91Èȱ¬

UPDATE: 21:29hrs TUESDAY 12 JANUARY 2010:

So, things now progress largelyÌýas expected from the modellingÌý(see new radar image from Met Office, below, snapped at 21:15hrs and compare to the last one from 19:45.... note how the progression north into parts of Wiltshire and NE Gloucestershire has been very much held-back and how the orographic effect remains very evident elsewhere): the northeast spread of the heavier snowfall is largely - if not entirely -Ìýnow restrained, albeit not finally resolved; the intensity for snowfallÌýtending to wane for the rest of tonightÌýbut still some heavier pulses to come on a more local scale. The trend will now see snow turning increasingly back to sleet and rain from the extreme SWÌýas the precipitation rates fall.... that process is already underway.

Fingers-crossed we'll escape serious disruption by tomorrow morning on present reckoning, albeit some areas have seen a fair dumping of snow and not least some of the upland areas duly warned of this likelihoodÌýin earlier updates.

Ìý

UPDATE 20:10hrs TUESDAY 12 JANUARY 2010:

Latest Met Office radar imagery (below, at 19:45hrs)Ìýfor the West/SW shows very clearly how theÌýnortheast extent of heavier snowfall into Wiltshire, Gloucestershire and beyondÌýwill remain rather uncertain but equally, how the orographic influence (see below update!) is having a profound effect on snowfall intensity and distribution in the south-easterly flow.

UPDATE 19:00hrs TUESDAY 12 JANUARY 2010:

The Met Office prediction has proven very accurate, with quite heavy snow falling as I write in South Gloucestershire and already around a fresh cm or so of new snow.Ìý Full marks to the folk at Exeter: this was a tricky one to call and they elected to issue their early warning way back on Sunday evening.

I'm especially impressed withÌýtoday's accurate timing of snow arrival northwards: the past three variants of the Met Office's high resolution modelling (called NAE: North Atlantic Evolution; we use this to 'drive' the raw graphics you see on TV) has proven extremely accurate. Indeed in many respects with one or two exceptions, it has proven extremely reliable for us throughout the winter so-far.

So, now the story becomes one of potentially disruptive effects.

We could see various amounts of snow continuing to fall throughout the night (waning later)Ìýand I suspect for some of you - not least atop the Mendips; Quantocks, Blackdown and Brendon Hills; some southerly parts of Exmoor especially; uplands of Gloucestershire and Wiltshire -Ìýthis could all prove a real pain by early hours of Wednesday morning.

Clearly, lower ground is also set for some degree of problems tomorrow morning too, albeit of varying severity... as I might be able to attest first-hand when trying to drive into work at 03.30hrs, so I will keep you all updated!

Ìý

UPDATE 16:05hrs TUESDAY 12 JANUARY 2010:

So, here we go.... time to watch just how closely the forecast matches reality.

It's somewhat of a moment of trepidation for those of us, myself included, who elected to 'go easy' on the forecast 'ramping' element this morning and instead took the wait-and-see approach... That's the nature of striking a balance when forecasting a few hours ahead of weather such as this and it can, obviously, go very right or very wrong!

We have recent reliable reports of light snow falling inshore at Burnham-on-Sea as the front continues to move northeast into Somerset and up the M5 corridor, with the expected degree ofÌý'orographic modulation' - i.e., the effect of high ground on precipitation intensityÌý- proving quite apparent on the rainfall radar.

One thing worth emphasising is that even at this stage, I still expect many reports from the ground to suggest a wintry mix of rain, sleet and snow is falling.

As we get into the hours of darkness, an emphasis leaning more towards snow (as opposed to rain) is expected, with the temperatures falling sufficiently further to support this change in precipitation type.

Our latest Met Office model continues to slightly ebb-and-flow with expected snowfall intensity, but it's overall offering a story not dissimilar to that explained in my last update.

So, anywhere from 2 to 5cm generally of snowfall in lowland Somerset, Bristol, Wilts, Glos and environs this evening and overnightÌýbut (and I do stress this!) with considerable variation on a localÌýscale. And just how much will remain settled and apparent by tomorrow morning in urban areasÌýis anyone's guess.

The new model also hints at clearing the front northeastwards - and by then, as a greatlyÌýweakened affair - rather earlier. Effectively, it should haveÌýfizzled-out by around tomorrow morning's rush-hour.

Ìý

UPDATES: 10:20 & 12:00hrs TUESDAY 12 JANUARY 2010:

Our new high-resolution modelling from the Met Office remains keen on the notion of bringingÌýfairly widespread snowfall across the West Country through this evening and tonight.Ìý Latest suggested accumulations (see Met Office map, below,Ìýfor general guide)Ìýare proposed asÌý2-5cm for lowland Devon and Dorset away from the south coast, with spot values around 10cm on higher ground; a fresh 10-15cm snow on Dartmoor above 200m;Ìý2-5cm snow by the end of tonight (and locally higher on upland areas)Ìýacross parts of (I stress 'parts of')ÌýSomerset, Bristol, Bath, Wiltshire, Gloucestershire, inland Hampshire and also Herefordshire.

