Storm Forecast
Valid: Mon 06 Sep 2010 06:00 to Tue 07 Sep 2010 06:00 UTC
Issued: Mon 06 Sep 2010 05:17
Forecaster: SCHLENCZEK

A level 1 was issued for NE Spain and S France mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.

SYNOPSIS

The Omega pattern over the NE Atlantic tends to weaken during the following days as an upper trough over the Irish Sea will shift eastwards. Unstable air remains near the center of this upper trough and also over SE Poland / Slovak Rep. Subtropical air with weak to moderate instability should be located over S France, NE Tunisia and the E Mediterranean.


DISCUSSION

...Ireland and SW England...

In the wake of the cold front, upper level cooling and diurnal heating should provide some hundred J/kg SBCAPE in an environment with weak LL shear and locally 10 - 15 m/s deep layer shear. As the cloud base should be very low (only a few hundred meters AGL), an isolated funnel / brief tornado cannot be ruled out.

...NE Spain, S France...

Unstable subtropical air remains over parts of Spain and France with dewpoints near 20°C. This airmass should be strongly capped with CIN around -200 J/kg. Later in the day, the cap tends to weaken when 2m temperatures exceed 30°C in many places. Some isolated orographically-induced storms are expected to develop which tend to remain stationary in this weakly sheared environment. Locally, marginally severe hail is possible but the main threat is excessive rainfall with local flooding. An isolated severe wind gust event is not discounted either as a high cloud base will support downdraft acceleration by evaporative cooling. The large hail / heavy rain threat tends to decrease during the late evening / night hours as increasing CIN should reduce convective activity.

...Central Mediterranean...

Some 800 - 1500 J/kg CAPE are forecast over parts of the central Mediterranean / Tyrrhenian Sea. Convection tends to stay isolated as CIN is rather high but some storms may profit from 15 m/s deep layer shear and may become more organized. Isolated large hail and heavy rain near the coastlines is not ruled out but no threat level is issued as most of the precipitation should fall offshore.

...Other regions...

Low-end CAPE and weak vertical shear is present over Italy and Greece. Isolated thunderstorms may develop and low LCL heights combined with locally enhanced LL CAPE may allow an isolated funnel / waterspout near the coastlines. Over the eastern Mediterranean, instability is somewhat higher but organized severe storms are not likely. The heavy rain threat will mostly stay offshore. An isolated waterspout is not ruled out.