Storm Forecast
Valid: Thu 13 Sep 2012 06:00 to Fri 14 Sep 2012 06:00 UTC
Issued: Wed 12 Sep 2012 22:23
Forecaster: GATZEN

A level 2 was issued for the Ionian Sea and especially western Albania to western Greece mainly for excessive precipitation.

A level 1 was issued from the Ionian Sea to Libya mainly for very large hail and excessive precipitation and to a lower extend severe wind gusts.

A level 1 was issued for the Tyrrhenian Sea mainly for excessive precipitation and waterspouts.

A level 1 was issued for the southern Adriatic Sea, southern Italy and Sicily mainly for excessive precipitation.

SYNOPSIS

An intense polar trough cuts off across the central Mediterranean. The associated cold front will affect northern Africa and Greece during the period, while additional storms will form across the Ionian Sea.

DISCUSSION

Southern Tyrrhenian Sea to southern Adriatic Sea, Ionian Sea, western Greece and surroundings

A cold front moves southward across the southern Mediterranean on Thursday. Cool polar air will spread into northern Africa in the wake of the cold front, whereas a plume of hot desert air will spread north-eastward ahead of it. The strong temperature gradient will increase the mid-level jet streak at the southern flank of the cut-off to more than 45 m/s at 500 hPa across southern Italy at the end of the period.

At low levels, the cold front will become orientated parallel to the strong south-westerly flow during the day. Strong low-level convergence is expected by numerical models along the cold front from northern Libya to the western coasts of Albania and Greece, with increasingly southerly low-level winds ahead of the cold front. This will be associated with moisture pooling along the cold front and a 0-1 km mixing ratio of around 16 to 18 g/kg is possible according to latest GFS. The rich low-level moisture will be capped by the hot desert air mass that overspreads the region from the south-west. The strong low-level forcing along the cold front is forecast to support a linear MCS that may evolve into a parallel stratiform MCS given the strong mid-level flow.

The main threat will be excessive precipitation in the range of the linear MCS. The highest threat shifts from the southern Tyrrhenian Sea to the southern Adriatic Sea, Ionian Sea and surroundings during the period. Latest GFS indicates the greatest precipitation amounts at the end of the period from western Albania to western Greece due to upslope flow, where flash floods are expected. Additionally, storms may produce large hail given a large overlap of strong vertical wind shear and high CAPE. GFS indicates CAPE around 3000-4000 J/kg along the front overlapping with around 20-25 m/s deep layer vertical wind shear. Current thinking is that the southern portions of the convective line will be associated with the main hail risk, and even very large hail is not ruled out with supercells. Further north, the hail potential is expected to be lower as isolated cells are not likely, but stronger multicells may produce locally large hail. Severe wind gusts are also not ruled out with the stronger storms.

Central Mediterranean

In the wake o the cold front, the cut-off low moves into the central Mediterranean. Initially, Mistral winds will advect dry air masses into the west Mediterranean, limiting deep moist convection. During the period, increasing convective activity can be expected near the trough centre given steep low- to mid-level lapse rates due to the warm sea surface. The activity will be centred across the Tyrrhenian Sea late in the forecast period. Stronger storms may produce excessive precipitation. As vertical wind shear will be weak across the Tyrrhenian Sea, and given the increasing low-level moisture and associated low-level buoyancy, waterspouts may form especially on Friday morning along the coasts of southern Italy and Sicily.