fergieweather

Indeed so. Mixed signals then emerging in latest output onward into 1st half of December, re anticyclonic v cyclonic and milder v colder phase balance.

I assure you I'm hiding nothing... no magic bullet is hiding in eg GloSea5. The dice remain quite evidently loaded towards a higher risk of colder weather into December, BUT we're no clearer on the scope of how this may unfold and nothing in the Tues 12z ENS up to 30 Nov necessarily offers the clearest route by then, nor for that matter any guarantee that it will even be particularly cold at that stage. Plenty of days to get through this month and see the route of travel ahead...! :-)

The period by late Nov / early Dec is certainly not without interest, but whether we see technical SSW is another matter. In a way it's by-the-by, because the underpinning signal is conclusively for an exceptionally weakened/disturbed PV into that period and hemispherically, the 300hPa GPH fields really couldn't paint a picture more foretelling of the enhanced blocking potential.

Correct, and (as witnessed last year, in Jan if I recall), due suspicion - but not outright discounting - should also focus on GEFS leanings to showing marked zonal reversals/SSW's into extended range unless and until firm, decisive support exists in certain other key stratospheric modelling.

....unless, of course, the stratospheric PV suddenly goes on steroids later Dec/&/or on into Jan and we see a marked switch from an exceptionally weakened state into early winter, to a stronger than average one thereafter. Caution needed re extrapolating current PV status as definitely characterising how it will behave all of this winter...

Yes, I've seen no compelling signal for anything other than transient upland snow on leading edge. The windstorm potential is the main issue, really, but 30% prob at this range of a flatter feature running further south, with strongest winds for France.

We have the 10d meteograms already for 12z. The DET runs right at mildest end of T2 spread, but this sort of marked disjunction versus ensembles has been typical of some recent runs. The most important signal remains the conspicuous spread from Sunday onwards. As I've mentioned in previous posts, we're not expecting speedy resolution to this kerfuffle and raft of outcomes post-Sunday (and rest of Nov) remain possible.

Yes, I'm not suggesting 'mild' given numerous pointers in the other direction. What I'm stressing here is some people misrepresenting current signals as automatically equating to *profound* cold and suggesting via twitter that it's the outcome we are seeing.

Yes, and it can't be dismissed given ongoing ENS spread in EC right through rest of the month. If there is to be a swing towards more markedly cold conditions rest of Nov and early Dec, we're yet to see that evidence emerge decisively and unambiguously. It's unsurprising given huge uncertainty even by Sunday (some threatening looking output, yes, but could all prove a total non-event) and gulf in direction in 850hPa temps thereafter. Whilst extended range output into 1st half Dec is very interesting, we shouldn't automatically equate those signals with profound cold a la 2010, or blizzards, or any other unsupported extrapolations I've seen appearing on twitter. Given nearer-term huge uncertainty, the prospects further into the 10-30d period are really no clearer... at present.

Worth adding that latest EC Monthly again raises GPH/MSLP to our NW with mean N'ly flow as we switch into Dec. Interestingly it then also develops (2nd week Dec) quite marked -ve GPH anomaly from Scandinavia to the UK, coupled to equally marked -ve MSLP across S UK and directly eastwards into adjacent NW Europe, with attendant appearance of +ve PPN anomalies by that stage. Fascinating times indeed.

The cluster 1 of n=27 (bear in mind there's inevitably individual variance between representative members in each cluster) has low heights to our NNW between UK and Iceland; +ve GPH/MSLP to our NW/Greenland and another focus to our NE/Scandinavia, ie effectively as an omega with mean NW-W flow. It's blocked, just not blocked in same guise as per cluster 2 (n=)24, which also has the strong +ve GPH anomaly to NW but also across Iceland, with Scandinavian trough/low pressure to our E and a mean flow from N-NE/E. Of course, the individual clusters contain members with flow between these main signals (virtually every compass point is signalled to some degree by 28th, with T2 spread suddenly more marked then, despite majority below avg) but on the lead-up through period 23rd to 27th, it's noteworthy how most members fall into a broad flow between N to NE. We shall see. EDIT: ENS plumes of both EC Op and new EC version still agree on marked spread developing Sunday 20th onwards. Watch this saga run and run.... ;-)

...and by 29th, 27 members have us in a chilly cyclonic NW-Wly; 24 in blocked, cold N-NE'ly.

