cannot be said for the southern hemisphere, where it is much more erratic. Guessing here, but I wonder if this is due to inherent differences in the strat vortex for each hemisphere in winter, and may be related to the reason that SSWs are very rare in the southern hemisphere, and this leads to lower predictability. The second point of interest relates to the way the models are perceived in summer and winter on here, because I think most would probably expect them to be worse in winter not summer. And I think this is down to the weather most people want and look out for. In winter, people are looking for setups that are the exception to the zonal rule, i.e. the models find the high latitude blocking patterns very difficult, but that isn’t reflected in the stats becuase they are comparatively rare (now becoming exceptionally rare it seems!). In summer, people are looking for the build of high pressure over the UK, and this is usually modelled well in advance because it involves a slow moving high local to the UK, these are sometimes even seen coming as a 1030 hPa high on the ensemble mean at 10 days away and usually count down. But here, the good prediction locally is the exception to the rest of the northern hemisphere, so again the stats don’t show this.

Summer8906 SSW on 21/2/89.

WYorksWeather Interesting SST anomalies there. I would say there is some work to do to get into a good position for summer from there. I am expecting an abrupt change to warm/hot weather to manifest itself at some point, but I would not expect it before the last third of April. The reason I say this is due to the SSW, with a signficant reversal of winds in the stratosphere providing some continuing influence as the effect continues to downwell - the renewed westerlies predicted above at 10hPa will act to force this and we probably have to wait it out before the more benign spring patterns can take effect. By that time the sun will be much stronger, and, as I say, an abrupt change is possible - at which point the SST pattern should start to change as well. So spring very much a slow burner this year, which is in contrast to the years when the strat vortex merely peters out into the final warming without a late SSW, and spring patterns can take hold much earlier.

I think a UK high as per GFS 12z around the 8-9 day mark is one plausible evolution, but it is as part of a transition towards blocking further north later as the effects of the SSW start to have some impact, so if it happens I don’t think it will herald the start of a proper settled spell unfortunately. And there are other plausible evolutions that don’t have the high pressure go through the UK anyway.

It is a fair question. The evolution up to and after a SSW seems to be predictable on a much longer timeframe than other things affecting our weather in the winter. And when that includes snow for us at some point during that evolution, everyone is interested to discuss it! But in recent years, BFTE18 excepted, there have been interesting evolutions in the stratosphere but they have done nothing to halt the run of diabolically poor winters. This year, the strat has been absolutely fascinating, and quite different to many recent years, and yet it has done nothing to help promote any decent cold spell here. It is far too soon to say whether this has just been down to bad luck, or whether the bar to clear to provide the forcing necessary to get blocking in the right place is now getting so high with climate change that SSWs are going the way of favourable MJO phases - i.e. just don’t seem to deliver any more. Time will tell… summer blizzard I agree. It looks to me like there is perhaps more of a signal for some high latitude blocking in the 10-15 day range on the models in the last day or two, but it looks a bit anaemic - so the SSW probably will have an effect, but nowhere near enough now to turn the clock back to winter (for those who hanker after that!) but also, as you suggest, will still probably manage to hinder the progress of decent spring weather that most now want. Ho hum!

ECM clusters not offering much insight out to day 10, just one cluster from T120-T240. But in the extended we have this from the 12z: Cluster 1 looks fairly standard +NAO, i.e. wet. But the others look moe interesting, cluster 2 has a weak block just north of the UK. Cluster 3 goes for the cross polar ridge, as per the GFS 12z. Cluster 4 develops a strong block just south of Greenland.

Nick F Interesting. I really can’t see the 1/250 year event! It has been a very odd year in the strat, but both the first and second SSWs were by the barest of margins. There is not enough time elapsed since the second one, for the third one to count as a separate event as I understand it (needs 20+ days westerly winds in between) - but there is no denying that the latest one is by far the most significant reversal - and it may have some effect later into spring - but I can’t see it being anything spectacular.

GFS 12z is interesting in the second half of the run, from T192: First, a welcome period with high pressure in charge over the UK, which would be long overdue and pleasant. Right at the end, the NH pattern looks very much as though the SSW is having an effect, and splits the vortex. GFS has been playing around with stronger blocking scenarios towards the end of the month, so one to watch as we count down to Easter.

TwisterGirl81 There are about 6 SSWs every 10 years. Whether that is considered rare or not probably depends on the point of view - but my post was about the training dataset for AI models, and for most of that training dataset there won’t be a SSW happening.

My bet is the AI models won’t catch on to any post-SSW patterns quickly. Given that SSWs are rare, it is unlikely that the AI models will suss the evolutions with an SSW in play from the current pressure patterns over the evolutions without, from the training dataset that they are based on. No sign on the latest AIFS run, of the kind of pattern that GFS has been playing with in deep FI, T360:

I’ve been wondering for a couple of days now whether we are starting to see the effects of the strat reversal now in the extended clusters: I think if we do, the likely area for blocking appears to be Greenland, as shown in clusters 2 and 4, maybe also 3 by day 15. One to watch, if this does happen it will be against the backdrop of the flabbier pressure patterns of spring rather than winter, and of course very late for any cold, but if the right pattern evolved, an interesting Easter weekend is still very much a possibility.

Well looking at the zonal winds, I’m don’t think the required 20 days has elapsed since the winds went back positive at 10hPa 60N after the last ‘SSW’, (19th Feb looking at this chart). So the upcoming evolution should be recorded as the aftermath of that SSW, not a new one. Which seems wrong, but I believe those are the rules.

Rain All Night I think it probably will return positive before a final warming in April. A period of reverse winds rarely lasts longer than a month, and so there’s still time left to return positive before the vortex peters out for the summer.

ECM clusters this morning T192-T240: Cluster 1 hangs on to a continental flow to the UK to T240, the others essentially a return to westerlies. The key thing seems to be the low out in the Atlantic at this time, if it is kept further west and shallower as cluster 1, then the block is stronger and the Atlantic held at bay. The zonal wind reversal in the strat has now happened, and the peak reversal about -14m/s expected in about 4 days - it will be interesting to see whether there are changes in the week 2 modelling as this goes through. If it is having an immediate effect, I would expect to see the block showing more resilience in runs to come. If its effect is still some time down the line, that probably wont happen.

ECM clusters T192-T240, there are just two options this evening: Interestingly, the rain fest of the GFS is in neither of them, so that model seems to have gone off on one this evening. Cluster 1 (contains the op, 32 members) stronger block, maintains the flow off the continent until T240. Cluster 2 (19 members) brings in a southwesterly flow courtesy of the low in the Atlantic. My impression is there’s still quite a bit of uncertainty but it seems to me like gradually, the runs with strong Atlantic influence are reducing or getting pushed back - slightly - it isn’t a strong signal.

Quite a difference as far as the UK is concerned by T168: The first low goes under according to GEM and UKMO, the next low, UKMO it’s a case of ‘to be decided’, the GEM starts to inflate it, the GFS is well on the way to a wet and mild thereafter. So UKMO this afternoon with the better block and cold option, with chance to add.

Very cold run, great alignment, from the GEM this morning, T180 GFS chalk to the GEM cheese! Small margins earlier make a big difference to the UK blowing mild or cold.

ECM clusters T120-T168: Support for the easterly on this timeframe growing, with clusters 2 (contains the op) and 3 - both take the low well south under the block. Cluster 1 more inclined to hold it out west, but this is the solution on the decline, I think. T192-T240: I think this timeframe is still work in progress! The UK rather in a no man’s land that could go one of a number of ways. More runs needed…

Rain All Night If each run takes 1 minute, 1000 member suites are easily possible…