Storm Forecast
Valid: Thu 11 Oct 2012 06:00 to Fri 12 Oct 2012 06:00 UTC
Issued: Wed 10 Oct 2012 20:23
Forecaster: GATZEN

A level 1 was issued for the west Mediterranean mainly for excessive precipitation.

SYNOPSIS

A polar trough affects eastern Europe, and cool/dry low-level air is present to the north-east of a line from the North Sea to the Balkans and northern Aegean/Black Sea. A westerly jet ís situated across the Mediterranean, where rather moist air is present. Given the warm sea surface, this air mass is rather unstable. During in the period, another trough moves into western Europe.

DISCUSSION

British Isles, France

A warm and moist maritime air mass is situated across the Atlantic west of the Bay of Biscay. This air mass is convectively mixed and is advected north-eastwards ahead of the approaching trough. Along the axis of warm air, a well-developed low-level trough is present and will move into France and the British Isles. At the surface, backing low-level winds are indicated by latest GFS model across southern France and the British Isles, leading to low-level convergence. A weak mid-level trough will also lead to some QG forcing.

Daytime heating will result in weak instability across France. Instability over the British Isles is questionable, but elevated CAPE is likely given the well-mixed air mass. Given the lift along the trough axis and low-level convergence, at lead weak CAPE is likely. Together with CAPE and forcing, low-level vertical wind shear is expected to be around 10 m/s in the lowest km and low-level hodographs are rather large across the British Isles. Current thinking is that tornadoes can develop in this environment. Given the weak instability, chance of tornadoes seems to be too low for a level 1 forecast. Stronger storms may also produce excessive precipitation. Otherwise, severe storms are not expected across the Birtish Isles and France.

North-west Mediterranean

A warm and moist air mass is situated across the west Mediterranean and spreads north-eastward during the period. The warm sea surface and rather cool air aloft results in weak instability and storms are forecast especially over the sea along of land-sea-breeze convergence zones. Most storms will likely be non-severe. However, vertical wind shear is stronger at the northern flank of the warm air plume, where 0-3 km vertical wind shear exceeds 10 m/s. Multicells and mesocyclones besome more likely there with excessive precipitation the main threat. However, every mesocyclone the forms will have a potential of producing large hail and tornadoes.