fergieweather

Depends. The alignment (northwards) of the capping baroclinic zone threatening PPN to the S/SE has continued to show great flux across all modelling. It's one of the core forecast difficulties this weekend.

The key difference with 12z GFS versus it's preceding run is the modelling of a closed circulation and moreover, running this further south. Despite the initial awkward and finely balanced forecast rain-sleet-snow phase in much of south, this current prognosis would then readily entrain colder boundary layer around N-NW flank and - depending on how this phases with occlusion remnants/forcing/WBFL - would readily give snow for a while as the centre moves S. However, it duplicates various earlier ENS members so albeit a plausible outcome, remains but one of many - some more nuanced than others.

Hang on! Talking *solely* about the initial Mon-Tues evolution (albeit EC has some snow further S too): NOT the whole sequence through rest of week!!!

Reasonable agreement in assessment of 00z UKMO and other output for the scope of Mon-Tues evolution but still only broad-brush detail possible. Without describing all the rationale and diagnostics, we expect essentially a risk of (perhaps significant) snow Wales/Midlands northwards. Marked uncertainty how long the cold weather lasts before a return to around average temperatures.

...but a wide spread in solutions in the EC stamps, including at all stages post-weekend and ditto in EPS clusters further ahead. Low confidence re PPN distribution and phase throughout the sequence, starting with the Mon-Tues developments.

Yes, often folk take the 850hPa prognosis too literally. Chilled continental boundary layer critical.

Nope. Just a rational, measured look at all the evidence. Buckets of uncertainty prevail, as should be massively apparent from current output. Posting and over-analysing zillions of GFS PPN phase charts is an utterly pointless waste of bandwidth at this tentative juncture.

E.g. re PPN details Sunday; re E'wd extent of frontal progression Mon-Tues; re threat of cut-off to S leading to more pronounced E-NE solution...rapid occluding of any warm sectors in Atlantic lows expected, with 522/sub-522dam air prevailing, with attendant awkward forecast period re wintry risks.

As others note, UKMO regard EC DET as progressive outlier. Strong signal for colder or much colder weather next week, albeit extremely low skill on snow risk in any one area (not even worth speculating!). MJO phase root cause for the synoptic 'flip', they say. Some solutions look ominous, but a while before ANY worthwhile deterministic detail emerges for day-to-day prospects after the cold, cyclonic weekend has passed...

About a 3-5 day phase based on EC-EPS before mobility resumes. But nought reliable either way: too much volatility of current output, with fine balance between the likes of snowy 12z EC DET into 6-10d versus some clusters rather more akin to 12Z GFS. No one solution is convincing enough to be taken as ultimate outcome. There's sufficient f'casting problems even by Sunday to trouble us.

Agreed. Trough circa 3am may extend snow showers across even to S of London (hence UKMO warning amended). Meanwhile, latest UKV and MOGREPS signal for snow remains particularly strong for Exmoor and Mendips especially. W Country remains under the 522 dam thermal trough until about 0900 tomorrow.

No. Exeter add signs of mobility return later in trend period. I've no idea. I believe those are a PWS task.

UKMO indicate it is thought to be MJO-related in their latest assessment. They note the risk of a more blocked scenario establishing (as per some DET runs and also EC-EPS members), at least for a while, with enhanced likelihood frost/snow, especially in E re latter. However they also advise on 'fine balance' UK sits in re any blocking to E versus mobility from W... no high confidence yet on which will prevail.

The raw UKMO solution only currently a 20% PROB based on EC and MOGREPS EPS. So for now, while noting the possibility, Exeter have modified operational GM towards the inter-model consensus.

Risk of too literally. We can only broad-brush the main risk areas (notably Exmoor, Mendips/Mendips Plateau, Broadfield Down, Cotswolds/FoD).

Cb tops to ca. -45C in profiles. Could be lively on coastal/adjacent fringes where very efficiently fuelled by 8C SST's.

Somewhat contradictory! The BBC graphics (0-48hr) are the Euro4 output but crucially, it's the output as modified by the Chief Forecaster at Ops Centre... NOT the raw E4 seen on websites. So ironically, in the TV version, you are seeing both high-res AND after forecaster intervention. Still, we can't please everyone, I guess...!!

