Winter 2009 / 2010 in Western Europe?

10Z Update -21Feb

Developments overnight planned out much as expected, though with slightly more Snow over N Ireland and far SW Scotland than expected, though accumulations are quite small in general. Overnight a 110Knt Jet has developed over Southern Britain, this has created increased surface and upper baroclinicity across the UK. At the jets left exit diffluence zone a small surface depression has developed. This currently over N Wales and will move ENE and slowly fll as the Jet moves away. The increased ascent has occured at night with a cold airmass firmly in place over the Northern half of the UK, hence we have seen heavy Snow developing. 2-5cm generally, but reports of 6-9cm in a few places. At 09Z both the 12KM NAE and 16KM ECWMF have a good grasp of the precip zone which stretches from E N Ireland to Morcambe Bay acrtoss through Leeds to Hull and down to tCambridge and Suffolk. Its largely rain south of 52N. Heavy Rain has moved across the more southern Counties overnight.

The Snow will continue to push E and NE through today, tending to become lighter and turning to rain over lower levels with time, though several CMS to come across N England. Rain hanging on close to the SE and infact ECM is not keen to clear from far SE at all today, hinting at a return of some precip to Hants, Sussex and Kent later today. NAE is similar and hints at outbreaks of showery rain returning from Portland Eastwards this afternoon.

Tonight most places become dry and clear with another cold night across Scotland and N England under Clear Skies and Snowcover (ECM has temps below -12C again across Scotland and below -8C across N England)

However cloud persists further south as another depression approaches. This one again has the potential to bring more snow to Wales, Midlands and Eastern England. The 00Z GM, NAE, GFS all bring the low through the English Channel nr the English Side, However the 16KM ECMWF brings it ENE closer to the FRench side effectively shunting the precip further SE away from the Midlands and confining it to SE England with Snow threat reduced and confined to the Chilterns largely. The Hirlam is similar to the ECMWF (though this not surprising as it gets its boundary data from that model)

I do not like going against the ECMWF, but given the available evidence from just about every other model has the precip further NE and as the NAE has done a superb job with the Snow overnight, its hard not to do anything else, but with lower confidence than normal for a 36hr forecast.

So rain comes back into the south later tonight and spreads ENE tomorrow, and turns to snow from SE Wales, across the Midlands into E England. NAE quite bullish about a period of Mod Snow developing giving generally 2-5cm on low ground and 4-10cm on ground above 150m. Heavy rain in the south for a time. It should be noted the huge discrepancies between Models (GFS has precip reaching Manchester and ECM barely has it north of London, though the NCEP experimental FIM has it more like the NAE)

Another low comes towards the SW on Tuesday (this one the 2nd low currently north of the Azores). All models return north the frontal band on Tues. However because the ECM has not dragged as much milder air north (for reasons above on Monday) it is quicker to turn it to snow on Tues. The GFS which has the greatest incurson of milder air is slower to turn to snow *(an example of where early errors in a forecast can lead to further problems down the line) The ECM turn the rain to snow more bullishly on Tues across Inland Southern England, S Wales, Midlands to produce a snowy afternoon indeed across Central areas. Another 3-6cm possible as it pushes north.

The Snow pushes on North into N England and N Ireland with less cold air moving norht for a time and lots of heavy rain/showers across the south as a deep low moves through the Channel Hail and Thunder possible into Weds across the South.

Beyond that, another strenthening Jet Core comes across the Atlantic and develops another low close to the AZores in the highly embryonic area of the north Atlantic, this low turns NE and heads towards the South of England. There is perhaps not surprisingly another mirrad of solutions for this low, from winding up to the West of the UK to running across N FRance.

The UKMO GM, ECMWF and NCEP experimental FIM run the low through the Channel (Operational GFS has it further NW), which engages cold air again and sucks it into the Depression. More rain moving into the South and turning to Snow across Wales, Midlands and East Anglia (and Southern England if ECM is correct)

However given we have problems with the detail 24hrs ahead - we can have very little confidence in the forecast for 120Hrs, except to say it looks like staying very unsettled and cyclonic with a good deal of Snow for many - esp northern parts.

EDit - 06Z GFS is more like 00Z NAE and increases the Snow signal over the Midlands tomorrow. However NAE has moved SE too and is moving closer to ECM ( only has 5mm North of Oxford vs 19mm in 00Z) Clearly there is scope for sig errors in the forecast, but the trend is clearly SE, but it may be the SOuth Midlands & Northern parts of Southern England are at risk of Snow.

P Blight 09:53