Storm Forecast
Valid: Thu 20 Sep 2018 06:00 to Fri 21 Sep 2018 06:00 UTC
Issued: Thu 20 Sep 2018 04:50
Forecaster: GATZEN

A level 1 was issued across Sardinia, Corsica, Tunisia, Italy and the Tyrrhenian Sea mainly for excessive rainfall.

SYNOPSIS / DISCUSSION

An ill-defined vortex is still present across the central Mediterranean. Below rotating PV features, low-level convergence will support a weakly capped environment. Initiation of storms is forecast along the land-sea-breeze, that is mostly over the sea due to the late time of the year. However, some storms will also affect the shores and may form in the afternoon over the islands and mountains.

Latest soundings show skinny CAPE profiles with plentiful moisture and high relative humidity. In addition, the flow is weak through-out the troposphere. Hence, thunderstorms will be capable of producing excessive rain and flash flooding. Main threat is in the afternoon, when activity shifts from the sea towards the land, in particular Sicily, southern and central Italy, and northern Tunisia. Late in the period, activity will again shift towards the sea and will go on until the morning.

Over north-western Europe, a strongly-forced synoptic situation goes on. On Thursday, a frontal wave will cross the southern British Isles. Ahead of a sharp trough that rapidly moves east, the wave will evolve into another intense cyclone that moves towards the North Sea. In its warm sector, a unseasonably warm air mass will be present. Although there will be CAPE, convection initiation is not likely since the main synoptic forcing stays well west of the frontal boundary. Behind the cold front, strong low-level cold air advection will likely limit chances for thunderstorms in the strongly-sheared environment. Best chances exist across north-eastern England into the central North Sea with weaker cold air advection and rich low-level moisture. However, low-level convergence is ill-defined and storms are not expected to organize. Main threat will be isolated storms capable of producing tornadoes and severe wind gusts as low-level vertical wind shear is strong. A similar scenario is expected across Finland that is crossed by another intense short-wave trough in the afternoon.

Further thunderstorms may develop along the shores of Norway in the warm air advection regime. CAPE will be marginal, and severe thunderstorms are not likely.