Extended Forecast
Valid: Thu 21 May 2009 06:00 to Fri 22 May 2009 06:00 UTC
Issued: Tue 19 May 2009 18:37
Forecaster: TUSCHY

A level 1 was issued for France, Belgium, Netherlands, Luxembourg, Germany, parts of Poland and the Czech Republic, NW/W-Austria and Switzerland mainly for severe wind gusts, large hail and tornadoes.

SYNOPSIS

Upper streamline maps still feature an elongated trough just west of Europe and ridging over the Mediterranean. A belt of strong SW-erlies affects western/central Europe and embedded disturbances result in widespread thunderstorm initiation.

A cold front over N-France and Benelux moves southeastwards during the day and serves as focus for thunderstorm development. Weak surface pressure gradients prevail over the rest of Europe and diurnal driven thunderstorms are expected over most parts of Europe.

DISCUSSION

... France, Belgium, Netherlands, Luxembourg, Germany, parts of Poland and the Czech Republic, NW/W-Austria and Switzerland...

A broad warm sector, filled with warm/moist air covers an extensive area over west/central Europe beneath steep lapse rates due to a northeastward moving elevated mixed layer. Quality of the boundary layer moisture ought to be no problem as the airmass had enough time for modification during the past few days. Widespread 500-1000 J/kg MLCAPE are expected with some fluctuations possible in respect of the surface moisture dispersal.

The whole warm sector is strongly sheared with 0-6km bulk shear of 20-30m/s and 15-20m/s at the lowest 3km. Combined with augmented 0-3km SRH and steep lapse rates, there is a large hail risk. Even an isolated very large hail report can't be excluded and for now this is the most certain hazard in the level-1 area. Both the tornado and the severe wind gust risk depend on smaller scale features which can't be discussed that far in advance. In fact we expect still some potential major model output changes due to the uncertainty of the convective activity in the day-1 outlook, so right now there is no real concentrated swath of damaging wind gusts or tornadoes forecast. This may be included in later outlooks however.

Initiation will likely occur along a slowly SE-ward moving cold front, crossing Benelux during the noon/afternoon hours and this front is placed somewhere over central Germany and central France during the evening/night hours. The enhanced wind field at the lowest 3km also points to the potential for organized squallines/bow echoes next to more discrete supercells ahead of the front.

The angle between the front and the background flow is minor, so a slow motion of the front hints on locally excessive rainfall, especially if storms train along the boundary.

An extensive level-1 was issued to highlight the potential magnitude of that event, but it is still too early to say, which region could see an upgrade.

...General thunderstorm areas ...

Diurnal driven thunderstorms develop but either shear or instability or both are too marginal for organized thunderstorms. Those storms pose a marginal hail/strong wind gust risk. Storms decrease in intensity and coverage after sunset.