Storm Forecast
Valid: Tue 25 Sep 2007 06:00 to Wed 26 Sep 2007 06:00 UTC
Issued: Tue 25 Sep 2007 03:00
Forecaster: VAN DER VELDE

SYNOPSIS

A deep longwave upper trough stretches southward over western/central Europe during this period, after which it becomes a large cut-off low. The cold front stagnates over Poland-Slovakia into the Alps by the end of the period. Ahead of the cold front, in the warm airmass, some unstable profiles can be expected northeast and south of the Alps.

Behind the cold front a cool airmass induces convection over the warm English Channel, southern North Sea and Kattegat. Advection of colder air from the north over the British Isles occurs by the end of the period, steepening near-surface lapse rates over sea, but with ridge building from the west suppressing deep convection.

Shortwave troughs affect N-Netherlands through Denmark in the morning, another one affects S-UK and NW France in afternoon-evening and France at night, and northern Italy will be under the continuous influence of QG lifting, increasing as cold upper air mass arrives. Other zones of deep convective activity are the trough over the southern/central Mediterranean the region of eastern Spain/Balearic Islands where the cold front arrives.


DISCUSSION

Only two areas see significant shear (15-20 m/s deep layer shear vectors, 200-400 m2/s2 SREH 0-3 km): E-Spain/Balearic Islands and southern/central Mediterranean (around Sicily).
Storms may develop mesocyclones and intense updrafts forming large hail, but low level shear remains on the low side, limiting tornadic potential.
The Thyrrenean Sea could see some waterspouts though, because of large 0-3 km CAPE and good 0-500m lapse rates predicted by GFS in the morning of Tuesday, in a weak convergent flow. GFS predicts large precipitation sums.


English Channel, North Sea:

Pretty weak (0-3 km) MLCAPE in GFS and strong convective precipitation signals suggests convectively well-mixed airmass. As 0-500m lapse rates are not yet very large the chance of waterspouts seems not worth issuing a level 1 at this moment.