Storm Forecast
Valid: Tue 02 Sep 2008 06:00 to Wed 03 Sep 2008 06:00 UTC
Issued: Tue 02 Sep 2008 02:42
Forecaster: VAN DER VELDE

SYNOPSIS

A low filled with unstable airmass covers Great Britain and Ireland, with several shortwave troughs circling around it. The cold front belong to this depression affects northern France, the Benelux, NW-half of Germany, Denmark and SW Norway during this period. Large geopotential height gradients flank the front and create strong vertical shear. However, the part over Benelux and western Germany is according to GFS overspread by subsidence, and equilibrium levels of any convection along the front barely reach sufficient depth for mixed-phase growth of precipitation. This should be different over parts of France.
Other (weak) lows with enhanced convective activity include southern Italy, and southwestern Turkey.

DISCUSSION

...northern France, Belgium, Netherlands, W Germany...

Slight CAPE is generated in GFS for the afternoon, and strong deep convergence along the front. While tops will probably not be high enough for thunderous convection, the area should be monitored for signs of it. Any storm in this shear environment (15 m/s LLS, 200 m2/s2 SREH-3, 20 m/s DLS) could develop tornadoes. NW France is indicated by a level 1 for the same reason and better chances for instability release in the morning (uncapped parcel layer depth convergence), though it may come from elevated levels. 0-2 km shear has a 8-14 m/s component perpendicular to the gradient which supports also strong line convection with gusts (max 20 m/s).

...central France...

In the outskirts of the strong shear region to the north, but some few hundred J/kg CAPE to work with and steep mid level lapse rates, isolated cells, developing in the GFS-predicted convergence line, may rotate and would be strong enough to produce large hail, and perhaps a tornado.

...NE Spain...

The southward extending surface trough/low induces inland advection of Mediterranean moisture, with some 1000 J/kg MLCAPE in a moderate shear environment (15 m/s DLS, 100-200 m2/s2 SREH and steep lapse rates, some multicell and marginal supercell storms can develop with large hail as primary threat. Given dry low-mid levels, cold pools with be strong.

...England and Scotland...

Predicted strong 0-3 km CAPE, relatively humid air and weak wind pattern within the low support spout-type tornadoes/funnel clouds on vertical vorticity sources... given GFS difficulties to simulate subtle profile differences these may well occur also outside the level 1.

...SW Norway...

Again, high shear conditions along the front (similar to those in NW France) and a band of instability provide a chance of tornadoes with any well-developed cell, augmented by the low LCL heights.

...N Algeria, Tunesia...

Moderate/strong DLS (20 m/s) and SREH (100-200 m2/s2) through >1000 J/kg MLCAPE should create multicells and supercells with large hail and severe gusts.