Storm Forecast
Valid: Thu 06 Nov 2014 06:00 to Fri 07 Nov 2014 06:00 UTC
Issued: Wed 05 Nov 2014 23:38
Forecaster: GATZEN

A level 2 was issued for Italy and Sicily and surrounding areas including the coastal regions of the Balkans mainly for excessive rain, but also for tornadoes and large hail, the latter mainly in the southern portions.

A level 1 was issued for large parts of the central Mediterranean surrounding the level 2 area mainly for excessive rain.

SYNOPSIS

Eastern Europe remains under the influence of a broad ridge as the sharp amplified west-European trough forms a cut-off centred over Tunisia. Its northern vort-max will move across Germany into the Baltic Sea. A weak ridge will evolve upstream before another Atlantic trough follows from the north-west. At lower levels, rich Mediterranean moisture is in place ahead of the forming cut-off low. This moisture partly overlaps with steep lapse rates originating from the Atlas mountains. Drier air has spread into central and south-western Europe.

DISCUSSION

Sicily and surrounding Mediterranean, southern Tyrrhenian Sea, Italy to Adriatic

With a strong low-level jet, warm and well-mixed air masses from the Sahara desert are advected into the Mediterranean. Across the Mediterranean, this elevated mixed layer overlaps with substantial moisture as sampled by latest Trapani sounding. CAPE can reach values near 2000 J/kg as indicated by latest GFS model run.

Forcing for initiation is expected at the frontal system moving slowly north-east into Sicily and southern Italy during the day. Additional storms can form within the low-level jet due to upslope flow along the coasts. Back-building storms can help to get sustained and up-scale growing clusters across the Adriatic region.

Main severe threat is excessive rain given moist profiles and the chance of back-building storms. Additionally, strong vertical wind shear and favourably veering winds (0-3km bulk shear around 10 m/s) will assist for supercell formation and large hail is forecast. Furthermore, tornadoes are expected with isolated storms given strong low-level vertical wind shear around 10 m/s. The main limiting factor is rather poor low-level buoyancy due to the cool sea surface under quite warm African air masses, but even strong tornadoes are not ruled out given the strong wind shear. Chances for tornadoes generally decrease in the northern parts where low-level buoyancy will be marginal.

Clusters of storms are expected to go on until Friday morning, capable of producing excessive rain and flash floods. As storms become more embedded in stratiform rain, convective contribution will become smaller during the night.