Not mentioning yet as support still inconclusive. We've seen a signal for potential gales/severe gales for a few EC runs (ENS) lately, as I mentioned in a post a few days back, but cursory inspection of 12z stamps suggests circa 15 members running-up something of similarly bothersome look to the DET & CTRL, smack across the country. Many more show the deep-ish cyclonic outcome but with typically with more northerly bias to their genesis. A few run south of UK (just). Ensembles into 15d still trickling out.

Not always the case. When the signal for significantly colder temperatures is overwhelming, it can indeed be reflected. Take GloSea back in Nov 2010, for example: ...and equivalent probabilities:

Yes, it's been the similarities between ECMWF and GloSea 500hPa GPH and MSLP anomalies that grab the eye foremost (GloSea's respective ENS mean fields and +ve anomaly probabilities below).

No, not publically.

Now look at the ENS means for 500hPa and MSLP.... & bear in mind the potential for 3-month anomalies re temperature etc to be skewed (because of ongoing uncertainty re outcome 2nd half of winter). (PS also bear in mind the consistent signal has been for blocked/colder propensity into *early* winter... indeed as Adam Scaife recently blogged, to put perspective on the signals, this does NOT necessarily reflect winter as a whole; moreover the signals have not strongly leaned towards 'very cold' category in GloSea ENS, merely a bit colder than average, with higher potential for E/NE'ly outbreaks. The latest output remains consistent and shows strong parallel to latest EC Seasonal in broadscale pattern).

At present there's really little change to the dichotomy that has prevailed for a few days now and if anything, the 00z runs have exacerbated the uncertainty more (versus some continuity that seemed to be appearing prior). This weekend remains the 'fork in the road' of the medium range dichotomy: will NW Europe remain under blocked and generally colder conditions moving ahead, or will we move to a somewhat more progressive story of ridge-trough-ridge etc into the 15d period, bringing an oscillation from mild phases to colder and so-on? What *does* have higher confidence, in broadscale terms, is the clear tendency towards a very convoluted upper pattern hemispherically, making it prone to amplification and blocking. With -ve AO/NAO the preferred outcome on into December (and GloSea5 repeatedly and unwaveringly going for broad +ve MSLP anomalies across the UK into turn of month and a wholly blocked pattern into early Dec), there will come a tipping-point where we see better continuity in ensembles, but we're certainly not seeing that with any great faith....yet.

No 'train' in sense of last year, BUT continued signals for some deep cyclonic outcomes (a minority of which would yield gales / severe gales) from a broad W/NW flow bias from next weekend on into following week. The cyclonic signal, whilst still a majority in clusters by w/c 21st, are juxtaposed by then against a minority that go fully blocked. As I noted earlier, these latter solutions (dominated by a more NE/E flow bias) then become more numerous out into the 15d period (= last few days of Nov), by which stage GloSea5 switches to it's more +ve MSLP signal on into turn of month. So, considerable spread post d10, and we are entering a finely-balanced phase where I'd expect model volatility (and doubtless wild swings between mobility v blocking) to become very apparent for outcomes ca. 20-21st onwards.

ECMWF Deterministic by T+240 sits in a smaller cluster (n=10) with a focus amongst ENS towards colder patterns thereafter. Whilst confidence is invariably low on this extended range period, it's nonetheless noteworthy how we are seeing increasing signs now of clusters emerging that tally more broadly with the repeated signals from both GloSea5 and EC Monthly for a blocked tendency characterising the opening of December.

Yup. I see WSI have tweeted the latest EC extended range and the resurgent blocking signal I mentioned, for those who otherwise can't see that output.

Worth adding that we've seen strong agreement emerge now in past 24hrs for the cold outbreak later next week. We see a tipping point then in latest EC ENS: A direct split between those that go on to stall the pattern, thereby maintaining the cold cyclonic feed; versus those that revert to something a bit more mobile. By final days of Nov, the upturn in colder (N/NW or broad E'ly) solutions is quite striking v earlier runs, which may offer credence to the colder cyclonic offering for early Dec in latest EC Monthly. But we must remind ourselves that's still about a month away...

Beyond that (to 12 Dec), the run develops +ve PPN anomalies consistent with a cyclonic broad northerly flow (N/NE Scotland/England; N Wales). Weak signals as would be expected at this lead time.

Well, latest EC Monthly still broadly favours the northerly quadrant outcome through early Dec, with troughing E of and across the UK; +ve heights to our NW. So, a general consistency, but we are of course still only at 10 Nov. Early days...

Gotcha. Yup, admittedly not 'cold' as many on here might define the word.