Storm Forecast
Valid: Wed 13 Aug 2008 06:00 to Thu 14 Aug 2008 06:00 UTC
Issued: Tue 12 Aug 2008 22:01
Forecaster: GATZEN

SYNOPSIS

Unseasonably intense trough present over north-western Europe, and a very strong jet moves across Bay of Biscay region, north-west Mediterranean, France, Alps, Germany, Czech, Poland, and Baltic, reaching western Russia during the period. Winds will reach more than 60 in the jet axis. To the north of the jet, cool maritime air mass will affect north-western Europe north of a frontal boundary from southern Bay of Biscay to Alps, south-eastern Poland to eastern Baltic States. Some instability will likely develop in the range of the trough centre. To the south and east, warm and unstable air mass is expected from south-west Mediterranean to northern Italy region and northern Balkans, while only weak instability is forecast from south-eastern Poland to western Russia.

DISCUSSION

Northern Italy to western Hungary

In the lee of the Alps, a tongue of warm and unstable air mass is expected to remain during the day. Latest soundings indicate rich boundary-layer moisture centred over southern France. This moisture is expected to spread eastward and southward ahead of the approaching cold front that will lead to dry Mistral winds over southern France. Aloft, a strong jet streak is expected to move across west Mediterranean. QG forcing in the range of this jet streak is expected to lead to destabilization from north-western Italy to western Hungary in the afternoon hours. Convective cells may initiate over the region given some convergence along the leading front of the Mistral over northern/north-western Italy as well as further east along the frontal boundary from southern Austria to western western Hungary. Limiting factor is rather strong capping inversion, and storms may be isolated. Given very strong vertical wind shear with around 30 m/s DLS over northern Italy, storms will rapidly organize. Large hail is not ruled out as well as severe wind gusts. A tornado is not ruled out with more isolated cells, while limiting factors are rather weak low-level vertical wind shear below 10 m/s even in the evening hours, when forcing will decrease over most of Italy in the wake of the vort-max. An upgrade may be needed when low-level vertical wind shear will evolve stronger than expected at this time.

Czech Republic to south-eastern Poland to western Russia

Ahead of the cold front, a warm air mass characterized by quite steep mid-level lapse rates but only poor low-level moisture is forecast across eastern Europe. Latest observations indicate that this air mass will not build significant instability during the day even with strong daytime heating. However, given strong low-level convergence along the propagating cold front, a frontal rain band is expected, and a few strong wind gusts may evolve given strong vertical wind shear. An upgrade will be needed when significant moisture will develop and convection becomes more likely along the front.

Baltic Sea region, North Sea region, north-western Germany

Unseasonably strong low pressure system will affect a large area leading to strong low-level winds and associated low-level vertical wind shear. 0-1 km SRH values are forecast in the range of 100 to 200 m²/s² along the North Sea coasts as well as western Baltic Sea. Lower values are predicted further east. At the north-eastern flank of the strong upper jet, strong QG forcing is forecast and maritime air mass will likely destabilize over the North Sea and Baltic Sea region as well as over southern Scandinavia. While vertical wind shear will be rather weak near the trough centre, shear and instability may overlap at the southern edge of the unstable air mass, where storms will likely be organized. While severe hail seems to be unlikely given weak CAPE, severe wind gusts will be possible in the range of a strong jet streak reaching 25 m/s at the 850 hPa level from the channel to Benelux and north-western Germany, as well as Denmark, and southern Sweden later in the period. Along the gust fronts of thunderstorms, a few tornadoes may also develop. To the east, threat seems to increase late in the period.

South-west Mediterranean

Given low-level cold air advection and weak QG forcing, it seems that deep convection will not develop along the cold front that will not go on further south during the period. Given strong vetical wind shear, storms that may develop will have severe potential.