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UPDATE, 09:30hrs, WEDNESDAY 20 JANUARY:

The Met Office warnings proved very timely and accurate for this one. It's usually at this stage us weatherfolk breath a collective sigh of relief, but it's hardly a 'relief' for those people trying to get to work this morning...

Visibility during the rush-hour on stretches ofÌýthe M4 between Bristol and ChippenhamÌýhas been described as "terrible" due to fast-falling snow, with reports of it settling fairly readily in some areas (such as Swindon and elsewhere in north Wiltshire).

It's a very similar story up the M5 through Gloucestershire, with a good deal of snow falling there and settling steadily away from the major road surfaces.

That oh-so-tricky-to-forecast transitional boundary - where snow is falling and starting to settle, as opposed to more a rain-sleet-snow mix - seems to be lying somewhere just south of Bristol,Ìýjudging by Highways Agency live traffic cameras. Snow has been settling too - patchily, at least -Ìýinto parts of Bath & NE Somerset, towards the Wiltshire border. The regional thrust of the forecast was pretty close to the reality; now we need to await just how much snow actually falls. I'm seeing little sign of any significant settling until moving north and north-east of Bristol, as per expectations.

The Met Office rainfall radar image, snapped at 09.15hrs (below), shows the dual-banded nature of this system developing and the fairly broad swathe of precipitation falling across much of southern England. We expect those two bands to merge today, become largely stationary and then weaken effectively in-situ as the forcing aloft eases awayÌýboth northwards and offÌýinto NE France. Our current expectation is for some snow to still fall across parts of Gloucestershire and Wiltshire even by mid afternoon, so any clearance will be slow and characterised by the snow rates getting steadily lighter, patchier and resorting to rain-sleet as the day progresses, as opposed to skies suddenly clearing.

The Gloucestershire/central-North Wiltshire emphasis still looks a solid bet for the heaviestÌýof the snowfall.Ìý Please do keep your reports coming and post them here on the blog!

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UPDATE, 22:15hrs, TUESDAY 19 JANUARY:

My final thoughts, having seen the latest Met Office modelling...

The frontal progress will prove slightly ahead of original expections - thus any snow risk effectively expectedÌýbetween around 0800 and 1200hrs tomorrow.

TheÌýnotion of snow - noteworthy totals, at least - beingÌýlargely restricted to Gloucestershire and parts of Wiltshire remains steadfast. So, I'm sticking to my original comments on this (doubtless to be proven wrong?).

Indeed, I'll suggest further refinement to this theme: effectively NE GloucestershireÌýand extreme north/upland north Wiltshire most likely to see snow (i.e., if taken from all the West Country districts) and even in those areas, rather patchy in terms of amounts.Ìý

And in any case, it'll all be replaced by heavy rain and strong winds by Thursday!Ìý Far easier to forecast....

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UPDATE, 19:55hrs, TUESDAY 19 JANUARY:

I've beenÌývery impressed how the Met Office modelling has retained bothÌýcontinuity and the regional essence of this forecast ever since I started penning this blogÌýentry.

Other forecast models are now catching up with the general theme, albeit - as earlier noted - the French versionÌýhas offered support for a while. So now, we await reality.

Below isÌýthe latest Met Office prognosis for likely snowfall distribution and amounts. Blur the edges, please, but it does make interesting comparison with the chart I reproduced hereÌýon Monday. The broad theme remains largely similar.

Incidentally, we expect road surface temperatures (RST's) will not fall sufficiently tomorrowÌýfor snow to settle widely or for any prolonged timeÌýon most roads, other than where the rates of snowfall are greatest.

Settling onto grass, yes.

This willÌýprove a key point in determining the level of travel / road network disruption... I'm still not expecting anything like last week's widespread scenes of snowbound chaos, but there will be local exceptions, for sure.

UPDATE 13.00hrs, TUESDAY 19 JANUARY:

My last update explained - as simply as possible - how the Wet Bulb Freezing Level (WBFL) plays a critical role in our snow forecasting. If you've read that, then the chart below will (hopefully) make some sense...

