Storm Forecast
Valid: Wed 18 Apr 2007 06:00 to Thu 19 Apr 2007 06:00 UTC
Issued: Tue 17 Apr 2007 21:51
Forecaster: GATZEN

SYNOPSIS

Relative complex weather situation affects Europe. A cut-off low is present over north-western Africa/southern Iberian Peninsula, yielding an easterly flow over southern/western Mediterranean. Another upper trough is present from Black Sea region to Greece and southern Italy that propagates eastward. A high remains over British Isles, providing a northerly flow with cold air advection from Scandinavia into Germany, Poland, and Baltic States. Associated cold front is expected to reach from western Russia over central Belarus into western Romania, and northern Balkans on Wednesday, 18Z. Thunderstorms are not expected in this cold air mass. Ahead of the cold front, a tongue of warm and slightly unstable air mass is present from southern France and Iberian Peninsula to southern Alps/northern Italy/northern Balkans, and farther to western Russia on Wednesday, 12Z. Latest soundings indicate well-mixed low-levels over most places, and boundary-layer moisture is poor, but nevertheless allows for some CAPE. While vertical wind shear is weak in the range of the warm air mass, a strong upper jet streak rotates around the cut-off low and will affect northern Africa late in the period.

DISCUSSION

Northern Tunisia region

Strong upper jet streak that travels around the cut-off low over northern Algeria will propagate into southern Tunisia late in the period. Associated quasi-geostrophic forcing expected at the cyclonic flank of the jet streak will affect northern Tunisia region during the night hours. Affected air mass is characterized by steep low to mid-level lapse rates, yielding some (elevated) CAPE over places, but latest models do not show instability due to dry and rather cool boundary-layer. However, I won't be too pessimistic as latest surface observations do indicate dewpoints around 10 to 14°C that spread westward into Tunisia/Algeria, and it is not ruled out that sufficient instability will form due to daytime heating. Current thinking is that mid-level height falls and increasing forcing will assist for convective initiation over northern Tunisia late in the period. Given strong DLS as well as favorable veering profiles, supercells are not ruled out, capable of producing large hail and severe wind gusts. Tornadoes are not likely given weak low-level moisture and rather high LCL heights. Additional convection is expected to the west underneath the trough axis, where vertical wind shear will be weak.