fergieweather

Just to explain: The wording of the UKMO forecast is a nod towards latest GloSea, which (after the mid-late Dec shenanigans and slight bias towards blocking) then leans marginally in favour of zonality into early part of Jan.

Or in this case a clear split between the more mobile, milder outcomes versus those that are blocked/colder. As UKMO suggest, what may actually unfold is a hybrid of both: "At this stage, a return to a more blocked pattern is marginally favoured through the second half of the month; however this may manifest itself as slowly evolving transitions between more settled, colder and unsettled, milder periods."

Yes, but they are used seriously but NOT on pure basis of an anomaly chart. Inspection of the individual stamps is essential. Anomaly fields are effectively meaningless viewed in isolation.

UKMO point-out the increasingly bimodal distribution of members in ECMWF Monthly by week 4. This explains the sudden appearance of bland temperature anomalies into that lead time, through a process of cancellation. Interestingly, latest GloSea5 also goes bimodal into same period, with the result that both above and below avg temps are signposted as more likely than climatology and confidence remains very low.

Well, the issue we just discussed at work is how evidently *something* is driving the similar pattern to resolve between those models, but we watch and see...

Hmmm. It's an interesting quandary, given how EC Monthly today goes ultimately (after a pathway effectively as presently signalled in op models) towards a solution mirroring seasonal output pattern in EC Seasonal & GloSea (by late Dec--early Jan, indicative of a broadly NW-SE axis to storm track west of UK, with +ve GPH/MSLP anomalies building across Scandinavia into same lead time, as +ve temp anomalies suddenly then fade to become indistinct... all indicative of mean SE-E flow between high to NE/E and low to SW/S). Stamps of course show wide spread. However, at least some parallel to the last crop of seasonal suites (albeit EC Seasonal updates again this week).

Vast majority of members between S to W by 20th. Two clusters in final phase of run (as per 00z). Flip-flopping continues between these two very equally balanced broad solutions.

Notice however how NCEP's parallel 06z suite heads a very different direction, more in keeping with e.g. GloSea's continued - if somewhat less bullish - favouring of the blocky set-up resurrecting after mid-month (a return to blocking also the majority outcome via EPS products using the DECIDER tool run by UKMO). There remains strong suspicion (and this is an old story) that current GFS remains very prone to going beserk with mobility at longer leads, thus must remain viewed with due caution. Either way, confidence is of course very low further into the month and we await this evening's EC Monthly update with great interest to see what crossover (if any) it retains with GloSea.

See my addition to post above.

...erm, that's the output for De Bilt near Utrecht! EDIT The 00z EC ENS (for Reading) retain signal for switch back to avg temps around 15th, with trend then to below avg thereafter. So at least some inter-run consistency emerging in that respect.

Worth noting EC 12z deterministic and control sit in a cluster of.... just 5. Trend heads to slightly below average temperatures post-13th-ish onwards.

Yes, but there's the key points encapsulated together in your post: the mean fields will always mask the members that are less energetic/stray from climatology. Inspection of individual stamps/clusters is critical to appreciating the actual breadth of potential outcomes... and in this case, the reasons for such marked uncertainty being expressed!

The quandary that unfolded earlier this week (disconnect between operational models at longer lead time versus 30-d/seasonal models) is suspected to be a case where quicker, more volatile response in the former - to solving solutions based on the relative dominance of initial starting values - reaches a crossover with the slower-responding climate-forcing factors in the other suites (GloSea5/EC Seasonal etc). Where we can't now be clear - as outlined by Matt - is whether the dog's dinner of solutions now offered by operational models is a sign of them coming around to realign with the (surprisingly steadfast) seasonal model output for 2nd half December, or instead resolving into foretelling a very different synoptic direction (mobile/zonal of sorts/milder). Whilst we've seen a gamut of overreaction to recent operational runs by some folk, as Matt and Nick both stress: it's currently impossible to offer worthwhile clarity at that extended lead time, so just not worth the raised blood pressure!

I was referring to the original comment suggesting a de facto mild/2015-esque phase all December was signed and sealed. It's not. After a (in places notably) mild period, EC trends back to average temperatures, with latest ENS ultimately favouring no dominant synoptic signal with mean flow split almost equally from any quadrant between S'ly around to E'ly. This highly mixed/uncertain signal post mid-month has characterised each run since midweek. Although a colder, blocked outcome is still favoured at present, it's only marginally so and certainly considerably less than a few days ago.

Yes, but 12z EC EPS are starting to lean back towards a different mode. Early days.

