Storm Forecast
Valid: Mon 24 Aug 2015 06:00 to Tue 25 Aug 2015 06:00 UTC
Issued: Mon 24 Aug 2015 04:49
Forecaster: VAN DER VELDE

A level 2 was issued for Belgium, Netherlands and NW Germany mainly for tornadoes and severe convective wind gusts.
A level 2 was issued for NE Italy mainly for large hail and excessive convective precipitation.
A level 1 was issued for S Scandinavia for large hail.
A level 1 was issued for S Germany for large hail.
A level 1 was issued for Italy mainly for large hail and tornadoes.
A level 1 was issued for NW Algeria and Tunesia mainly for severe convective wind gusts and large hail.

SYNOPSIS

A vigorous low pressure system with sharp pressure gradients moves through the English Channel into the North Sea. Its strongest rising motions occur over N FRance, Belgium, Netherlands and NW Germany. A cold front should roughly be situated over NE France, Belgium and SW Netherlands by 12Z (GFS 18Z run), occluded and bending back to the center of the low via southern UK. The warm and especially humid airmass harbors 200-700 J/kg MLCAPE. Unstable airmasses also are found over southern Scandinavia (1000 J/kg), Germany and over N Italy (1000-2000 J/kg).

DISCUSSION

...France, Belgium, Netherlands, Luxembourg, N Germany, S England...

Strong lift will likely trigger widespread thunderstorms. While most low-level wind is found behind the cold front, the region along the cold front seems to develop more than 10 m/s 0-1 km shear and enhanced SREH which can assist in tornadogenesis, particularly together with the low LCL heights in the humid airmass. One or more tornadoes could occur, as well as severe gusts. Isolated excessive rain may occur over S England where the occlusion and embedded thunderstorms can drag slowly.

...S Germany and also S Scandinavia...

Indications of moderate DLS but very localized SREH suggest that an isolated supercell may form with a primary chance of large hail. 0-1 km shear seems enhanced in GFS over the Kattegat/Skagerrak region which supports tornadoes.

...N Italy anf NW Balkan...

This region has some lift from shortwave troughs superposed on the southeast flank of a large upper trough. Warm low level airmass is advected northward from Tunesia to the Alps. A cap exists until mid Italy. Weak capping over N Italy should result in numerous storms, some of which can be split/supercells with large hail, given 15-20 m/s deep layer shear, with SREH enhanced only along coastal convergence zones. LaMMa and Modellzentrale WRF models predict very large precipitation accumulations over northeastern Italy, for which a level 2 is issued. Also waterspouts are likely across the level 1 region.