Shuttler

I don't want to be a spoiler here but I think the chances of a split SSW this winter are LOW. It really is only the split SSW that has the power to override the inherently mild Arctic Sea Ice signal this winter and give us the HLB and -NAO most people crave for. A displaced SSW rarely delivers sig cold for Europe and can actually enhance the westerlies in many cases. My reasoning; 1. wQBO + low solar combo is not ideal. When looking back a split SSW occurred 3 times in the 15 winters that combination was in place in the last 60 years. Giving a roughly 1 in 5 or 20% chance. 2. However, we are at risk of transitioning to a eQBO before the end of the winter, such are the pecularities of the QBO these days. But, looking back we really need to be 'entrenched' in that eQBO phase well before the winter to give max attack on the PV. Most eQBO split SSWs had transitioned into the eQBO phase by at least the summer before. 3. Aleutian low + Scandi block is the ideal combo for wave breaking and Strat disruption. But, approx 10th Dec onwards we're moving back to the negativePDO/La Nina base state with Aleutian blocking/high pressure, and in 2-3 weeks time the Barents-Urals block is also likely to weaken (latest ECM46 confirms both). 4. There is no sign of a very active MJO phase, just weak and Phase 4/5 next couple of weeks (maritime continent). Again, a precursor to a split SSW event is often a very active MJO wave running from Phase 2/3 thru to 7. So I have an inherently mild winter in Europe with any cold largely limited to NW'ly polar interludes rather than any beast from the east type stuff. But, I could easily be proved wrong.

QBO has just transitioned into the easterly phase (at 30hPa). Looking back at analogue years, coupled with low solar, when that happened - 4 in total. 50% of these saw a split SSW in the March which brought resulting cold in Apr (and May), the other 50% failed to register a SSW at all. In most of the low solar years we've seen a split SSW event the QBO transitioned well before the winter i.e. at least the summer before. So by the winter we were firmly entrenched in that phase. It's looking less likely we'll get a split SSW in time to bring the sig cold this winter season (i.e. beast from the east type stuff) but perhaps for part of the spring, though of course less significance for markets, etc. Otherwise we remain at the mercy of low Arctic sea ice driven patterns, and as per most winters since 2010 outside of split SSW events. As some have alluded to on here, I think we may have a greater chance of split SSW induced cold next winter providing QBO remains E'ly.

I don't see much downwelling of lower heights from the Upper Strat in the past 3 months or so. Compared to earlier this year for example, as is fairly evident I think from the time series plot. Just trying to understand what causing the perpetual positive GpH in the lower Strat and Trop (and Arctic blocking) since Sept.

Hi. Can the more learned on this forum possibly explain why the Trop and lower Strat has been so de-coupled from the Upper Strat since Sept ? And set to continue for much of Dec by the looks of latest Ens and weekly fcasts (ECM46). I expect phases of this but it seems unusual to see such longevity in the winter half of the year. I'm trying to understand what may be driving the perpetual Arctic blocking and -AO signature in the absence of any sig downwelling from the upper Strat. Are we looking at a greater influence from the reduced Arctic sea ice or SAI this season, perhaps ? I do wonder if a split SSW (as I'm expecting this Jan) would break the Strat-Trop de-coupling we're currently seeing, though of course this is likely to induce further Arctic blocking for the second half of the winter. Any suggestions appreciated - with the ultimate aim of improving seasonal guidance !

Sorry, didn't mean to post this here - please delete topic

Hi. Can the more learned on this forum possibly explain why the Trop and lower Strat has been so de-coupled from the Upper Strat since Sept ? And set to continue for much of Dec by the looks of latest Ens and weekly fcasts (ECM46). I expect phases of this but it seems unusual to see such longevity in the winter half of the year. I'm trying to understand what may be driving the perpetual Arctic blocking and -AO signature in the absence of any sig downwelling from the upper Strat. Are we looking at a greater influence from the reduced Arctic sea ice or SAI this season, perhaps ? I do wonder if a split SSW (as I'm expecting this Jan) would break the Strat-Trop de-coupling we're currently seeing, though of course this is likely to induce further Arctic blocking for the second half of the winter. Any suggestions appreciated - with the ultimate aim of improving seasonal guidance !

