Singularity

Just seen the +96 which makes the difference clearer than the +90 I was posting about .

I'm not so sure; LP has cleared more cleanly (no shallow feature trailing behind) and the LPs over Canada are closer together which means a better chance of them merging further west in 2-3 days time, which would help going forward. There is also marginally better alignment of the flow in the western flank of the ridge but that's very slight indeed. Still - even small deviations have impacts that grow a lot over time...

Hello - I've not seen strat for ens yet today but I think he was saying 'im just showing the op here' rather than that the ens show no support - most likely ens are more mixed timing-wise which dilutes the signal too much to clearly show what he wanted to. GEFS reportedly more progressive than EPS with strat warning as of yesterday evening - would be surprised if much has changed

https://kachelmannwetter.com/de/vorhersage/2643743-london/ensemble/euro That's very neat, thanks Sebastiaan

With the Canadian vortex lobe looking to try and invade Scandinavia it seems that some form of 'lean' of the ridge toward the Svalbard area is going to hard to avoid, in which case there's the potential for a period of NE to E flow if the blocking can put up a fight in that area. I'm watching out for any signs of LPs splitting away from the Canadian lobe as opposed to the whole thing shoving its away across. Gradually accruing low heights in the Scandi/NW Asia region without wiping out the blocking. For all we know, the ridge could still align better and give us the almost-cross-polar ridging via linkup with the Arctic High, but this really is the ultimate outcome representing the best of luck having gone our way. Even if the cold spell doesn't make it beyond Thu/Fri next week, it seems the blocking will be sufficient to seriously perturb the stratosphere. The usual slowing down of the progression toward an upper-stratospheric ridge making its way poleward is in evidence in the GFS runs of the past few days, but this by no means represents a reduced chance of occurrence; I have seen plenty of these slowed down before and they've tended to come through in the end - but you see, that's tended to be during Jan-Feb, when the delays place the response within an increasingly undesirable time period as we head away from the prime winter weeks. On this occasion, time looks to be on our side, which makes for a nice change .

The 12z GFS elongates the strat. vortex more than previous runs and even pinches off a secondary trough/lobe for a time, but having moved into lower-res ('FI'), this secondary feature moves east and merges with the main lobe again and by some unpleasant irony this period of extra elongation seems to slow the rotation of the main vortex around to Siberia and hence the progress of the warming emanating from above E. Asia. In light of this possibility, I really do hope UKMO/GEM are right to maintain a better angle of WAA in a week's time so that the tropospheric blocking pattern can continue to put up a good fight while the stratospheric pattern re-arranges itself into something more helpful for us longer-term. Glad that UKMO is on the good side here.

Lovely tail there

Broadly represents what's possible if the vortex is actually split as per the direction of recent ECM runs

I suppose it's not surprising to see some issues with the 850s given how far north the low travels at the weekend; this means a period in which we both see less cold air journeying north and very cold air not journeying west or southwest toward us. A classic southward adjustment would do very nicely. I know - I'm asking for a lot and it's only early doors!

So as fortunate as the ECM 12z with the upstream amplification, it's also very unfortunate with the way that first the weekend storm stalls out for about a day while so far N. (mixes out a lot of the cold air before it can reach us), and second the associated trough drifts just a bit too far east for the slider low to merge with it and turn the flow to between NE and N across the UK and pull that cold air across, rather than drop down to the S. of the UK and produce a dank easterly with spells of rain. Goes to show it's not just the ingredients you have, but what you do with them! If the trough was a bit slower moving east, we'd get the merger but the cold would mostly go west of us at least to begin with, so the best upgrade would see the sliding low made faster. ...and that, everyone, is how you sniff at some very interesting weather patterns for late November Even with all these issues, the run still finishes with some decent WAA heading between Iceland and Greenland and only a weak trough on the other side of the ridge, which should mean the ridging N of the UK gets at least a small boost, helping to keep that cold airflow in place across the UK. Such details are just for fun at this range, of course

I wonder if we could have an 'eye-rolling' reaction option to model development posts as well . Wholesale movement is indeed what GFS goes for, but compared to the 06z it also makes more of the usual troublemaker in these situations; residual segments over Canada. THis being why I'd rather see disrupted troughs breaking off and being shoved about a bit by the blocking high(s) that the vortex is looking to displace. We've rarely managed that in the past half-decade though. Liking the stratospheric warming on this run. That makes it two 12z runs in a row to have made the most of the signal. Coincidence most likely but I'd like to see some of the other runs getting in on the act. As much as this seems slower in terms of the progression compared to the last two ECM runs, it does at least represent an agreement over the general theme of giving the vortex a hard time during late Nov and going into Dec.

Oh yeah... it's GFS going into lower-res so what was I thinking? Of course it will, in response to the stratospheric vortex displacement, look to move the whole polar vortex across to Scandinavia or thereabouts rather than breaking smaller pieces off and feeding them across. Not to say it can't pan out that way, though!

Cheeky little ridge appending itself onto the Asian blocking high with cold air stagnating over Scandinavia. If that trough moving through Greenland could disrupt and slide as they often do after passing the high terrain...