I'm not sure which channel low you mean. However the Friday solution that was looking v bothersome a few runs ago in UKMO-GM for S England now rated as 10% prob. Meanwhile, some further (mostly modest) snow accumulation does show again Thurs-early Fri in E4 for parts of upland S England (W Country/S-Central England/some S coastal districts).

Latest assessment was based on 00z suites. Yes, broadly average/colder than average next 10-15d; generally cyclonic with further windy phases and more markedly colder ones with snow at times, mostly but not exclusively in N. Best of any settled weather in S. Mention of possible more settled weather late Jan hasn't been characterised by detailing any expected temperature regime... because it may vary even if anticyclonic weather does occur. The 12z EC clusters offer prospect of further more (perhaps windy) NW-N'rly outbreaks with trough to E. I haven't yet seen where latest MOGREPS heads.

Righty-Ho. This can't pass without comment. Below follow just a random snapshot of detailed medium range briefings from Ops Centre made in first days of Jan, looking towards what was then the 10-15d trend (and now current-short term) period. I will leave it to others to judge if my UKMO colleagues have got things "so massively wrong".... Written 1 Jan: "The unsettled conditions continue into the trend period but with some evidence of increased meridionality in the upper pattern and so allowing for some greater space between systems and perhaps increased signal for some clearer/colder/showery conditions between." Written 3 Jan: "Latest f/c output continues to indicate a continuation of the predominantly mobile, and often unsettled, theme, with periods of wet and windy weather interspersed with clearer/colder weather. Some EPS members continue to indicate the risk of very deep areas of low pressure crossing over or close to the UK at times, bringing the risk of locally storm-force winds mainly, but not exclusively to N’ern areas. A few of the more S’ly tracks (currently about 10% of members take lows across Wales and England) would allow the entrainment of colder air into these systems, with a lower risk of heavy snow developing. All output does allow for transient ridges, but the wavelength of the upper pattern is such that any drier/colder interludes would be short-lived, and there is no strong signal for any prolonged settled spells, or easing of zonality..." Written 6 Jan: "The zonal, cyclonically biased spell looks set to continue for the majority of this period. Further very deep areas of low pressure are signalled to cross the UK at times, bringing the risk of disruptive weather with them, these in turn being driven by a powerful cross-Atlantic jet. Colder Pm airmasses (originally Pc airmasses with origins from N’ern Canada, though heavily modified) are likely to cross many parts between systems, bringing below normal temperatures, and a risk of snow/frost." Need I go on?

Yes, but with a sizeable number of volatile/deeply cyclonic and periodically colder scenarios returning to EC clusters now out to T+360, nothing can be presented with any confidence on outcome. As it happens, we have more than enough to focus upon during next few days, without worrying about stuff post-D10...!

Just to reiterate for those following E4 snow accumulation prognoses: divide the raw amounts by factor of 2. I mentioned this yesterday.. it's to do with how model 'over-clumps' showers (problem thus in convective regimes but not re dynamic snowfall). I've seen some folk on Twitter taking 10-20cm raw signal literally. We go for 2-6cm generally, as per UKMO warning text.

No significant changes at this juncture from UKMO albeit clearly development later Weds remains a key uncertainty with regards to wind strength detail/distribution. Ditto PPN phase across S England late Tues; ditto what unfolds later Fri. So little to add until tomorrow.

We await another assessment shortly from Ops Centre based on their 18Z output. The 12z stuff didn't look at all pretty with inland gusts widely to 50kts.

Brief update. High confidence now on snow for parts of W Country later Tues/Tues night. Several cm possible higher ground, notably (as it stands) Exmoor, northern Mendips Plateau; parts of Cotswolds. Likely the shower line (to Cb) extending down roughly M4 to give some surviving all the way towards London. Fairly high depth temperature should help prevent much low-lying accumulation. Different story on uplands aforementioned. Meanwhile, the troubling-looking 12z UKMO GM for Friday, which would yield substantial snow across SW/W Country, considered low probability. Low is most likely to head to Portugal, around the high, Exeter reckon. That's it from me for a while...cheers.