Latest 00z Ensemble Mean output for days11-15 (23-27 Aug) confirms the likelihood of High pressure dominance, and as we stand centred close to the UK & NW Europe. Breaking it down into individual days for each model suggests this will run throughout the period, indeed the transition point to this pattern seems to commence around the 20th. There is also a consensus on above average temps through this period, though anomalies (both 850 and sfc) are not great, around +1 to +2C overall. So, yes we seems to be heading into a drier, lighter wind and very pleasant possibly sunny phase for the latter third of Aug, but at this stage not a major heatwave.

Hi, I am wondering if the current supremely negative AO (including the outlook for the next 2 weeks+), HLB and the relative mid-latitude cold (including much of Europe), is a continued hangover from the SSW we had in early Jan, and the resultant displacement/split of the PV ? It looks to me like to Strat PV has returned to a nearer normal state, and with downwelling this may have brought us a greater incidence of +AO by now. Or, is the negative pattern more of a response to MJO Phase 7 or 8 ? Just trying to understand the possible driver of this pattern. Thx

I'm sure you'll be pleased to hear: the ECM Ens 240-360h is not as bullish as the weekend models, which were returning a re-intensifying PV polewards. The latest run has back-tracked to a large extent, and now maintains a split vortex (in the mid-upper strat) with centres of action in the vicinity of Canada, as well as close to Siberia, and polar ridging from both the Pacific and Atlantic sides, albeit weaker than currently. There also appears to be further warming from the Canadian sector into the polar region, totally oposite to previous. The GFS Ens output at this timescale does something very similar, with both vortex centres in a similar position and also in a very weakened state. This adds weight to the expectation of no rapid return to 'normality' in the near future, and as picked up by last nights ECM 32dayer, which maintains the negative pattern and Greenland blocking through to first week of Feb, and relative cold/spells of snow for UK and much of central/northern Europe throughout.

I only see the Ens mean for a couple of levels, namely 10 and 50mb. I'm pretty sure full subscribers to the ECM output can get to see each individual Ens member, as well as Ensemble clusters. It would be fantastic to see this, including the control run and Ensemble spread, as I see for the tropospheric output. Now, regarding the accuracy: the Ens forecast from 15days ago nailed the current situation extremely well i.e. splitting vortex with centres over Canada and Siberia. And also got the SSW timing spot on. Will be interesting to see which of the GFS or ECM will be closer to eventuality in 15 or so days time. I've done very little verification of the ECM Ens at these timescales, so can't vouch for reliability against the GFS Finally, a strange and probably very obvious question, but can someone tell me what 'FI' is short for. I've got a rough idea, but can't put the words to the acronym !? Got to get this off my chest.....!

The ECM Ens for 240-360h declines the cross-polar ridging, allowing the more intense PV over Canada to gradually edge back towards the pole, though by 360h (21st) it's not quite pole-centric. This would seem more likely going into the last week of the month. The split part of the PV over Siberia gradually loses its identity. Polar temperatures at say 10mb are back below seasonal norm in this period, with no evidence of any further upper strat warmings. Just want to add my congrats to all involved in this superb, informative thread....

I only see data for a couple of fixed levels from the ECM Ens, namely 10 and 50mb. I can't answer your query, but have quizzed my sources and will come back if/when I have an answer. For information, the ECM Ens at the 10 and 50mb levels between 240 and 360h maintains a split PV in pretty much the same locations as the op at T240 i.e. Hudson Bay and Siberia, with no great re-location in this time frame. We therefore see the Polar High extending through from the Aleutians to Greenland/Iceland throughout.

This doesn't tie in with the GFS Ens mean at this point, which maintains a displaced vortex N.Russia, and closer to expectations. Also, the warming moving through the China/siberian sector is too aggressive. You can never rule this scenario out, but seems unlikely at present.

Sorry, meant since 00z Wed 19th............!

Has been a consistent pattern for the last 3 ECM Ensemble runs, since 00z Tue 19th. Bear in mind it's developing towards the end of the run, so can't really check before that. I suspect there is a marked drop off in accuracy and consistency of the GFS strat forecasts, be good to see some verification if anyone has a useful link. ECM Ensemble mean less prone to swings between runs as you'd expect, but of course a more muted output at this timescale.