Wales could well see the highest snowfall totals and disruptive impacts: with the topography playing a key role in modulating how much snow falls, we could see 15-20cm snow on the high ground rising north of Cardiff and -Ìýaway from the immediate coast -Ìýeven 5cm or soÌýin some of the districts actually around Cardiff itself.

The rates will lessen as the night continues, much as per earlier forecast expectations.

Ìý

UPDATE: 09:00hrs TUESDAY 12 JANUARY 2010:

We continue with various forecast uncertainties this morning. And much as I expected, it's been a very difficult few hours hereÌýat the 91Èȱ¬ Bristol weatherdesk broadcasting forÌýboth regional TV and local radio forecasts, attempting to strike the right balance.

It'sÌýbeen allÌýabout emphasising the chance snowfall later today and overnightÌýfor many districts - especially our upland areas -Ìýwhilst stressing how we could seeÌýgenerally smaller (and perhaps rather transient) accumulations at lower levels.Ìý A damb squib? Sleet or rain for some and no snow? Or very heavy snow? It's all possible.

However, the broader thrust of this morning's updateÌýisÌýto downplay, somewhat, the expected amounts of snowfall whilst reserving the strong likelihood that any precipitation falling later this evening and tonight - aided thenÌýby dropping temperatures - will do so as snow, at least for a fair part of the night.

Some of our forecast models are still eager to suggest up to 6cm of snow could fallÌýinto areas around Bristol by the end of tonight. Conversely, some other forecast models (including the French one, called Arpege) greatly downplay the totals... indicating perhaps 2cm for much of our lowland districts, at best. In fact, if some models are to be trusted, most precipitation could rapidly become a wintry mix of some snow, sleet and rain at lower levels. It's a real mess.

So, with such a complex set-up and the strong likelihood of pronounced local variation in the outcome, it's been a case of striking a middle ground. Thankfully, I've been able to offer much more explanatory detail to listeners through this morning on 91Èȱ¬ local radio forecasts.

Of course, I'm also mindful that after all manner of recent disruption,Ìýmost peopleÌýwill happily live without any extra snow causing additional problemsÌýin the middle of a busy working week!

For that reason I'm rather hoping we see nothing disruptive... albeit the potential for this certainly exists and it remains a case of watching developments hour by hour...

Ìý

UPDATES have been added atÌý19:10, 20:45, 21:45HRS, MONDAY 11 JAN 2010:

21:45hrs: New high-resolution Met Office modelling tends to downplay snow totals - perhaps around 15-20cm highest ground of the SW but more likely 2 to aroundÌý5cm at lower levels (and for many, evenÌýless thanÌýthat, I fancy).

Increasing likelihood that this will become a far more complex rain-and-perhaps-some-snow scenario, with major emphasis - at leastÌýfor many districts, especially anywhere northwards of Somerset's higher ground -ÌýonÌý the word 'perhaps'.Ìý

However, there's a broader chance of some light to moderate snowfall later into Tuesday night across many parts of Somerset, Gloucestershire, Bristol and Wiltshire.

My own punt? I'm not going to be at all surprised if we see nothing exceptional other than over the uplands of the SW Peninsula and southern Wales. But then it might have all changed by the time I'm back at work, 4am tomorrow..... keep reading; goodnight meantime!

Ìý

(EARLIER UPDATES FOLLOW)

As is so often the way in my profession, nothing is for certain.Ìý

Faced with a complex forecast still someÌý24hrs away, we'reÌýkeeping abreast ofÌývarious changes and tweaks to regional emphasis from the latest Met OfficeÌýmodelling.

Some are subtle, but - magnifiedÌýfor regional and local forecasting -Ìýthey canÌýmean major differences in getting things right or wrong!

The latest prognosis keepsÌýthe risk of heavier snow through much of the inland SW Peninsula across higher ground, but tends to lessen the snow accumulations quite markedly to the north and east of Devon and Somerset (refer to my earlier caveats in this respect!).

In many regards, it'sÌýan expected modification. I've copied (below) an updated 'best estimate' map of likely snow accumulations fromÌýthe Met Office (but don't take this as the final, definitive version of forecast reality...!).Ìý

We expect many southern coastal districtsÌýcould see temporaryÌýor indeed no snow whatsoever, but this shouldn;t be taken for granted - the wind direction will prove very critical.

So - The red lines suggest zones seeingÌý20 to perhaps 30cm of snow (and deeper drifts, it must be stressed) above around 200m above sea level.

