Storm Forecast
Valid: Thu 29 Mar 2007 06:00 to Fri 30 Mar 2007 06:00 UTC
Issued: Thu 29 Mar 2007 09:04
Forecaster: VAN DER VELDE

SYNOPSIS

Shallow low pressure area is present from the North Sea over France/Germany into the western half of the Mediterranean Sea. Cold upper air in combination with sufficient low-level theta-e leads to convective temperatures in the 12-15 C range over land evidenced by 00Z soundings, while tops can reach -40 C. GFS has confidence LFC will be reached over a large area. Hence, widespread thunder from mostly single-cells (weak shear environment), like the previous day over northern France, is likely over much of the Benelux, France, Germany and Switzerland. An isolated landspout and small hail are possible. A zone of slightly enhanced theta-e in weak SW-ly flow and plenty of solar irradiance moves slowly eastward over France and Benelux, while a small but high vorticity upper low enters the western part of the thunder area, behind the theta-e tongue. This may trigger additional convection later into the period, possibly in the form of a comma cloud over western France.
Ahead of this feature the tight upper pressure gradient causes very good deep layer shear, but thinking is that air only unstable enough behind the shear zone and over sea, so a marginal level 1 is drawn only for a small coastal strip, with some chance of marginally large hail and a tornado.

Within the Mediterranean low pressure zone, several low level convergence features trigger convection, as well as a more dynamic trough coming out of Tunesia. Most absolute moisture is over the Italian waters, while an upper jet touches the unstable airmass over Malta, Sicily and southeastern Italy. With deep layer shear over 15 m/s, low level shear around 10 m/s and 0-3 km SREH up to 150 m2/s2, storms developing under the jet can develop rotation and cause large hail and tornadoes. However, instability is higher to the northwest and GFS is rather confident that convective precipitation is not going to occur within most of the level 1 region until the night, apparently due to capping, and only sparsely so.
Elsewhere in the Mediterranean, an isolated waterspout is not ruled out.