Storm Forecast
Valid: Thu 04 Oct 2018 06:00 to Fri 05 Oct 2018 06:00 UTC
Issued: Thu 04 Oct 2018 05:15
Forecaster: GATZEN

A level 2 was issued for the Ionian Sea, southern Italy, and southern Sicily mainly for heavy rain and severe wind gusts and to a lesser extent large hail and tornadoes.

A level 1 was issued for the Tyrrhenian Sea and surroundings mainly for heavy rain.

SYNOPSIS

Broad area of high pressure covers central Europe, its center shifts from west to east during the period. To the south, a cut-off low affects the central and southern Mediterranean. Cold mid-levels above the warm sea have lead to unstable conditions and thunderstorms are currently going on. Farther north, a strong mid-level jet extends from the northern British Isles to Scandinavia, curving southward towards the Black Sea at the flank of a trough that is centred over western Russia. While the trough moves east, a vort-max will curve around it, spreading across Belarus and the central / eastern Ukraine and than entering southern Russia late in the period.

At low levels, dry air masses affect the Iberian Peninsula, northern and eastern Europe. Although lapse rates are forecast to become steep in response to diurnal heating, only marginal CAPE can evolve, with coldest equilibrium levels near the coldest mid-level air mass over western Russia and parts of the Ukraine. Better moisture but stable lapse rates are present from the Bay of Biscay towards the Baltic Sea. Rich moisture across the Mediterranean overlaps with an elevated mixed layer east of the cut-off low.

DISCUSSION

Ionian Sea to southern Italy and Sicily

A strong southerly mid-level flow is directed towards southern Italy ahead of the cut-off low that will be quasi-stationary during the period. