91Èȱ¬

UPDATE 10:30HRS, SUNDAY 31 JAN. 2010:
Good morning! The grey skies above Bristol reflect the forecast developmentsÌýI described yesterday - and in many respects, little has changed in terms of our expectations today.

Some areas are still likely to see a dusting of snow - anywhere up to 1cm typically - spreading down from Wales into parts of southern-central England and probably across to the likes of Bucks, Beds and Herts.

A chance of some localised heavier snow exists; primarily however,Ìýtoday's feature is oneÌýof (indeed, giving only sleet and perhaps justÌýrain for some, but mostly it'll offerÌýsnow). There will be pockets of - perhaps even someÌýCumulonimbus cells - offering heavier snowfall rates in a few areas, mostly up towards the NW of the swatheÌýaffected.

I've added below the Met Office's high resolution (1.5km) prognosis from early this morning,Ìýsuggesting the areas most likely to be seeing / have seenÌýsome precipitation by around 6pm this evening.Ìý Note the colour key on the left and the green signal (rain or sleet) around some Bristol ChannelÌýcoastal areas, parts of Somerset, SW Wales etc. The inland signal is also a mixed rain-snow combination albeit we think the model might be overplaying the 'wet' emphasis a tad too much. This Met Office product only shows the type of expected precipitation, not the intensity or accumulations...

Either way, it'll probably prove a patchy and mostly light spell of precipitation for most of our districts affected - notably parts ofÌýGloucestershire and north Wiltshire.Ìý

UPDATE 21:30HRS, SATURDAY 30 JAN. 2010:
Note (below) some key differences in the very latest NAE modelling for tomorrow, not least the later and somewhat more restrictive spread of snow showers.Ìý The essential theme and driving force (a weak baroclinic zone, see earlier entry) remains the same, however.

Worth adding that the raw NAE is very keen to show most of these showers as liquid precipitation (rain / sleet mix) rather than snow, but the Met Office 'human' view holds the opinionÌýthatÌýmany of these will appear as snow inland.

Nonethless, this later arrival of the showersÌýinto (northern) parts of our region is very critical: the precipitation rates are mostly rather light in any case; the daytime maxima will have just occurred and and the Wet Bulb Freezing Level is generally over 200m... so, will we see much snow?

Well, high into the cloud tops forming over parts of the Midlands and Gloucestershire, the temperatureÌýcould be around -20C and we could still see snowfall boosted - through convective processes -ÌýtoÌýoffer a dusting up to 1cm beneath the broader span of cloud cover and locally accumulating to around 3-6cm.

UPDATE, 19:15HRS, SATURDAY 30 JAN. 2010:
At this rate, I will need to start a new blog entry!Ìý But meanwhile, please bear with me...

My last update discussed tomorrow's prospects andÌýhere's some further thoughts, based on the latest from the Met Office.

The general thrust remainsÌýlargely similar: i.e., snow showers spreading east during Sunday morning, with generally light accumulations for mostÌýdistricts eastwards of the Severn.

In many respects - and I do stress this - the risk of ice remains our major concern, NOT the expected (mostly, but not wholly, small-ish) volumes of snow. Yes, there will be local exceptions, inevitably, but hopefully I've made the point!

So what is going on?

Upper air activity - in this case, an upper ( if you are into the detail!)Ìýtrough - will descend southeast from around the NW of Greenland on the cold (northerly) side of the prevailing jetstream. Eventually, it will comeÌýtogether with the being caused by an upper ridge (of higher pressure) creeping eastwards on the cold side of the jetstreamÌýand steadilyÌýsqueezing a . Does it sound like complex atmospheric physics?ÌýOh, yes...Ìý

So, do spare a thought for the Met Office scientists interpreting all this in ExeterÌýand trying to get your local forecasts spot-on...!

The end result, however,Ìýis a forecastÌýwe expectÌýwill differ - for some, quite markedly soÌý- from the original notion (as you likely sawÌýbroadcast a day or two ago)Ìýof a 'crisp, blue-sky'Ìýsort of Sunday.

Yes, many parts of the UK will have exactly that; but for some us in the West Country, it will surely prove a much more mixed (and cloudier)Ìýset of fortunes....

I've added below the latest Met Office (NAE)Ìýmodelling of wintry shower progression eastwards into the morning but also, note the high-resolution (4km modelling) prognosis graphic following thereafter, suggestingÌýeventual snow depths from midnight through to 21.00hrs on Sunday.....

It's hardly anything truly spectacular, is it?. Which brings me back to my original point about a broaderÌýice risk, rather than loads of snow!Ìý But locally, there will be a good deal of variance in the amount of snow you could see lying.

UPDATE: 11:45HRS, SATURDAY 30 JAN. 2010:
So far, so good with the forecast handling of the this morning's weather - we'llÌýsee a continuation of snow showers feeding down off the Irish SeaÌýinto North Cornwall through the day, but otherwise it's looking totally glorious everywhere.

The latest Met Office modelling for Sunday throws-up some potential complications to muddy an otherwiseÌýdry-sunny-cold story.

We'll be keeping an eye on developments during the morning,Ìýextending from the northwest (caused by a partially , if you are interested in the technical reasons behind it!). The upshot is likely to come in the form of snow showers, giving light to moderate accumulations across from Wales and into parts of central-southern England; some extending perhaps as far east as some of the 91Èȱ¬ Counties.

Below, the latest run of the Met Office's NAE (North Atlantic Evolution) - as tweaked by their Chief Forecaster at Exeter - gives an idea of how these showers could spread steadily eastwards tomorrow towards midday. It'll be fascinating to see how this element of the forecast develops in successive runs of the Met Office supercomputer.

Ìý UPDATE: 18:40HRS, FRIDAY 29 JAN 2010:

In case you wondered why I've been so keen (effectively over nearly 3 days of local radio and TV broadcast forecasts as well as here on the blog!) to talk-up the chance of snow showers affecting parts of West Somerset / Exmoor and perhaps as far east as the M5 during tonight and tomorrow morning, I've included a graphic, below, to stress the point.Ìý

This is the Met Office's high-resolution prediction for accumulated precipitation during the early hours of Saturday morning (1am-7am) and you'll notice the signal for a stream of showers to extend southwards into parts of the SW Peninsula off the Irish Sea (note, incidentally, the far more potent snow risk along NE coastal counties and into East Anglia).

As I write, we're now watching the predictedÌýIrish SeaÌýfeature forming on radar. For a while overnight, we suspectÌýsnow showersÌýcould brush just a tadÌýfurther eastwards from Exmoor & environs, perhaps extendingÌýto the likes of Burnham-on-Sea, Weston-super-Mare and even Bristol, which is why I've used the M5 corridor as a rough guide.... i.e., in the broadest sense,Ìýdistricts west of it within SomersetÌýare most likely to see some snow showers tonight.

Up over Exmoor, we might even see accumulations close to 10cm in some spots by the end of Saturday.

UPDATE: 15:45HRS, FRIDAY 29 JAN. 2010:

The showers shown just north of Monmouth in the radar image below just reached me here in Bradley Stoke, South Gloucestershire.Ìý You might remember a blog I wrote about how to visually spot hail falling. It's a similar story for spotting snowfall from showers - I've taken a photo (below) to make the point visually. Those billowing white areas beneath the cloudbase are areas of snow actually falling - still about 1,000ft up at this stage but within about 2 minutes of taking the image, snow began falling steadily.... and still is.ÌýÌý But as I mentioned in my blog this morning, these showers are blowing-along quickly and so (for most areas - Exmoor excluded)Ìýany accumulation from snow, graupel or hail will prove a temporary point of interest!

