Storm Forecast
Valid: Fri 05 Oct 2018 06:00 to Sat 06 Oct 2018 06:00 UTC
Issued: Thu 04 Oct 2018 22:20
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for S Italy mainly for flash floods and to a lesser extent for large hail, tornadoes and severe convective wind gusts.

A level 1 and level 2 are issued for the Ionian Sea for large hail, tornadoes and severe convective wind gusts.

A level 1 is issued for the E coasts of Corsica and Sardegna mainly for flash floods and to a lesser degree for (non-supercellular) tornadoes.

SYNOPSIS

A "high index" pattern is present over Europe. Central Europe is placed under a broad and warm mid-level ridge, which deflects the frontal zone to the north, and in the warm sector of a strong cyclone over the Norwegian Sea. The cold front of this cyclone will inch southward across the North Sea and the British Isles.
A cut-off low over the Tyrrhenian Sea sets the stage for another thundery day over the central Mediterranean region. Little change is expected to the previous forecast period.

... Ionian Sea, S Italy ...

The severe weather threat is highest at the forward flank of the cut-off low, where robust CAPE, enhanced vertical wind shear and synoptic lift overlap in the warm air advection regime over the Ionian Sea. Forecast models predict CAPE on the order of 1000-3000 J/kg offshore, though it will likely stay much lower at the coasts (700 J/kg at Trapani was the maximum in the Thursday 12z soundings).
Convection in this environment may organize well and grow upscale into large clusters, which could also contain supercells at their tail ends. All kinds of severe weather are possible with these storms. When they move onshore, flash floods become the dominant risk, especially in Calabria where 15 m/s SE winds at 850 hPa pump plenty of moisture towards the coast and the mountains.
The timing, frequency and placement of convective initiation and the extent to which the storms move onshore are difficult to predict. At the time of writing (Thu 22 UTC), intense and probably well-organized storms are moving northward in Calabria and just east of Sicily, earlier and further east than predicted. This may indicate that their tendency to build side- to backward towards the higher CAPE regime and stay offshore may be a little higher than forecast models suggest, at least in the first half of the forecast period. However, the model pool largely agrees on peak accumulations up to 100 mm in 24 hours with repeated rounds ofconvection in coastal areas, and severe weather, especially flash floods, should be expected with any storm that moves onshore.
Scattered convection may also spread northward over the Adriatic Sea and adjacent coastlines as the warm air advection regime proceeds in the course of the forecast period. Vertical wind shear is weaker, CAPE is limited and a strengthening cap may force convection to become increasingly elevated and/or embedded. Therefore the severe weather risk decreases betimes.

... Tyrrhenian Sea, Italian W coast, Corsica, Sardegna ...

Under the core of the cut-off low, cool mid-levels on top of the warm sea water create some hundred J/kg of CAPE without much shear. At least scattered thunderstorms will pivot around the cut-off low throughout the forecast period. A few flash floods may occurr at the E side of Corsica and Sardegna. The center of this risk slowly moves northward, following the foreseen track of the surface cyclone. A few waterspouts are not ruled out in case pronounced convergence zones form, though the synoptic wind field could suppress their formation.