It's again from the Met Office and continuing to tell a largely similar story to their last forecast output; i.e., rain turning progressively to snow during tomorrow morning as it moves northeastÌýand especially towards midday.

Shown here - superimposed against those all-important WBFL contours in greens and blues -isÌýwhere the signal for rain (green) versus sleet and snow (blue) is expected to occur by midday tomorrow.

Note again howÌýthe geographical emphasis by this timeframeÌýisÌýprimarily on SE Wales, the SW Midlands and parts of southern-central England. If anything, we're leaning towards a slightly further NE focus, such that Birmingham and environs (including Oxfordshire)Ìýcould well see up to around 5cm of snow through tomorrow.

I still stick to the notionÌýof upland Gloucestershire and upland Wiltshire seeing most ofÌýany West Country snow angle to this. Below around 100m above sea level,ÌýI wouldn't be at all surprised if we see only rain and sleet -Ìýperhaps mixed with some very transient snow -Ìýin the likes of Bristol, Bath and most ofÌýSomerset.

With rain turning to snow above about 300mÌýover Wales early in the morning,Ìýwe expect it toÌýform a cold 'pool' of air at lower levels in adjacent Gloucestershire through evaporative cooling (the latent heat exchange process...remember this fromÌýmyÌýlast update?). This shouldÌýhelp the WBFL to drop - progressivelyÌýthen aided by colder air advecting in from the near continent later in the day.

The output I've reproducedÌýhereÌýis from the (called NAE, the North Atlantic Evolution)Ìýand has been expertly 'tweaked' in this version by the Chief Forecaster working at Exeter. This sort of detailed mapping, benefitting from hands-on modificationÌýat the Met Office and updatedÌýthroughout theÌýday, - and indeed directlyÌý'drives' the ever-changing graphical representation of rain, sleet, snow, cloud and wind you see on our TV bulletins.

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UPDATE 10:35hrs, TUESDAY 19 JANUARY:

The continuity between successive runs of the Met Office forecast models has proven exceptionally good for developments into Wednesday. Indeed our confidence is helped by the French, whose own forecasting model, called Arpege, is keeping pretty close company with the Met Office on this evolution.Ìý

The area of greatest uncertainty involves just how much snow will fall. It's unlikely to be on a par with the disruptive events of a week ago - broadly speaking, at least.

Much as expected (see my previous comments below), for many of you at lower levelsÌýthis will prove a very marginalÌýsituation with less rather then more snow likely. However, it's by no means impossible we'll see up to 10cm falling on higher ground of SE Wales and perhaps 5cm across parts of the SW Midlands, including uplands of Gloucestershire and probably across into Wiltshire and Oxfordshire.

I doubt the snow will fall or for that matter linger on the groundÌýfor very long, however. The signal for a return to sleet and rain from the west remains very solid, so a likely outcome will be rain andÌýsleet arriving at first during Wednesday morning, turning heavier and ever-more-wintry in natureÌýas it spreads northeastwards towards midday.

The period between late morning to early afternoon holds the greatest potential for seeing snow, especially as the system progresses anywhere north and east of Bristol... so I'm staying with my hunchÌýthat parts of Gloucestershire and Wiltshire have the greatest chance of snowfall exceeding, say, 2-3cm. For a while, at least...

By mid-afternoon the heaviest precipitation will have departed and lighter outbreaks of rain and sleet follow from the west. Thursday, meanwhile, still looks distinctly grim: very wet and very windy; some pretty significant weather that day, for sure, and so worth highlighting this!

For those of you interested in the science behind our forecasts, I've added a little insight belowÌýinto just how tricky snow forecasting can prove for us, not least on a sub-regional scale.

