Storm Forecast
Valid: Sun 10 May 2009 06:00 to Mon 11 May 2009 06:00 UTC
Issued: Sat 09 May 2009 13:05
Forecaster: TUSCHY

A level 1 was issued for W-France mainly for large hail and severe wind gusts.

A level 1 was issued for Poland mainly for large hail and severe wind gusts.

SYNOPSIS

A progressive through affects N-Europe during the period, bringing unsettled conditions for most parts of Sweden, Norway and Finland. Another trough sweeps in from the west and approches Spain/Portugal during the night hours. The rest of Europe is situated under a "dirty" ridge with numerous disturbances moving ENE-wards. The atmosphere is quite unstable and therefore thunderstorms will be a good bet over most parts of central Europe.

DISCUSSION

... SW-W France ...

A northeastward traveling upper wave crosses W-France during the day while weakening. Colder mid-levels accompany that feature and keep mid-level lapse rates at least moderately steep. Dewpoints in the lower tens at the surface, coupled with those environmental conditions result in moderate MLCAPE release on the order of 500-1000 J/kg. DLS between 15-20m/s helps thunderstorms to gain some organisation and multicells will pose a large hail and strong to severe wind gust risk. After sunset, CAPE diminishes but a combination of some MUCAPE and weak forcing keep thunderstorms going over the Bay of Biscay during the night.

... Poland ...

Embedded in a WSW-erly flow, a short wave travels to the ENE. This disturbance and attendant cold front are the foci for initiation during the afternoon hours. Surface dewpoints around 10°C, moderate lapse rates and some diabatic heating foster scattered thunderstorm development. DLS around 15m/s and 400-800 J/kg MLCAPE overlap so storms pose an isolated large hail/strong to severe wind gust threat. The background flow is near parallel aligned to the SE-ward moving cold front, so thunderstorms will have a hard time to stay more discrete, which dampens the overal severe risk somewhat. Right now we expect a loosely organized squalline with aforementioned hazards, which affects most parts of Poland during the day. Thunderstorms go on during the night while moving eastwards but intensity and coverage decrease betimes as boundary layer stabilizes.

... The rest of the thunderstorm areas ...

Either instability or shear or both are too weak for long-lived, organized convection so no severe risk is forecast.

Most thunderstorms decay after sunset despite the activity over Portugal. As the main trough finally approaches from the west, strong forcing and some MUCAPE could generate a few storms in a strongly sheared environment over NE Portugal/NW Spain after midnight, but confidence in anything organized is still too low for a level area. Otherwise, thunderstorms under the base of the trough affect Portugal during the night hours with marginal hail/strong wind gusts.