South West England Cold Spell Discussion

[Jonesywilliteversnow]

hehehe im currently in nottingham ng11 and its snowing heavy - visiting family woot snow!! so much better than bristol

[SteveB]

We've had a shocker down here in the south west, easily the worst place in the country for snow. Apart from a little flurry on Thursday, that's all I have had.

And by the look of things, nothing is coming our way.

[Dartmoor_Matt]

As I look north over the valley and to the hills opposite it does seem to be snowing, at least there are bands of white in amongst an otherwise grey cloud.

Its still only 0.6c with a DP well below -2C.

Thats 3 days without reaching 1c (I'm not counting the 1.5C it got to last night) haha!

[jazzy]

There does appear to be a few small showers that have appeared on radar in the last 1/2 hour. Just 5 minutes of snow would be better than nne at all

[❄️❄️❄️❄️❄️❄️❄️❄️❄️]

Frozen rain + hill + cars =

[Paul T]

There will be lots of showers over Devon and Cornwall from 8/9pm this evening; the question will be temperature.

I would think that above 250-300 meters there could be a fair sprinkling. Below that level i would expect it to be slightly too mild....CLICKY

[jazzy]

What temperature would be considered as too mild for snow?

[Snowfan50]

The met office had snow for north devon from 6am to midday, then sleet, but we got nothing at all. Frickin pants

[Ian Fergusson]

There has been the hint of that channel low for some time however it has looked like it was going to be a south east event yet again but maybe he has some new information to sugest that it could well hit the west country tomoro? I dont think it will but it would make a change for it to move in our favour in the short term for a change.

For a while today (mid to late morning) the NAE was eager to take a tongue of wintry precip up off the south coast tomorrow PM and modelled snowfall ((some of it heavy) into the evening rush-hour, affecting an area stretching from east Wilts / extreme east Glos along the M4 and into the Home Counties / SE / London / East Anglia. This model run proved a pain to those of us working for BBC Weather earlier today as it markedly differed from the NAE's handling in the 00Z run. My colleague Laura Tobin had to re-hash various graphics around the changes but then soon afterwards - based on the GM's more consistent interpretation of things - the Chief Forecaster at Exeter modifed the NAE output in our last run. Essentially, it wants to take the ppn up through Hants / Wilts / Glos as primarily (perhaps almost exclusively) a rain event, albeit with a prospect of snow on the leading edge into more easterly / SE districts. We'll watch how the next runs handle it, especially given the previous flip-flopping of cold bias (or not) to the ppn field. But for now, we expect tomorrow to manifest itself as rainfall and sleetiness for many near the south coast and indeed the Westcountry, rather than snow.

Meanwhile, later this eve, as the flow becomes ever more westerly, watch for showers - aided by convergence and increasing shear vorticity - to become more frequent, feeding off the Bristol Channel (SST there is ca. 10C) and drifiting inland into parts of Somerset, perhaps Bristol/Bath/S.Glos and northern Wilts. We expect some of these to deliver small, localised snow accumulations by dawn but obviously, the distribution of this will be tightly aligned with the convergence..... essentially its a dry and clear tale tonight either side.

Beyond tomorrow and it's uncertainties / icy start for some, potential for freezing fog Tues AM becomes the next regional concern.

[Dartmoor_Matt]

I'll let ya know if we get any here... certainly cold enough at the moment (and only going to get colder as the darkness comes in)

You don't want the DP above freezing for snow and the temp should be 2c or lower... although you can get snow from higher temps.

[dancing daisy]

Right we have a huge Orange, grey cloud covering for miles Its fulll of snow quite clearly So lets just see.

[Paul T]

There will be a very slightly milder air flow coming in with the showers. (upper temps 1 or 2 oC higher).

Plymouth is circa 4oc currently and falling away fairly rapidly - but i would still guess that the temp will be around 1.7-2.5 when the showers roll in.

[jazzy]

I'll let ya know if we get any here... certainly cold enough at the moment (and only going to get colder as the darkness comes in)

You don't want the DP above freezing for snow and the temp should be 2c or lower... although you can get snow from higher temps.

Thanks, Dew point is -1 and temp is dropping every 5 seconds so I won't give up all hope lol!!!

[Dartmoor_Matt]

Right we have a huge Orange, grey cloud covering for miles Its fulll of snow quite clearly So lets just see.

Or the local nuclear power station has just broken...

Edit: there is an orange cloud here, but I've sussed it... its the sun reflecting from behind the hill onto the cloud.

Edited December 20, 2009 by Dartmoor_Matt

[MP-R]

I have sighted my first cu-nimb for today directly to my south and moving eastwards. Can't tell if it's snowing from it or not because there is a hill in the way but certainly a wispy top and a line of heavy clouds developing behind it.

Temp here is 0.9C.

[anvilhead]

What temperature would be considered as too mild for snow?

good question. If you take the mid point between the dewpoint and the actual temperature (eg. dewpoint -2, temperature +2 - mid point = 0c) then as long as this midpoint temp is below freezing (edit: or at freezing), your odds on for snow.

