Storm Forecast
Valid: Fri 09 May 2008 06:00 to Sat 10 May 2008 06:00 UTC
Issued: Thu 08 May 2008 19:40
Forecaster: TUSCHY
SYNOPSIS
A strong ridge over central Europe is framed by a well evolved trough over SW Europe and a broad trough over eastern Europe. Weak pressure differences are present resulting in a broad area of thunderstorm probabilities over most parts of Europe.
DISCUSSION
... UK and parts of Scotland ...
A stalling frontal boundary is the focus for scattered to widespread convective initiation from S - UK to Scotland. Yesterday ( 17Z ) surface data behind the slowly NNE - ward shifting cold front revealed a plume of a notedly more humid airmass with dewpoints between 10 and 15 °C over far NW France / extreme SW UK spreading NE - wards. This airmass overspreads the forecast area during the night hours and will reach far northern Scotland at Friday, 06Z. As high pressure east of UK strengthens during the day, a somewhat warmer airmass approaches UK from the SSE wich should weaken mainly mid-level lapse rates a little bit. However, a moist BL should offset those weakening lapse rates. Some diabatic heating will help to overcome weak cap at lower levels and numerous showers / thunderstorms will develop. A combination of low LCLs and enhanced 0-3km CAPE release favors a few funnel / short lived tornado reports over the area of interest beside small hail and strong wind gusts and a low - end level - 1 area was issued. There is a distinct risk of significant rain amounts as storms should cluster betimes. A moist BL and slow storm motion all point to an enhanced flash flood risk mainly over central / N - UK and Scotland . This threat is not included in our risk scheme.
GFS hints at a short wave which approaches S - UK from the south around 18Z , moving rapidly northwards. This forcing supports thunderstorm re -/ development during the night hours mainly over N - UK and Scotland while activity rapidly diminishes over S - UK after 18Z - 21Z.
... Portugal and Spain ...
As strong upper - level low approaches Portugal from the NW during the morning - midday hours and SW Spain during the evening hours, widespread thunderstorms are forecast. Neither shear nor instability hint at any enhanced severe thunderstorm risk although an isolated funnel / tornado report is possible over SE Spain where LL CAPE is somewhat enhanced. Otherwise, strong wind gusts and marginal hail will be the main risk, the latter one mainly over Portugal and SW Spain as atmosphere rapidly cools down.
... NE - Algerian coast ...
A short wave crosses the area from the SW during the evening hours. Scattered to widespread thunderstorms are anticipated. Both shear ( DLS around 15 - 20m/s and SRH - 3 locally up to 300 m^2 / s^2 ) and instability ( 500 J/kg and plus ) favor an enhanced large hail risk and even an extreme hail event with hail larger than 5cm will be possible. Thunderstorms will keep going until 06 Z while spreading east / northeastwards. A small level - 2 may be issued later on if following model data hint at a more significant hail risk as currently anticipated.
... N - Finland / N - Sweden / N - Norway ...
A strong cold front crosses those regions during the evening and night hours from the west. Kinematics are impressive as 50 m/s mid -level speed max races eastwards ( 35 m/s at 700 hPa and 20 m/s at 850 hPa ). Bad timing and limited moisture should keep convective activity quite limited but any stronger convective line could pose a severe wind gust risk. Fow now we did not issue any highlighted areas as cold front should be too inactive but the region has to be monitored and a level - 1 may be issued with / without a general thunderstorm area in subsequent updates.
... Other areas ...
There are numerous other regions with thunderstorm chances but weak shear / limited instability release should preclude an organized thunderstorm risk. Those areas were highlighted with a general thunderstorm area and risk for thunderstorms should diminish after sunset.
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