One ofÌýthe most important forecast products supplied to us by the Met Office predicts where the boundary will lie for the Wet Bulb Freezing Level (WBFL). Without overbearing youÌýon the physics, as snow falls it evaporates -Ìýby varying amounts -Ìýinto the surrounding air. The latent exhange of heat from this process cools the atmosphere and bear in mind, billions of falling flakes are all contributing to theÌýoverall effect.Ìý The WBFL, measured in metres, offers us some idea how far up the snow is theoretically likely to change to rain (e.g., 200m up, 400m up etc.) and critically, just how easily this level canÌýbe brought downwards to the surface.

This lowering of the WBFL - and thus the extent of evaporative cooling -Ìýoften occurs when the rate of snowfall intensifies sufficiently to bring colder air downwards from aloft (albeit strong winds can effectively stop the process).ÌýIf conditions are right, the WBFLÌýcomes steadilyÌýdown from,Ìýsay, 200mÌýabove your headÌýandÌýsmack to the ground, allowing snow to penetrate all the way to the surface even when your car thermometer might still show +3C.

So, heavier rates of precipitation can make a very big difference in 'marginal' situations; you may recall, for example, how some very light precipitation a couple of weekends ago barely arrived asÌýdrizzle for many of us, let alone as snow. It was a real forecasting headache and the WBFL played a critical role.

All manner of additional small-scale factors: relative humidity; windspeed; terrain; the type of cloud providing the snow - play key parts in determining if snow falls, versus sleet or rain. Tiny variations in any ofÌýthese can make the difference to a snow forecast being perceived asÌý'right' or 'wrong'!

Now, the chartÌýabove might look a scrambled mess to many, so to help you see the British Isles I've highlighted the Bristol Channel for orientation! The cluster of round circles swathed across the chart show where frontal precipitation - rain - is expected to be falling at midday on Wednesday and how heavy it will be (larger circles =Ìýheavier rain).Ìý TheÌýstar and x-like symbols in blue are a signal from the computer modelÌýfor precipitation falling as snow (again, larger stars = heavier snow). Bear in mind this is a prediction, not rainfall radar here-and-now reality, so we're always mindful not to follow such charts too slavishly!

Note how the broader shading of the WBFL valueÌýchanges from greens to blues, with the boundary lying somewhere into the likes of SE Wales and northern parts of the West Country/SW MidlandsÌý(Glos, Wilts).

The lighter blue colour startsÌýwhere the WBFL contourÌýwill be at 400m; the next darker blue at 200m. As explained above, given heavy enough rates of precipitation (which we expect for a time tomorrow),Ìýit's possible to bring the WBFL right down to the deck from these heights and so it's over in the blue side of that boundary where snow could prove the likely outcome. And adding to the overall effect will be colder air moving across from Germany, helping to lower the WBFL more widely as Wednesday progresses.

Did this make sense? Phew....!

Ìý

UPDATE 21:30hrs, MONDAY 18 JANUARY:

Here'sÌýmy first update, as promised (see original entry below).Ìý

Our Met Office partners have synthesised data from another computer run and the mapping below, courtesy of their expertise at their Exeter HQ,Ìýgives a 'best guess' (and we do stress 'best guess' at this stage!) of the likely snowfall distribution and amounts for Wednesday.

If you followed my blogs through the last two snow 'events' here in the West Country, you will have seen similar predictive graphics I'veÌýreproduced from the Met Office. In many respects, their accuracy has proven very striking and reflects on how the scientific effort behind meteorologyÌýhas evolved not only on a broadscale, but on a much smaller scale, too. And so it continues...

If you superimposed this level of geographic weather prediction onto, say, a map of the USA, the level of fine-scale forecasting accuracy we've seen this winter in the UK would prove all the more compelling.

Yes, of course we get it wrong sometimes, but then these errors will always seem so much more magnified when viewed locally across the cities, towns, villages and hamlets of theÌýcomparatively small islands we inhabit; encircled by a massive ocean on one side and a vast continent on the other.

And no - that's not an excuse for when we get things wrong; it's merely a reasonedÌýinjection of scientific reality!Ìý

I expect this to prove aÌývery marginal event in terms of any meaningful snowfall; i.e., many will see only rain or sleet in the West Country, so I'm not getting my cat's snow hopes up (refer below!) too highly....

