Storm Forecast
Valid: Sat 21 Apr 2012 06:00 to Sun 22 Apr 2012 06:00 UTC
Issued: Sat 21 Apr 2012 07:20
Forecaster: VAN DER VELDE

A level 1 was issued for eastern France and western Germany for large hail and tornado/severe gusts near the Alps.
A level 1 was issued for northernPoland mainly for marginally large hail.
A level 1 was issued for northern Spain mainly for large hail.
A level 1 was issued for northern Turkey mainly for large hail.

SYNOPSIS

A large low pressure area covers Europe from Ireland to Ukraine, northern Italy to central Scandinavia. The low pressure centers are found over the western North Sea and western Ukraine. A cool unstable airmass with steep lapse rates and slight to moderate CAPE covers almost the entire area. Almost everywhere the small-hail chances will be enhanced due to a low freezing level and not too low LCL. The eastern half of the area sees warm air advection into Belarus and a weak trough coming in from Romania. Flow and shear are weak there. Near an old front over Poland some 15 m/s deep layer shear can enhance storm dynamics and larger hail chances. More dynamics and vertical wind shear are encountered over northern Turkey and over central France through Germany, where a second cold upper airmass enters, with linear forcing by a marked PV maximum.

DISCUSSION

...northern Turkey...

a high PV / low dynamic tropopause nose pushes into mild CAPE under a 30-40 m/s jet streak at 300 hPa. 20-25 m/s 0-6 km shear will be present and perhaps 200 m2/s2 SREH. Supercells can form with a large hail chance.

...central/eastern France, northwestern Switzerland and southwest Germany...

Albeit MLCAPE will remain a few hundred J/kg at best, the new cold front is backed by advection of steep horizontal PV gradients and associated upward motion. The maximum of CAPE will be found behind the upper front though, but is still crossed by 20-25 m/s deep layer shear. Near the cold front low level winds are quite strong and 0-3 km SREH as well as 0-1 km shear are enhanced in Hirlam and GFS models. In GFS even to >200 m2/s2 and 15 m/s respectively. This increases potential of tornadogenesis with storms over eastern France/Switzerland in the afternoon or early evening. LCL heights will be 600-1000m. The rest of the area has mostly only deep shear with enhanced large hail chance.

...northern Spain...

This area is under more or less straight 20 m/s shear and >1500m cloud base heights with enhanced storm rotation and hail chances. Lifting mechanism is not so well defined except orographic factors.