Return to climatology or exceeding that in some members (albeit with a minority going at times Dec-Feb the other way, into reversal). The persistence of that stronger signal (i.e. the sudden change coming after a period of exceptionally weak PV) was an underpinning consideration in how the initial 3-month outlooks were worded, when they started covering the early winter period and talked of a higher risk of colder/blocked set-ups for *early* winter but - as Adam Scaife blogged - a shift away from that sort of story could then dominate further into the season (latest outlook focuses this more towards Feb). I'm not sure re signals other than at 10hPa.

...precisely so, and precisely echoing UKMO thoughts after some recent flip-flopping of output (including yesterday's ECMWF Monthly, which suddenly offered substantial weakening of the previously strong +ve MSLP anomaly north/NW of the UK that it yielded in the run only 3 days prior and in all previous recent runs, for later December. Is it wholly trustworthy, given marked swing? No, not until we see the sense of direction offered in runs on Mon-Thurs next week. The situation currently is that whilst ECMWF now offers just a bland, weak +ve MSLP anomaly to the west of the UK, it equally - and quite starkly - offers no signal either to suggest a markedly zonal, +ve NAO set-up either.... In other words, whilst GloSea still remains more inclined to return a blocked story later through the month (and ever since output run way back in Sept, has not budged from indicating a massive ramp-up in stratospheric zonal winds later Dec, as now being suggested by GEFS), the forecast outlook by the UKMO remains quite reasonable in the current circumstances (not least as it would be easy to overreact to latest ECMWF in isolation). Thus, a return to blocking still remains the more favoured outcome, BUT whereas this always had low confidence (based purely on lead time), we now can add a further tier of uncertainty given ECMWF yesterday and the fact that no clear synoptic regime is obviously favoured after mid-month. Some reactions I've seen on social media have been bordering on hysteria, but there's nothing that suggests 'Dec 2015 Revisited' has suddenly manifested in model output. So it looks set to get milder through next week.... but does that really automatically, unquestionably characterise a whole winter ahead??? Deary-me ;-)

Yes, in fact you will actually see the 850hPa values rise under axis of the high cell... due to effects of subsidence at that level. This is not reflective of surface temps rising.. (yet)!

Looks generally in keeping with the script of last 2 days however. Whilst there's been fine balance over what degree of mobility asserts into early Dec, the currently favoured outcome remains for the high to slip SE; some degree of mobility taking over initially, into especially NW/N (possibly more rapidly nationwide: but 30% chance high remains dominant more widely for a while)... This followed by a more concerted period of limited mobility, with greater W-E frontal progression, but this phase looking temporary before blocked patterns re-establish around mid-month onwards. That remains the broad UKMO sequence into medium range. EDIT ie in keeping with Matt's explanation above

But who is offering *deterministic* predictions?? It's one thing outlining a model signal. Quite another saying .. " and that's exactly what will happen by week 3/4 and the resultant sensible weather will be x/y...". At that range we are merely interested in broad trend/probabilistic output; if folk then run off and interpret that as de facto/fait accompli outcome, then that's hardly a reflection on the model... they are merely guides, NOT gospel!

Actually I think the broad pattern match on GPH anomalies wasn't at all bad 3 weeks ago, considering lead time; inevitable weakening of signals at that range and the 'iffy' phase across all models that swung to and fro, as we all recall, as they modelled the late Nov into early Dec period. Here's EC Monthly 500hPa anoms from 3 weeks ago versus 12z EC today for same period (now-next few days). Note low heights towards Spain and +ve anomaly to our NW; plus -ve one Scandinavia-W Russia.

??????? Eh? The next 7 to poss more days are signalled as below average!! The next 2 nights look firmly set to be coldest of this autumn!! I suspect there's people so obsessed with snow forecasts that they're not seeing wood for the trees. No cold: no snow. And as I posted above, usual suspects in media aside, nobody sensible has given a snow prognosis 10+ days ahead... because the forecasting science to do so with any reliability doesn't exist. Honestly, compared to some recent winters(!), I fail to see anything in longer range models to make cold lovers moan.

Nobody (sensible) should be automatically equating the signals with snowmageddon (or lack of snow for that matter). The broad pointers are to a cold set-up. That is very clear. Issues re snow are a matter for much shorter lead times.

EC Monthly consistent with Glosea and it's last runs. Strong resurgence of +ve MSLP/GPH anomalies north of UK further into Dec, especially Xmas week, with marked -ve temp anomalies developing across adjacent contiguous Europe as N'ly flow veers to mean easterly late Dec. Good to see both suites in such firm agreement, despite inherent issues of confidence at such lead time.

Yes, re majority signal. Happy days. :-)