Storm Forecast
Valid: Thu 11 Oct 2007 06:00 to Fri 12 Oct 2007 06:00 UTC
Issued: Thu 11 Oct 2007 05:38
Forecaster: GATZEN

SYNOPSIS

A high pressure system over southern British Isles ridges into central Europe. To the south, low geopotential is present over the Mediterranean. An embedded upper low will be centered over western Iberian Peninsula on Thursday. To the north of the high, strong upper jet is forecast over Scandinavia. A intense trough is expected to move southeastward into southern Baltic Sea at the end of the period. Rather cool and stable air mass has entered most of Europe east of the high, while maritime air mass is advected into British Isles and southern Scandinavia. Unstable air mass is expected over most of the Mediterranean in the range and east of the trough.

DISCUSSION

South-west Mediterranean

A rather unstable air mass is present over southern Mediterranean Sea. Some boundary-layer moisture and steep mid-level lapse rates are indicated by latest soundings. While the main frontal boundary remains over northern Mediterranean, low-level moisture is expected to increase near the African boarder underneath the capping inversion. This moisture is advected westward at the northern flank of a low over northern Africa. In mid-levels, a strong westerly jet is present over northern Africa. An upper jet streak curving around the cut-off low over Iberian Peninsula will eject into this jet during the period, providing QG forcing over south-west Mediterranean. Low-level forcing will be maximized near the Spanish coast where low-level convergence is forecast by latest models. However, there are no indications for a well-defined frontal boundary.

Expected scenario is that some convective cells will already be in place in the morning hours that will go on during the period that are concentrated from eastern Spain to Balearic Islands. Vertical wind shear will be quite strong reaching 25 m/s DLS, and given favorable veering profiles especially near the African coast, some of the storms are expected to become supercells. Large hail and severe wind gusts are forecast with the stronger cells, especially in the range of bowing lines. Tornadoes are forecast especially to the south of the Balearic Islands, where low-level veering and buoyancy are strongest.

Northern Algeria, Tunisia

Some instability is expected over northern Africa although capping inversion will be too strong over most places. However, upper jet streaks may lead to some QG forcing during the day and locally weakening inversion. Best chances for initiation is along the Atlas mountains as well as over northern Tunisia, where upslope flow and a weak low-level convergence are expected during the day. Thunderstorms are forecast to develop during the day and will likely go on in the night. Given strong vertical wind shear, mesocyclones are not ruled out capable of producing large hail and severe wind gusts. Threat of tornadoes is greatest over northern Tunisia in the range of the low-level convergence, where low-level moisture is forecast to be rather rich.

Greece

Ahead of the Mediterranean trough, increasing vertical wind shear is expected over Greece. Thunderstorms are expected to develop and may organize, while threat for severe weather is rather low given only weak vertical wind shear and buoyancy.

Central Mediterranean

Underneath the low geopotential, strong low level buoyancy and weak shear are forecast, and isolated waterspouts may occur.