UPDATE: 15:15HRS,ÌýFRIDAY 29 JAN. 2010:
As per the forecast detail offered in my original blog (see below), this afternoon's crop of wintry showers have started appearing into the West CountryÌýessentially on-cue. The Met Office model runs since yesterday lunchtimeÌýhad called for these to appear from around 3pm - not bad accuracy!Ìý Recent distribution of these is shown in the rainfal radar image below. They'll continue sinking southwardsÌýfor a while this afternoon and turn increasingly more to a sleet-snow / snow emphasis as the Wet Bulb Freezing Level continues to drop further...

(ORIGINAL BLOG ENTRY FOLLOWS)

Remember February 2009?ÌýÌý

It's by no means impossibleÌýthat we'llÌýsee similar weather establishing across the same parts of the UK in a week or so.... and possibly more widely, too.

Usually, IÌýrestrict my blog to matters of forecast interest within a few days of the here-and-now, but the growing signs - and growing forecast model consensus - signalling a return to much colder conditions by the end of next weekÌýdeserves an early mention. More on all this in a moment...Ìý

Bristol's Kellaway Avenue - along with much of the south of England -Ìýwas almost impassable on this morning in February 2009. Colder weather set to return later next weekÌýcould offer a repeat performance... at least for some parts of the country (Photo: Ian Fergusson)

For the time being, we'll also see colder conditions set-in through today, courtesy of a northerlyÌýflow and a pronounced windchill developing into the weekend. It'll take a little while during today (Friday) for this to become truly apparent here in the West Country; the overnightÌýrain and cloud is already departing off to the south and behind it, the colder conditions are just starting to descend readily from the north, bringing the Wet Bulb Freezing Level (refer to this blog for details) steadily down towards ground level this afternoon.

We're anticipating a period into the mid-afternoon when a secondary feature running southwards (a trough) could just pep-up some wintry showers forÌýaÌýfewÌýdistricts - especially parts of east Gloucestershire and north Wiltshire - so don't be surprised if you see some temporary (not disruptive, I stress!)Ìýsnowfall from these as they pass quickly by on the brisk, chilly breeze later today.

By this evening however, for most across our regionÌýwe're intoÌýa noticeably raw, starry-sky dryÌýnight to come, as temperaturesÌýdrop to around -2 to -4C in and around the likes of Bristol, Bath & Gloucester's urban areas, accompanied by a significant windchill.Ìý

Out in West Somerset (and especially up on Exmoor), you could catch some snow showers feeding southwards off the Irish Sea - indeed, the risk of these will continue for your districts throughout the weekend and I'd not be surprised if we see a reasonable accumulation of snow up on parts of Exmoor by the time we reach Sunday.

For the rest of us, especially anywhere east of the M5 corridor, much of the weekend looks bright,Ìýdry (bar the odd snow shower) and certainly feeling cold, with widespread overnight frosts.

So, onto next week's developments....

The week actually starts with a temporary return to something a tad milder.

B y Tuesday, we expect a spell of fairly widespread rain - with hill snow on it's leading edge, tracking eastwards -Ìýbehind which,Ìýtemperatures will peak somewhere around the seasonal average. But thereafter, the trend is one for cold anticylonic weather to steadily exert itself across theÌýcountry andÌýinto the second part of the week, a return to something a good deal colder will become apparent pretty much everywhere, with the exception of the far West and NW.

ÌýBetween Friday and Sunday, developments are expected to yield a bitterly coldÌýeasterly flow off the continent, bringing the risk of snowfall - some potentially heavy - into parts of the SE initially, with a chance of this spreading further westwards to affect other areas. And beyond that, the south-easterly flow could well be replaced by a east-to-northeasterly version, thereby continuing to offer a growing risk of further snow.

Incidentally, if you're seeking to understandÌýthe detailed technicalÌýmatters behind longer-range weather modelling and indeed climatology, it's well worth dipping into (Paul is the weathermanÌýat 91Èȱ¬ Look North based in Leeds).

It's not impossible that we could see a synoptic set-up not dissimilar to the one of February 2009. It's all farÌýaway time-wise, of course, and much could (and doubtless will) change... but for sure, some very interesting - and surely very cold -Ìýweather lies ahead for the first half of February 2010...