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(ORIGINAL POST FOLLOWS)

I drove from Bristol to Hertfordshire over the weekend and it was interesting seeing the varying amounts of lying snow still left along the M4 corridor.

With milder conditions prevailingÌýsince Friday, much of the snow - as expected -Ìýhas melted to be replaced by a good deal of standing water. However, some appreciable amounts still cling to shady parts ofÌýfields and woodlandsÌýalongside the motorwayÌýin Berkshire and Wiltshire.

And a snowy top-up, not least for those very same fields and woods,ÌýisÌýforecast for mid-week...

Any fresh snowfallÌýwillÌýbe greatly appreciated byÌýmy cat, Pickle - seen here last week enjoying a 'snow leopard-wannabe' moment. But are snow-mad cats rather a rarity? I'm really not sure. Some clearly hate the stuff, avoiding putting even a paw in it!

Over the past 36 hours, the Met Office's supercomputer has been keen toÌýrepeatedly forecastÌýup to 5cm of snowfall arriving on Wednesday morning, spreading eastwards to affect some partsÌýof the Westcountry, central and eastern England - possibly including more western parts of London, too.

The Met Office has issued an advisory - .

Those of you who follow my blog regularlyÌýwill know I'm reluctant to get bogged-down in the finer-scale detail at thisÌýrelatively early stage. But suffice to say, this will prove yet another of those 'what if?' situations for much of the West Country. Some of you areÌýlikely to see a fair amount of snow and others... none.

The broad theme suggests around 1 to 3 cm of wet snow (the type we experienced last week)Ìýfalling at low levels for a while through Wednesday morning -Ìýwith 5 or more centimetres in quite a few districts -Ìýall becoming progressively lighter into the afternoon, as it moves away eastwards. The mechanism, rather as we saw last week, is courtesy of a frontÌýswinging off the Atlantic, overriding colder air lying across much of central-easternÌýBritish Isles.

The recent snow caused all manner of transport problems but whether Wednesday's forecast heralds similar levels of disruption remains to be seen. It's quite likely someÌýfurther snow will fall here - at Panshanger Aerodrome in Hertfordshire - whereÌýthe frigidÌýscenes shown here a fortnight ago have since melted awayÌý(Photo: Ian Fergusson)

And again much like last week,Ìýthe local fine-detailÌýforecastÌýaccuracy will prove a real headache. For usÌýproviding the forecasts, aÌý'win or bust' outcomeÌýis likely to involve distances of only a few miles,ÌýpredictingÌýthe transitional boundary where rain changes to sleet and then to snow.

Not easy, but we've already had two attempts at thisÌýin 2010...!

For now, it looks likeÌýdistricts from around Bristol eastwardsÌýare most likely to see some snow, but I'm expecting upland areas inÌýeast Wiltshire and east Gloucestershire to beÌýprincipally in the crosshairs for any more substantial falls. But this is speculative:Ìýlet's see what the next set of higher-resolution Met Office predictions make of things. I'll provide updates here, as ever, and look forward to reading your thoughts and opinions.

By Thursday - withÌýanother Atlantic incursion above usÌý-Ìýthe snow has been largely or whollyÌýreplaced by very wet, windy and milderÌýweather; indeed, perhaps 20 to 30mm of rain across much of the West. Heavy snow, perhaps even blizzards,ÌýisÌýa moreÌýlikely outcome up across northeastern parts of the British Isles, where the colder air remains.

Incidentally, the longer term outlook always holds a lurking -Ìýif small -Ìýrisk of the bitterly cold east/northeasterly influence returning laterÌýinto next weekÌýto 'block'Ìý any milder Atlantic weather from readily progressing across the country (as it did so effectivelyÌýa fortnight ago).

But for now, after this week'sÌýexpected snow, wind and rain clears, our broad expectation is for the West CountryÌýto experience a settled spell from Friday andÌýintoÌýweekend; withÌýfurther wet, windy and close-to-average conditions then dominating into the start of the following week. Will this become reality? We shall see!

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