South West England Cold Spell Discussion

[Thermohaline Conveyor]

"This kind of set-up relies on advection of cold air at the surface to lower the isotherm. What we look for is a high precipitation loading event from the East to drag down the 0oC isotherm to the surface. Ideally, we need a weaker surface/mid level flow (light winds) in the lower level and a relatively cooler and drier ingestion of air to enhance evaporative cooling induced from melting and release of latent heat. Effectively, this scenario will bring snow to some lower levels if (1) Precipitation loading is moderate/heavy in intensity (2) Relatively prolonged in nature through stalling (3) Cold and dry air is advected at the surface below a relatively marginal mid level (4) A weak airflow or steering flow (wind flow pattern) in the lower levels to limit mixing of warmer air at surface. We call this the isothermal melt layer theory and it will play a role in this scenario. Note marginal theta-ew in the region of theta-e =+16/+18 along the Northern extent of the trough axis and where colder air undercuts from the East. This alone would support a 0oC isotherm to 400M without any evaporative cooling effect. If we had a strong steering flow projected, mix out would occur and evaporative cooling would not be effective. In this instance we have a relatively weak flow which indicates if we can get the PPN to fall moderate/heavy for a short period of time, this could in turn lead to a lowering of the WBPT Theta-ew gradient. Note also the additional CAA undercutting from the East spinning and stalling to the SE dragging in colder/drier continental air - this drier air undercut at the surface becoming our friend giving the ideal ingredients for Isothermal Drag to occur.

Note: Theta-e WBPT is projected at +20 NAE 12Z 09Z (9am Wednesday) which is more the wrong side of marginal - so a rain to start with event. Moving into the 12Z (Midday) framework colder air undercuts as gradients drop from +20 to +17 along a long approximately West Country into Oxfordshire/South East Wales into Gloucestershire. NAE PPN guidance indicates at this stage a more substantial risk South of the M4 but I don't by it based on a WBPT gradient of +20. All in all then we have some agreement on theta-e between GFS and NAE despite the disparity on frontal ovemnet and progression East. The +16/+18 by GFS relates to NAE guidance and +17. On this basic guidance alone an isotherm in the region of 400/300M can be attained. We then look to PPN intensity phasing and longevity phasing through stalling position of the front. GFS is really underplaying the progression East, whilst also lowering the overall intensity - which is perhaps the main fundamental reason why it is holding back on breaking out more wintry PPN in its snowfall accumulation and phasing simulations found in the Wetter3 suite.

We have to assume that PPN intensity will be stronger than what is being projected. Overall on current guidance an area immediately North of the M4 around Oxon/Gloucestershire running SE Wales would be favoured. Why? Well a strong signal for PPN to fall heavy/moderate in pulses occurs across SE Wales on both NAE and GFS guidance - both leading to accumulations - there-fore we can ascertain some certainty in this area. It also indicates that generally this is where the Northern proportion of the frontal boundary will be. ESE of this areas would mark the wavering/stalling point (approximately) and I do stress approximately. Judging by cold air will be undercutting from the South East/East along the frontal boundary one can assume that this additional injection of colder surface pooling advection will lower the WBPT combined with stronger intensity PPN enhancing the overall evaporative cooling through latest heat release induced by melting of mid level snow. This effectively could drop the colder boundary very close to surface. We await the 12Z NAE and GFS guidance merely for (1) further consistency in the NAE regarding positioning of the frontal boundary and progression East and (2) GFS to come to some sort of agreement in the intensity phasing and trajectory of the frontal boundary"

A complex evolution unfolds. We await with great anticipation the 12Z release guidance from both GFS and NAE

Edited January 19, 2010 by Thermohaline Conveyor

[Dartmoor_Matt]

BBC having none of it for here

Also, mainly just settling on grassy surfaces as the ground itself has warmed up a bit now.

Didn't seem overly confident about it being anything other than a high ground event any how, with perhaps 1-2cm on lower ground (on the grass)

[03jtrickey]

If this comes off, it will be an extremely impressive call by the Met Office - it was five days out when they issued the advisory!

My personal opinion is that we will see some sleet and transient wet snow at low levels (in Cheltenham), with accumulations on the hills, from around 150m. However as TC's post illustrated it's such a complex situation (thanks for the post btw), and I think forecasters are right to be warning of snow because the potential is there!

[snowking]

NAE 12z back towards yesterdays 12z and 18z guidance - the 6z was a little bit of a minor blip

2-5cms i'd think fairly widely through SE wales into glos, oxon, wilts and through into western most extremities of the SE (as defined by the Met Office)

Though as pointed out above, nothing but a very temporary covering around midday perhaps on concrete - grass should see a proper covering for a while

Forrest of Dean is going to be tricky tomorrow

SK

[Thermohaline Conveyor]

NAE 12z back towards yesterdays 12z and 18z guidance - the 6z was a little bit of a minor blip

2-5cms i'd think fairly widely through SE wales into glos, oxon, wilts and through into western most extremities of the SE (as defined by the Met Office)

Though as pointed out above, nothing but a very temporary covering around midday perhaps on concrete - grass should see a proper covering for a while

Forrest of Dean is going to be tricky tomorrow

SK

Hi Kris - Good to see you again

The 12 GFS guidance has come into line with the overall NAE 00Z and 06Z - as well as the NAE 12Z release. So, finally we have some degree of consistency to work with.

