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Winter 2009 / 2010 in Western Europe?

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Author Topic: Winter 2009 / 2010 in Western Europe?  (Read 27818 times)
martinastro
Martin Mc Kenna
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« Reply #390 on: February 14, 2010, 10:14:32 pm »

Update - 14th FEB 10.15Z

A forecaster's nightmare this week - with temps at the crucial level where 1 degree either way will make the difference between Rain/ Snow.

Synoptically - The slow moving high which has been anchored nr NW Scotland for most of the week, is now slipping away NW as a result of warm advection off the east coast of N America and strong cold advection and cyclogenesis occuring over the Norwegian Sea.  OVer the coming days this low will slip SE, and then rather against normal protocal will extend SW (comes up against raising contour heights in SE Europe, will be steered SSW over the British Isles, with a deep layer cold pool extending SW just to the west of the UK, whilst at the same time the old cold pool over Europe is steered NNW.  Overtime both pools are modified by convective latent heat release, but over the UK there will remain an awkward temp level which will mean that either rain/sleet or snow is possible - largely dependent on time of day, wind, altitude and precip intensity. The worse mix of all for forecasting.

Initially we have cold air hanging on in the SE, and another weak baroclinic zone is trying to bring more modified Atlantic air SE, and warmer air is trying to push the cold air SE. Snow being reported this morning from Biggin Hill, Odiham, Stansted, Gatwick.  Snow largely above 200M elsewhere an awkward mix of rain & sleet.

A very sunny morning over Wales, Midlands, this tending to give way to a more cloudy regime as the day progresses, with lght rain breaking out over the Irish Sea later today.

As the new low comes SSW, it extends a front SE across the UK, initially rain falling into 400-600 WBFLs so largely rain, except over the Grampians where more snow will fall esp as some of the cold air gets dragged in from the European cold pool as it moves NW.  The front moves SE into Tuesday and becomes slow moving from the IOW to the Wash (within 100 miles or so). The ECMWF has the rainband a tad further NW than the GFS, UKMO in between the two.

Its overnight into Weds that it becomes very difficult to forecast.  The front will not only be sat nearly stationary overnight, but will entrain slowly a more continental airmass.  So a mixture of overnight temp falls, latent heat release through precip and colder air and lightwinds will make the band increasingly transition to Snow. Areas N and NE of London see most likely - The ECMWF has lower dewpoints at the time than the GFS and is more keen on turning the front to Snow (though 06Z GFS has more snow by 06Z than its predecessor (better))

4-6cm possible and up to 10cm in places cannot be ruled out - though considered low prob at this time as front will be weakening. (20%)

The front slowly dying out on Weds before more rain spreads from the SW later in the day as a low comes ENE across England at the end of the week.  This low will have cold air to the NW again the ECMWF has colder 850mb temps and dewpoints than the GFS, meaning a high prob of snow to the N and W of the low as it moves across to the N Sea.  Areas around N Wales, NW England the most likely areas to see some heavier snow falling as we go into the end of the week.  GFS 00Z had a strong snow signal also - though 06Z less so (all very marginal and as stated above 0.5 degrees both in terms of low pressure positioning and temp levels makes a lot of difference this week) However There is reasonable agreement on some heavier snow around the Merseyside, Manchester areas of the Populated NW - 5-10cm possible (maybe more) needs careful watching - esp intp Thursday night and first thing on Friday.

Another front comes east into Saturday with again cold air firmly in places across the north, rain in west wales, and the SW will turn to snow across the N Midlands, N England. ECMWF has a strong snow signal into Sat night with rain turning to heavy snow in places esp over 100m, but in heavier areas down to sea level.   

It could end up being a fairly wintry week in parts - esp the north, but not only on high ground.  The coastal west and SW the most definate areas to stay on the milder side of the rain/snow boundary.

Paul Blight
UKww Manager - Education/Warnings
Associate Fellow Royal Meteorological Society
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