The orange area,Ìýwidely 5 to perhaps 10cm and again, deeper drifts possible; the yellow, perhaps another 2 to 5cm widely (but more could fallÌýlocally) and outside that, yes, still a risk of 2cm+ of fresh snow... but with much lower confidence.Ìý You might see nothing at all.

I've also added belowÌý(update at 20.40hrs) another somewhat finer-resolution Met Office chart (below) with the forecast snowfall shown as black-line contours marked by their equivalent values in centimetres. Again,Ìýthis tends to highlight a broad south-to-north swathe of the worst weather affecting districtsÌýfrom Dartmoor up into central-southern Wales, with perhaps a 'snow shadow' effect reducing localised accumulations quite markedlyÌýinto parts of north Devon and elsewhere, caused by high ground.

Ìý

Compared to earlier expectations, the latest Met Office modelling also delays the arrival (and probableÌýintensity)Ìýof snowfall into much of lowland Somerset and indeed across districts further northeastwards. So for many of you, expect thisÌýto start developing asÌýmore ofÌýa Tuesday evening / overnight into Wednesday story, rather than Tuesday afternoon / late afternoon version.

I'd estimate perhaps 2cm of snow is still possibleÌýinto many lowland parts of Somerset and North Somerset; plus maybe - maybe, I stressÌý-ÌýBristol & Bath and South Gloucestershire (albeit I'm yet to be convinced), plus elsewhere across Gloucestershire and parts of Wiltshire by Wednesday morning.

Upland parts of Gloucestershire, as is often the case, could see a fair bit more. But that's subject to major uncertainty for now.Ìý

WeÌýexpect Bodmin Moor, Dartmoor,ÌýExmoor and parts of southern Wales will get a lot of snow (drifting too) but other parts of our region remain subject to considerable doubt. Topography will play a key role in both increasing and decreasing (or even preventing almost entirely)Ìýthe snowfall witnessedÌýover fairly small areas. And that's even before we add other variables, not least the behaviour of the jetstream up aloftÌýand some other key dynamic factors.

So, I'm awaiting the start of my shift again tomorrow at 4am with a mixture of expectation and yes, no small degree of trepidation. We hate getting things wrong, but hey, such is our chosen field of science and public communication... and we're getting rather used to this level of routine professional stress through this winter of 2009/2010!

Ìý

(ORIGINAL ENTRY FOLLOWS)

And it could prove somewhat of a heavyweight bout...

Just as the snow from last week continues a slow thawing process, we turn our focus to the southwest and the , stretching from inland parts of Cornwall northwards and eastwards to Wiltshire, Wales and quite possible further afield.

This time, it's the eastern North Atlantic Ocean providing the 'oomph', with a quite potent area of low pressure moving towards southwest Ireland tomorrow, swinging a band of heavy rain eastwards towards Land's End during Tuesday morning.

After making landfall, the Met Office expects - with 80% confidence - the rain to start turning readily into sleet and then snow, affecting much (and possibly the majority) of the West Country with the exception of coastal fringes.

Our current modelling via the Met Office suggests a band of snow - perhaps rather patchy at first - crossing into Somerset by mid to late afternoon on Tuesday. The winds will grow steadily stronger;Ìýindeed theseÌýbecome a real feature of the weather tomorrowÌýand will doubtless cause snow to readily drift.

By evening rush-hour,Ìýsnow isÌýexpected to be affecting the likes of Bristol and Bath, before spreading further northwards and eastwards into Gloucestershire and much of Wiltshire.

Into Wednesday morning, snow could still be falling from Bristol along much of the M4 corridor and elsewhere, but readily waning in intensity as the day wears-on.

But as the Met Office predictiveÌýgraphic (above) suggests, between Tuesday morning and the end of Wednesday, we could have seen a lot of snow and a lot of disruption.Ìý Those lines on the chart are 'raw' precipitation (i.e., rain)Ìýin mm; multiply by 10 and you'll see anywhere from 5 to perhaps 20+cm of snow could fall quite widely, albeit the northeasterly extent for snowfall is presentlyÌývery unclear.

Somewhere around 2 to 5cm would seem a reasonable widespread estimate, but with a strong likelihood of far greater totals over the likes of Dartmoor, Exmoor, the Quantocks, Mendips, Cotswolds and Forest of Dean. We shall see, but certainly this has all the makings of an event every bit as newsworthy and troublesomeÌýas the snow of last week.

Whether the snow will tend to turn quickly back to rain or sleet is somewhat hard to say presently, but it's a likely outcome for much of coastal and near-coastal South Wales; ditto more lowland areas of Cornwall, Devon and Dorset. But we shall see...

Worth adding that the strong wind will play a very important part in this. It'll be falling;Ìýthis tends to readily stick to all manner of structures and especially when assisted byÌýthe wind. We could see hefty accretions of , for example, all likely adding to the broader range of disruptive problems.

More updates, as ever, will follow on how things are evolving....