Storm Forecast
Valid: Tue 19 Aug 2008 06:00 to Wed 20 Aug 2008 06:00 UTC
Issued: Tue 19 Aug 2008 02:37
Forecaster: VAN DER VELDE

SYNOPSIS

A depression is crossing the North Sea. Thundery convection is likely near the center of the low, and along its thermally weak cold front extending from Poland into Spain, including a zone of average 0-1 km mixing ratios of 10-11 g/kg sufficient for some 500-1000 J/kg MLCAPE. A dry intrusion well visible in WV and 700 hPa GFS fields follows this cold front. Deep layer shear is enhanced to values of 18-22 m/s all along the cold front.
Higher instability is present over in a zone over Ukraine and Russia, where shear is more limited (DLS 10 m/s), locally moderate.

DISCUSSION

...SE France, SE Germany...

Isolated to scattered thunderstorm activity is suggested by GFS fields, with most precipitation between Massif Central and Bavaria. As low level divergence and somewhat negative QG forcing are predicted over central Germany, initiation may be more inhibited there.
Moderate deep layer shear and patches of slightly enhanced SREH of 100-150 m2/s2 support multicells and a few supercellular storms, with mainly a chance of large hail. An isolated tornado is not excluded especially where 0-1 km shear >10 m/s, GFS reaches 8 m/s in most places but in reality this may locally be higher (also SREH).

...W Poland, E Germany...

In this area GFS predicts enhanced LL shear and SREH: 12 m/s and 200 m2/s2. CAPE is less at 15Z but this seems to be caused by rapid initiation of model convection in GFS. The area is also near the nose of the dry intrusion and could see enhanced ascent. If convection is discrete instead of MCS, an isolated tornado could occur, and isolated large hail.

...Spain...

The region near Tortosa has good chances of a thunderstorm as winds from sea converge over higher terrain. LCLs are high. Shear conditions are comparable to those described for southern France, which may yield a supercell or multicell with isolated large hail threat.

...Ukraine, Russia...

Isolated to scattered large hail may occur as conditions can locally be marginally good for supercells and multicell clusters, but forcing is only weak.
Over NW Russia a frontal wave may spawn an MCS with chance of strong gusts near the start of the period.