Storm Forecast
Valid: Thu 20 Feb 2014 06:00 to Fri 21 Feb 2014 06:00 UTC
Issued: Wed 19 Feb 2014 22:33
Forecaster: GATZEN

A level ! was issued for parts of the south Mediterranean Sea mainly for severe wind gusts, large hail, and tornadoes.

SYNOPSIS

A ridge moves east across Scandinavia and Central Europe. From the west, another Atlantic trough approaches over western Europe. Over the central Mediterranean, a third mid-level trough will slowly move east. Low-level moisture is poor in the northern part of Europe except of a tongue of Atlantic air spreading into western Europe ahead of a cold front. Across the Mediterranean, rich low-level moisture has developed over the central and eastern parts. Steep lapse rates are present from the central Mediterranean to the Adriatic, Aegean, and southern Balkans and will spread eastward during the period.

DISCUSSION

The Mediterranean trough will slowly move eastward on Thursday. A short-wave trough and associated jet streak will rotate around its southern flank and moves north-east into the Aegean late in the period. This will provide strong DCVA especially from southern Italy to southern Greece. At low levels, warm air advection will go on ahead of the cold front, resulting in strong QG forcing. Before, rather weak QG forcing is expected due to a short-wave ridge moving north-east ahead of the trough.

The affected air mass is characterized by rich boundary layer moisture over the Mediterranean Sea, with 0-1 km mixing ratios around 8 g/kg. Steep lapse rates originating from northern Africa will overlap with the moisture, and CAPE of several 100 J/kg is indicated by latest models.

On Thursday, some deep moist convection will likely go on across Italy and the Adriatic. Weak vertical wind shear is expected and severe storms are not likely. Late in the period, new storms may form from southern Italy to northern Africa, spreading north-eastward. These storms will form in stronger vertical wind shear, leading to better storm organization. Bowing lines and mesocyclones are not ruled out, capable of producing severe wind gusts, marginally severe hail, or a brief tornado. The overall threat is expected to be rather low initially and may increase on Friday.

Western France

An Atlantic trough enters north-western Europe during the day, and a south-westerly jet will move eastward. At low levels, a cold front moves east rather quickly and will be located underneath the anticyclonically sheared flank of the jet. Weak QG forcing and stable lapse rates are indicated by latest GFS model output, and deep moist convection is not expected. In the wake of the jet, a deeply mixed cold air mass will spread into the British Isles and north-western France. Especially over the warm sea surface to the south of the British Isles, thunderstorms are expected. Given the weak low-level convergence and vertical wind shear, severe storms seem to be not likely.