Storm Forecast
Valid: Thu 16 Apr 2009 06:00 to Fri 17 Apr 2009 06:00 UTC
Issued: Thu 16 Apr 2009 04:58
Forecaster: VAN DER VELDE
SYNOPSIS
A low pressure area with it center between Ireland and Spain affects a large part of western Europe with unstable weather. The main occluded front remains stationary over southern Great Britain, Belgium/Netherlands and Germany. The sharp convergence zone along the front provides a clear focus for thunderstorm development, and flow parallel to the front may cause training of storms through confined areas.
Elsewhere, over France and Spain, widely scattered convective cells will readily develop in the colder airmass. With steep low-level lapse rates and 0-3 km CAPE and weak flow, an isolated landspout is not ruled out.
Less defined is the instability over the area of northern Italy, but the presence of a strong jet creates lift and an environment of strong shear.
DISCUSSION
...northern Italy...
A marginal level 1 is issued for this area. Up to 30 m/s 0-6 km bulk shear is predicted, crossing weak instability. There is less shear in 0-3 km layer and patches of 8-10 m/s low level shear. Storm-relative helicity is enhanced to 100-250 mē/sē but does not appear to overlap during the period of most instability: after 18Z and the west side of the area. An isolated storm may produce large hail or severe wind gusts. The same goes for late at night in the Istria area, but with increased LL shear (>12 m/s inland) a tornado is not excluded.
...Germany, Benelux...
CAPE is currently not really large, but marginally enhanced deep layer shear (10-15 m/s) and deep dry boundary layers like in the 00Z Essen or De Bilt sounding (small warm cloud depth) support some chance of reasonably large hail, as well as gusts (evaporative cooling). Additionally, large scale rising motion may further destabilize the profiles. The forecast well-defined convergence zone with mid level flow mostly parallel to the line (posibly enhancing hail cycling), may cause some multicells to be effective hail producers.
There is potential for thunderstorms tracing along the path of previous cells which could locally cause large rain sums, but as profiles currently still seem rather unsaturated, the chance seems limited.
