Storm Forecast
Valid: Fri 18 Sep 2009 06:00 to Sat 19 Sep 2009 06:00 UTC
Issued: Thu 17 Sep 2009 15:58
Forecaster: TUSCHY

A level 1 was issued for the NW Mediterranean mainly for strong rainfall.

A level 1 was issued for S-Sardinia mainly for excessive rainfall.

A level 1 was issued for the SE coast of the Bay of Biscay mainly for excessive rainfall.

SYNOPSIS

The North American-style rex-blocking pattern over SW-Europe weakens as geopotential heights increase over central Europe. Upper low over SW-Europe remains in place with a marginal northward drift. Strong CAA affects NE-Europe with a cold and quite stable airmass.

DISCUSSION

... Noth of the Balearic Islands and the southern coast of France ....

Ongoing cluster of thunderstorms approaches S-France during the morning hours and noon from the south while weakening. Stratiform rain, probably enhanced by embedded thunderstorms, spreads northwards with strong rainfall. In the aftermath of this cluster, isolated to scattered storms re-develop over the western Mediterranean. Weak shear but moderate CAPE cause multicells to be the main storm mode. An isolated tornado event over the warm Mediterranean is possible, but overall conditions do not indicate an augmented risk.

... Sardinia ....

Increasing upper divergence causes another flare up of thunderstorms as strong subtropical streak draws near from the WSW after midnight. Numerous thunderstorm clusters evolve with an heavy rain risk mainly for the southern parts of Sardinia and offshore. Shear is too weak for anything organized, but strong winds could accompany the mature cluster. Thunderstorms keep going until 06 UTC and onwards.

... SE coast of the Bay of Biscay ...

Cold-core upper low loops northwards and again crosses parts of the Bay of Biscay. Thunderstorm chances increase during the forecast. Very weak background shear supports slow moving storms with backbuilding/training along the coast. Next to marginal hail, locally heavy rainfall will be the main risk.

... SE Europe ...

Daytime driven thunderstorms evolve in a very weakly sheared environment. Isolated higher CAPE amounts could assist in stronger updrafts with marginal hail and strong wind gusts. Thunderstorms decrease after sunset.