Storm Forecast
Valid: Thu 21 Aug 2008 06:00 to Fri 22 Aug 2008 06:00 UTC
Issued: Thu 21 Aug 2008 06:28
Forecaster: GATZEN

SYNOPSIS

Trough axis over western Europe extends from western Scandinavia and North Sea to north-western Iberian Peninsula. A strong south-westerly mid-level jet is present from Portugal and Bay of Biscay to Germany, Poland, Belarus and northern Ukraine into western Russia. A short-wave trough travels north-eastward over Belarus during the period. High pressure and increasing temperatures affect southern Europe. North of the frontal boundary from Bay of Biscay to western Russia, maritime air mass has spread into northern Europe.

DISCUSSION

Baltic States, Belarus and northern Ukraine into western Russia

Ahead of the approaching upper trough, a cold front moves eastward over Belarus. Warm air mass further east is likely stable over southern Belarus and Ukraine as indicated by latest soundings given poor lapse rates. To the north, low-level lapse rates are improving, and diurnal heating will likely be sufficient for CAPE east of the approaching cold front. Given QG forcing due to the propagating trough and associated strong upper jet streak, convective initiation is likely from Baltic States to northern Belarus and western Russia. Underneath the strong upper jet, vertical wind shear will likely be favourable for multicells and mesocylones along and ahead of the cold front. With weak low-level convergence and limited amount of CAPE and shear, potential for severe storms is not too high. Local hail reaching around 2 cm in diameter seems to be the most significant threat. An isolated tornado is not ruled out over western Russia, where low-level vertical wind shear will be stronger along the approaching cold front. In the wake of the front, latest models suggest that weak instability will remain over the Baltic States due to moist adiabatic lapse rates and rather rich low-level moisture. Latest soundings confirm this unstable air mass. Given some sunshine west of the cold front, instability is not unlikely to develop. Limiting factor is weak forcing west of the cold front, but some storms seem to be possible. Given strong low-level winds, some severe wind gusts are expected with this convection that will likely die in the evening hours as low-level heating stops.

Northern Spain into southern France

To the east of the southern trough axis, warm air mass is present over eastern Spain, while a cold front moves south-eastward into north-western Iberian Peninsula. Ahead of the cold front, models indicate a strong mid-level jet curving around the base of the trough and providing strong QG forcing. This will likely be associated with mid-level height falls and increasing lapse rates in the evening hours. At the surface, a low is forecast to develop over central Spain, moving north-eastwards during the night hours. Easterly surface winds will likely advect substantial moisture into northern Spain. As a consequence, latest stable thermodynamic profiles may improve significantly in the evening hours. With strong QG forcing, initiation becomes likely in the evening hours over northern Spain, and storms will spread north-eastwards into France. Convective cells will be well-organized given strong vertical wind shear (15 m/s 0-3 km) and also SRH, and large hail is expected with more discrete cells. Isolated severe wind gusts are also not ruled out. Storms will likely merge into a convective cluster moving into southern France, where severe potential is expected to decrease due to limited low-level moisture.