Storm Forecast
Valid: Sat 27 Mar 2010 06:00 to Sun 28 Mar 2010 06:00 UTC
Issued: Sat 27 Mar 2010 05:51
Forecaster: VAN DER VELDE

A level 1 was issued for central/southern France for a chance of an isolated tornado and marginally large hail.

SYNOPSIS

At 12Z, a large upper trough is situated over western and central Europe. Two main low pressure centers will remain stationary near the Shetland Islands and the southern Baltic Sea. Several shortwave troughs are circling around. Over France, southern Germany, Italy and Balkan these will activate thunderstorms in the slightly unstable airmass.

DISCUSSION

...France...

The jetstream tightly rounds the unstable airmass such that some overlap exists between CAPE and deep layer vertical wind shear. Predicted instability is rather sketchy in GFS but obvious in current satellite images. Strong advection of moist air in low levels and the forcing by the upper shortwave trough can keep this going until the afternoon.
A lobe of 0-6 km shear >20 m/s and 0-1 km shear over 15 m/s, and SREH3>250 m/s can interfere with convective development, with very low LCL heights (500m), this can lead to tornadogenesis and strong updrafts supporting marginally large hail.