The temperature drops as the water evaporates in the dryer air (to evaporate water, it takes heat and energy from the atmosphere to do this)- the dewpoint rises because the air is taking up moisture,and they'll roughly meet in the middle. or something like that!

sam

Edited December 20, 2009 by anvilhead

[Ian Fergusson]

Here's some key frames from the very latest NAE output, offering discrimination of wintry ppn (blue) versus rain for tomorrow evening, 21Z onwards. Note the geographic shift is once again slightly further westwards into central England and across to East Anglia (earlier frames, not reproduced here, want to introduce snowfall into north Hants / Berks / Oxon by around 18Z). The NAE is eager to propose 10cm of snowfall in a line between Oxford up to Kings Lynn, but there's a major caveat of this figure not being trustworthy.... especially given the variation of earlier runs today. It's a forecasting headache, for sure (and one I'll have to revisit when back in the office at 0400 tomorrow). I'd imagine the existing MO advisory will see further tweaks in the next few hours.

[Mark]

After a high of 4.2c today the temp has now dropped to 2.6c (DP -1.7c). Frost/snow has stayed complete in the shade and still slippery under foot. After a completely clear day there are now clouds developing to the NE...

Keeping an eye on the radar at the showers coming of the Bristol Channel and feeding SE across Somerset. It's gonna be close! Unfortunately I don't think they'll quite make it :lol:

[jazzy]

good question. If you take the mid point between the dewpoint and the actual temperature (eg. dewpoint -2, temperature +2 - mid point = 0c) then as long as this midpoint temp is below freezing (edit: or at freezing), your odds on for snow.

The temperature drops as the water evaporates in the dryer air (to evaporate water, it takes heat and energy from the atmosphere to do this)- the dewpoint rises because the air is taking up moisture,and they'll roughly meet in the middle. or something like that!

sam

Thankyou, that's explained it very well my mid point is currently 1c but temp is rapidly dropping, fingers crossed.

[anvilhead]

Bristol is currently 2c/-2c - so could well be snow there if anything falls. Winds seem to be turning more westerly now across the B channel, so I'd imagine the showers will start building now ..

sam

[Ian Fergusson]

Attached recent local radar image for those of you with no radar access..... gives idea of current distribution but this will tend to shift ever more towards a straight westerly feed off the channel into inland Somerset and environs later.

[anvilhead]

thanks Ian. your updates are invaluable to us, but dont get yourself in trouble!

[Ian Fergusson]

thanks Ian. your updates are invaluable to us, but dont get yourself in trouble!

No probs - the raw data is exactly as fed into our graphics system - as you then watch on TV - - and so you're not seeing anything that's not otherwise available (online too) albeit in somewhat different format (including NAE output now through Weatheronline, etc); obviously I can't offer you everything we see from our lovely MO colleagues for obvious contractual and commercial reasons, but a helpful snapshot here or there of their expertise (rather than entire timelines etc!) is in keeping with our nowcast / forecast effort anyway. And below is how the high-resolution predictive modelling want to feed showers into Somerset and environs by 21Z this evening.... note the snow bias manifesting not far inland (Somerset Levels / Mendips) and quite beefy cells feeding in sometimes on the flow....but don't take the distribution too literally!!! A hint, however, of what is likely later.

Edited December 20, 2009 by Ian Fergusson

[BristolBlizzard]

Here's some key frames from the very latest NAE output, offering discrimination of wintry ppn (blue) versus rain for tomorrow evening, 21Z onwards. Note the geographic shift is once again slightly further westwards into central England and across to East Anglia (earlier frames, not reproduced here, want to introduce snowfall into north Hants / Berks / Oxon by around 18Z). The NAE is eager to propose 10cm of snowfall in a line between Oxford up to Kings Lynn, but there's a major caveat of this figure not being trustworthy.... especially given the variation of earlier runs today. It's a forecasting headache, for sure (and one I'll have to revisit when back in the office at 0400 tomorrow). I'd imagine the existing MO advisory will see further tweaks in the next few hours.

thanks ian great post , do you think theres much chance of snowfall this week for westcountry !!

[dogs32]

No probs - the raw data is exactly as fed into our graphics system - as you then watch on TV - - and so you're not seeing anything that's not otherwise available (online too) albeit in somewhat different format (including NAE output now through Weatheronline, etc); obviously I can't offer you everything we see from our lovely MO colleagues for obvious contractual and commercial reasons, but a helpful snapshot here or there of their expertise (rather than entire timelines etc!) is in keeping with our nowcast / forecast effort anyway. And below is how the high-resolution predictive modelling want to feed showers into Somerset and environs by 21Z this evening.... note the snow bias manifesting not far inland (Somerset Levels / Mendips) and quite beefy cells feeding in sometimes on the flow....but don't take the distribution too literally!!! A hint, however, of what is likely later.

Core thats a nice radar....

would like one of them for xmas..lol

cheers for the update Ian...