Figure 1.1

As attached above, this graphic covers the latest risk based on the trough axis elongation from West Sussex to North Cornwall. Anywhere to the North of this line will effectively be tapping into the 1000-900mb cold/dry air ingestion from the East. The main zone as noted in the 80%/60% zones simulate areas that are running the stalling frontal boundary between +20> theta-ew WBPT and -17. Both of these values are around 4-5 degrees celcius the wrong side of marginal to support lower level snowfall. An isotherm of 400M would be more accurate. 60% areas take into account the main PPN intensity spread forming ahead of the frontal boundary and along it - thus areas that effectively [1] Ingest cold/dry air lowering the WBPT during the coalescence phase [2] Dry air is simulated in vertical sounding forecasts with subsidence at 1000-850mb with marked vertical forcing PVA(Strong) 850-300mb - peaking at 700mb. This supporting the theory of hydrometeor descent being substantial enough to fall through the drier/unsaturated lower level enhancing snow melt in turn increasing latent heat release deduction in the lower boundary. The perfect isothermal melt layer.

Like yourself mate, I would favour those areas immediately N of the M4 into the Cotswolds, Oxfordshire, Gloucestershire and South East Wales. The wet surface prior to the onset of a more wintry ilk precipitation will - to a small degree impair overall snowfall accumulations totals. I would say 2 - 5cm is a fair assessment - to lower levels. South East Wales above 300M should see closer to the 10cm margin.

[Nights King]

Hmm i still have no idea really if i am going to see any snow here. Each forecast seems different and looking at Ians blog it seems to include my area but who knows.

Height i do have plenty of but looks a little to far north and east for mendip area.

Edited January 19, 2010 by mullender83

[Bristawl Si]

Hmm i still have no idea really if i am going to see any snow here. Each forecast seems different and looking at Ians blog it seems to include my area but who knows.

Height i do have plenty of but looks a little to far north and east for mendip area.

Don't think you will M.

But then again i think BTL will only see spells of wet snow, with rain and sleet as well. Looks like Wilts, Oxon and parts of Glos will be best placed, plus parts of mid Wales.

This is no repeat of last Wednesday or the one before IMO. Even now there's only a 40% chance for some areas, which i believe is not v.high in METO 'warning' terms.

EDIT: Ian has just posted, in last 20 mins, an update to his blog.

Edited January 19, 2010 by Bristle boy

[rich101]

light snow in churchdown!

[textmum]

Can someone give me an idea of what time we are expecting the snow to happen/ Thanks

[loopydreamer]

light snow in churchdown!

really???

[Nights King]

really???

I also find that hard to believe Maybe some dust in the air or something similar? Just looked at latest graphics and it still includes northeast somerset in the snow graphics at times into tomoro and even richards forecast earlier said high ground towards bath so i shall hold out hope for the time being at least. <BR><BR>Just got a great birthday present from the other half today - Meteorology Today seventh edition by C.Donald Ahrens<BR><BR>Its a huge book and is going to take me a while to get through it. <IMG class=bbc_emoticon alt= src="http://forumcache.netweather.tv/public/style_emoticons/default/crazy.gif"> Edited January 19, 2010 by mullender83

[Dartmoor_Matt]

I can confirm at tennis it was icy rain... not fun. Back home with the elevation etc its... icy rain! 2c and raining... hard.

Anyway, there is a bit of sleet mixed in but essentially, no go here!

Happy birthday btw Mullender!

[dancing daisy]

Hmm i still have no idea really if i am going to see any snow here. Each forecast seems different and looking at Ians blog it seems to include my area but who knows.

Height i do have plenty of but looks a little to far north and east for mendip area.

Happy birthday!

Regarding Ian's blog, I have just read the comments section and I must live in a bubble as it never occurred to me that the BBC putting it's weather contract out to tender would mean that our forecasters jobs are in jeopardy.

That sucks big time!

[dogs32]

light snow in churchdown!

...tomorrow Rich..lol..

temp here 6.0 and falling

[textmum]

Can someone give me an idea of what time we are expecting the snow to happen/ Thanks

Answering my own question here, according to Ians blog, between 8 a.m. and midday! Hope we get some.

[Nights King]

I have been trying very hard today not to get any false hopes up but it is so hard not to!

Nowcasting event really i think and although i expect very little it would still be nice to see a few wet flakes around.

current temperature is 4.1c

[Dartmoor_Matt]

Dew points are well above freezing at the mo, pretty much the same as air temps. (which are barely marginal themselves)

[SteveB]

Looks like a rain only event for Weston today. I would be surprised in Pat or Trickey haven't got snow falling when the wake up later.

[towbar0]

Snowing like the clappers and sticking. Half an inch already. This is supposed to be sleet...

[skidders]

Snowing here in Winterbourne, Bristol. I have to give credit to the Meto, I didn't think this would happen! Snow on 3 consecutive Wednesdays now!! Not pitched yet but if it snows heavy enough, who knows!!

[Rich_Clements]

Heavy snow when I left the house this morning, which turned to rain once I reached Bristol

[03jtrickey]

Heavy sleet here, will probably turn to snow properly soon, it's becoming increasingly snowy as time goes on. Very good call by the Met Office.

[dogs32]

heavy snow here,which is settling everywhere...huge flakes

[Rich_Clements]

Now wet snow in central Bristol.

[dogs32]

Now wet snow in central Bristol.

going white here mate...

Infact it's settling much better than I thought it would even on wet roads

Local areas affected Warning type Valid from Valid to

South West England:

Gloucestershire

Swindon

Wiltshire

Heavy Snow 0800 Wed 20 1800 Wed 20

Rain is expected to turn to sleet or snow, especially over hills, at times today with accumulations of 2 to 5cm. Locally 8cm is possible over higher ground.

The public are advised to take extra care and refer to the Highways Agency for further advice regarding traffic disruption on motorways and trunk roads.

Issued at: 0521 Wed 20 Jan