Storm Forecast
Valid: Fri 25 Sep 2009 06:00 to Sat 26 Sep 2009 06:00 UTC
Issued: Thu 24 Sep 2009 19:12
Forecaster: TUSCHY

A level 1 was issued for N-Morocco mainly for excessive rainfall.

A level 1 was issued for the Ionian Sea mainly for excessive rainfall.

SYNOPSIS

Strong zonal flow has established over far northern Europe with various embedded disturbances affecting most parts of Norway and Sweden. Typical for such a zonal streamline pattern is a strengthening high pressure area along the anticyclonic shear side, which indeed causes quiescent and stable conditions over most parts of Europe. Two upper lows affect parts of the Mediterranean with showers and thunderstorms.

DISCUSSION

... NE-Algeria, N-Tunisia, parts of the Tyrrhenian Sea, the Ionian Sea, Sicily, S-Italy and parts of the Aegean Sea ...

A weakening upper low opens over the Ionian Sea but still assists in scattered showers and thunderstorms. Main focus for persistent thunderstorm development was highlighted with a level 1 due to an excessive rainfall risk. A warm and moist air mass continues to loop around the still intact mid-level circulation with deep convergence signals over the Ionian Sea. In addition, a favorable position of the subtropical jet to the south supports upper divergence, which ought to yield favorable conditions for numerous thunderstorm clusters.

Thunderstorms beneath the upper low and along its north and west side are forecast all day long but mesoscale boundaries and local topography will play a major role in distribution of convective activity. Overall, thunderstorm activity will probably remain on the weaker side south of Sicily, where somewhat drier and more stable air overspreads the Mediterranean.

... N-Morocco (21 UTC onwards) ...

Another upper impulse appraoches from the west. Increasing forcing and a northeastward pointing subtropical jet streak cause scattered initiation during the late night hours. It remains a bit uncertain how far north activity evolves, but gloabl models agree in a sharp cyclonic curvature of the steering flow over far S-Spain with decreasing support for thunderstorms due to decreasing instability. Stratiform rain could affect parts of extreme S-Spain, but no northward expansion of the level 1 became necessary. The main risk is local flash flooding over N-Morocco.

Daytime driven thunderstorms are forecast along the southern Alps. Marginal hail and gusty winds are the main risk.