I wonder if it's taking the Mickey... If I'm thinking about this right, there's a good flow alignment on day 9 for some wave breaking across Greenland, using the jet stream as an approximation;

Yes this does look pretty unpleasant for the NW with the rain band becoming quasi-stationary along a line pretty much as visible in the chart you've shown, leading to daily totals (as the Met Office outline) of 25-50 mm widely and higher (perhaps much higher with 100 mm mentioned in the CF's assessment) than that on west-facing slopes of high ground. With that event on the way, I'm hoping UKMO is wrong to make so much of the Fri-Sat low and track it so far north; that too may be a large quasi-stationary front type event capable of producing similar daily totals. GFS 06z has across the SE but not for too long, and ECM 00z only grazed the SE corner, so there's hope. Very interesting signs longer-term have developed over the past 48 hours. Need to see them maintained for a few days before properly raising the bar of expectations.

Thanks all - wow, talk about letting the cat out of the box! Trying to comprehend what that might lead to down the line... actually, I'd best not waste my time on that while it's just the later stages of a single run!

I don't recall seeing so much stretching and even near-splitting of the vortex at 30 hPa when the movement toward Siberia was in the 10+ day range? The warming and more importantly height rises in tandem have raised my eyebrows and BA's recent tease suggests they may well shoot off the top of my forehead tomorrow morning...?

Yes, unfortunate that the main trough drifted so far NW as otherwise those shortwave lows splitting off the disrupting trough could have made for some interesting interactions with the cold PM/AM airmass. Might be less of that trouble going forward from the UKMO 12z +144 chart though; The trough by us is further east here compared to GFS, but at the same time, the one SW of Greenland is not disrupting anywhere near as much. So a mix of wins and losses perhaps - making it unclear how things would really unfold during the weekend. More runs needed... maybe many more!

Whole pattern has jumped west. Looks better than 06z for the extent of trough disruption and heights are higher to the NW. Awaiting UKMO +144 with interest to see if upstream sees any similar adjustments.

Wow, that's even more different than I just anticipated in my edit on the earlier post . Not clear if it will achieve the split away of the SW extension of the trough like the 00z did, but an alternative of shortwave lows running into the base of the deep low to the NE would be even more interesting, though probably not in the south unless you're a fan of heavy rain.

Nasty little nipper in the flow from the GFS 12z for Wed evening. Any further east and it's not just the likes of Sidney facing some serious weather...! Could easily disappear again on the next run, of course. Secondary low on the SW side of the trough resembles the 06z more than the 00z so may see that hang-back of low heights to the SW again, leaving any cold weekend conditions down to the next trough off the U.S. to be slow and/or disrupted enough to permit mid-Atlantic ridging. Okay yes - as of +120, the hang-back is occurring. Could be a full working week of very balmy temperatures with the mid-teens being reached somewhere or other on each and every day! Edit: On the other hand, the UKMO +96 just updated and bears closer resemblance to both its 00z and the GFS 00z; We'll just have to wait and see how close to the UKMO 00z that run turns out to be.

That trough disruption to the west Fri-Sat of the GFS 06z is just the sort of thing that, during periods of increased westerly progression, ensembles tend to mask in the mid-longer range, even in cluster format, due to the differences in timing and placement of what tend to be fairly short-lived resultant ridges. So - it rings an alarm bell for me; one that says 'short-notice cold snap potential'. Not saying it's likely by any means, just that it's possible enough to cause an undesirable level of uncertainty. A common situation in the winter season of course!

When you consider just how much GFS loves to consolidate the polar vortex around Greenland, it's encouraging to see it failing to maintain such a setup even in the further reaches of lower-res ('FI') when climatological bias usually shows its hand the most.

So if I'm reading the above right, the interaction of the approaching polar vortex with the preceding blocking highs across N. Asia is primarily or entirely responsible for the vertical wave activity flux and subsequent warming in the upper stratosphere? Busy with other matters today so leaning on others to do some of the thinking for me - I hope you don't mind - cheers!

On and on it goes. UKMO not as door-closing as GFS as of day 6. It does feel like we're just picking out of a hat filled with ensemble runs. Typical of situations where slight differences in the placement of disturbances within the trough circulation at just a few day's range have drastic impacts down the line. It was on day 3 that I could see GFS leaning things less favourably than the 06z, leading to an expectation that this run would evolve into a ripe pile of dog eggs. If it wasn't for the displaced subtropical ridge over Europe, it might yet have been rescued in the midweek period, but instead the trough was held too far NW and so a ridge was unable to establish just east of Canada and hold the progression up. Not sure the Euro ridge would have been sunk even then, though. The UKMO 12z is tricky to gauge as of day 6, for it has a bit of trough disruption going on with the one SW of the UK, with signs that a shallow secondary low might peel off into Europe, but then it also has a very vigorous low by Canada which may simply power east as it engages with the cold air passing by to the N of the UK. That threat is clearer in the traditional view; It is getting to the point where I'd just like to get past this tricky period; this extreme level of uncertainty is a drain on even the most experienced of meteorologists; I expect there's been some stressful days at the UK Met Office given the lack of coherent movements in the model output either toward or away from their recent outlook summaries for the middle to latter part of next week.