13Z Update
The 06Z NAE is more similar to the ECM 12z and 00Z issues. The GFS is more bullish in pushing the front in a bit further before trough disruption occurs. There are going to be significant problems modelling the very potent jet core coming east North of the Azores. This needs precise modelling , too weak and the trough will disrupt to early, to strong and the model will disrupt too late. In the grand scheme of life this would normally not make much difference, however across the South tomorrow it makes a huge difference with regards Rain/Snow Mix. Any Snow in CS England is dependent on the wind backing Easterly (from SE'ly as the wind whilst will have cooled the WBFLs somewhat is still a wide portion of the Channel to cross hence southern coasts are likely to see rain & sleet, however if the wind back E'ly then Snow can be brought down to coastal areas.
Inland however a Sig Snow event looks possible for Exmoor, Dartmoor, North Devon, Somerset, Inland S Wales and as far NE as Bath, Bristol down to Salisbury. The GFS takes precip further east, However the ECMWF refuses to and has been very consisent in its positioning of the front by Weds Morning.
People should expect 5-10cm inland and 10-20cm on the moors and high ground. Sig accumulations may build up in inland CS England as the front grinds to a halt, but the precise phase mix by then as well as the orientation of the front by Weds Morning in assoc with any low in the Channel has BIG question marks. Highest totals appear from Exmoor across to Bath and to Yeovil as well as Dartmoor and the mountains of S Wales (though much progress North into Wales is questionable at this stage)
And before anyone asks - for those in this area (Bmth, Poole to Southampton) Its very Borderline... More likely Rain/Sleet - Possibility of Turning to Snow if a low can develop in the Channel at the right time and the wind comes more easterly.
Residual Rain,sleet and Snow is leaving N Ireland and SW Scotland. WBFL's around 300-450m in this zone having entrained some higher DP less cold air from the North Sea, somewhat cooler lower DP air is advecting into the South and SE from France, though its been a pitfully slow process with little advection anywhere of the air over England today with about a +/- 0.8 def change in temp doing well !
Synoptically things are beginning to change, a very strong Jet Core is crossing the Atlantic at a low latitude and coming up against the cold block over Scandinavia, the exact strength and position of this jet very important to the subsequent shape of the upper trough as it comes into the SW approaches. The ECM has been very consistent in the low turning to the North earlier and spinning it up to the west of Ireland and allowing an extending trough to be dragged ESE as the Jet overruns and punches into Iberia. The UKMO GM is / has been less keen on this idea and brings the low further into the SW, latest run has moved a little way towards the UKM but I still do not think its modelling the trough correctly. WV imagery indicates the low is moving to the West of Ireland, and supports the ECMWF. Its pleasing to see the 12Z GFS move towards the ECMWF, though its perhaps a little fast. The exact positioning of the low as it moves East is important - the latest trends indicate the low will run right through the Channel and then move towards KEnt and then Suffolk as it curls around and gets pushed north by another trough to the SW. This is important is drags precip a little further east and more importantly backs the wind from the land across most of Southern England instead of the Sea so meaning its very difficult to call the rain/snow mix on the coasts but 20 miles inland Snow is much more likely. Infact the ECM keeps rain/Snow going through most of Wednesday across SE and CS England with a slow rise in WBFL during the day from the SW, though the really milder air makes more progress North across Ireland than NE across England.
So the detail... Rain arriving in Cornwall during the morning and turning to Snow across Devon, Bodmin Moor, Dartmoor. The SE coasts of Devon likely to see Sleet/ then a cold rain. Inland across Devon the precip Snow, incl North Devon and Exmoor, arriving into SOmerset and S Wales during the afternoon, again sleet /rain on exposed SE Coasts and Snow inland.
Snow accumulations 3-8cm in most inland places of the SW, 10-15cm over the moors of SW England and the Mountains of S Wales 15-20cm isolated more.
Turning increasingly to rain across most of the SW during the late afternoon as WBFL's rise with much milder air into the SW by then - temps up to 6-8C
Now into the Evening we see the low forming in the English Channel and moving east, winds back easterly then ENE across CS England, Snow pushes into Somerset, Wiltshire, Bath, Bristol and Doret. Lighter amounts getting into Hants and Berks. 5-10cm inland across CS England, Coastal areas of CS England are very difficult, if the wind backs and the precip is heavy enough we could easily see snow, GFS Forecast Ascents show Snow to the surface easily in heavy precip and more generally away from the Immidiete Coastal strip. Its so dififcult to call and frankly for these CS Coastal areas we will just have to wait and see. Inland its an easier story and forecast ascents more generally support snow - so some heavy falls in places - the snow quite wet and sticky.
Weds sees a cold and grey day across SE england with snow / sleet slowly turning to sleet/rain and turning grey and misty, temps slowly coming up but feeling very cold. Much milder in the far SW and this milder air coming into West Wales and creeping up the Irish SEa. A slow process
Into Thurs and Fri and the Milder air pushes north right up the western side of the UK - temps 7-9C across the coastla SW by then and WEstern N Ireland IOM and Western Scotland, staying colder further east with a much slower temp rise.
Heavy Rain spreads east into the Weekend and this assisting a much quicker thaw, and it will clear the roads of salt, ice and grit - the front coming into the west but weakening as it moves east.
Looking longer ahead - the ECM remains very blocked again as its recent 12Z Runs have - Infact 12Z Runs in general have been shown a tendency for the block to extend west again. The ECM is very keen tonight, the GFS hints at this also with a developing cold pool heading west.
The fact the ECM continues to hint at this means we need to take it seriously though the lack of agreement from the GFS and oither models makes this uncertain at this time.
PJB
Fig 1 - Estimated Snow Depth Chart for EVent later tomorrow and into WEds - Estimates Only - not inplace of an official Chart.
Paul Blight
UKww Manager - Education/Warnings
Associate Fellow Royal Meteorological Society
