Message #14860

[martinastro]

19Z Update - 22 Feb 2010

General Situation

Synoptically nothing is changing fast. Aloft, a large high is located over Greenland and the Davis Straights into Eastern Canada. A deep trough is extending SW from the Barents Sea to West of the UK and a very strong combined Polar Front and Subtropical Jet covers the Atlantic and European Sectors (140Knts West of Maderia at 12z)

Over the coming days, little changes to the NW of the UK, but 300  &500mb heights rise over N Africa and the Med over the coming days forcing the Jet to veer towards the UK, this means it becomes less cold across the South of the UK, but with low level cold ever never too far away we can expect further snow - esp mid week over Northern parts of the UK.

The 500mbn pattern becomes less zonal with time over the Atlantic sector as more meridionality is returned.

Possible Severe Weather

Heavy Snow for Northern Parts on Weds / Thurs and Heavy Rain for the South

Discussion

Today's snow was quite extensive this morning - but weakened this afternoon. Still some heavy rain cl.ose to the Sussex and Kent coasts thisd evening, but largely clear and cold tonight across the north , this colder air advecting south again overnight and getting as far as about the M4. Another very cold night across Scotland. Still Snow showers around the North and NE of Scotland.

Main next event is the next frontal system coming into the South and moving north.  The frontal timing has continued to slow on succussive, GFS, NAE and ECM runs, but confidence is reasonably high for a fairly snowy spell of weather on Weds/Thurs across first Northern England and then Scotland. 

Tomorrow rain comes back into the South with WBFL, largely above 600m, however this falls into increasingly cold air which will be in place across Wales, Midlands and N England, it turns to snow over the Welsh Hills above 150m-200m and then expands NE as the thermal divide across the front increases.  Precip intensitiies pick up and both the GFS and NAE and inferred precip amounts in the ECM generate a fairly extensive spell of Snow later tomorrow into the evening across the Midlands, Wales and N England, most accumulations above 200m, and turning to rain from the south with time.   2-5cm in places, and up to 10cm over 200m or so esp N Wales, Peaks and PEnnines.

Much less cold - WBFL above 1000m move into the South on Weds, but it still stays very unsettled as the deep convective low (nr 20W) moves east along the SOuth Coast.  Models in agreement for some very heavy rain, hail and Thunder to move east giving local flooding in places.  At the same The snow pushes North into N Ireland, S Scotland giving several cms in places and the front actually slows down and the warmer air occludes out to the east, hence we keep cold air in the north maintaining the risk of Sleet and Snow right into Thursday. (though by then largely Scotland only)

Another deep depression with origins well south of 40N comes NE on Thursday and moves close to the South Coast again Around 975mbs overnight into Friday , another very heavy band of Rain comes NE (another 20mm possible) and this band then pushes N to rejuvenate the oclcusion by then almost stationary across Scotland giving more snow.  On Friday , the cold air spills back South again and the rain turns to Snow again across parts of N England and N Ireland

Windy at times in the South, though the strongest winds occuring over the Channel Islands and N France .

Beyond the end of the week, there has been discrepancies in the handling of an extremely low latitude low which moves ENE, then NE towards Western Europe.  Various models have deepened/ Not deepened this low depending on its speed NW at the same time as an active shortwave runs SE towards Iberia.  This low develops NE of Puerto Rico in the relatively deep tropics and moves ENE towards Maderia and the Canary Islands before turning NE, and deepening as it engages both an active shortwave and two Jets.  The low on the 12Z Models interacts with the LEft exit of a propogating NW;'ly Jet diving south of the Azores and the right entrance of a SW'ly Jet over the SW Approaches and English Channel.  This combination allows for massive amounts of upper level diflfuence and allows large scale ascent to take place forcing the low to deepen potentially explosively.  It will also have PVA and a large thermal divide(  Deep layer PM air diving SE and warm Subtropical air riding NE) (Theta W across the low goes from sub 6C to > 12C creating a huge amount of precipitation. 

12Z Models are in actual good agreement over the low, with it coming NE west of Iberia.  GFS has it furthest NW (earlier deepening) and pushes it across the SE of England, whereas the ECM and UKMO GM keep it over NE France (both have a deep low coming NE across NC France at T+144, ECM then moves it into Holland slowly filling) IN the GFS solution the low is closer to the cold air over Scotland and sucks it SE into the Low and the precip turns to Snow, however the more eastern movement in the ECM and the faster movement, means this struggles to occur.  Anyway - lots will change before thisd occurs - however the development needs watching

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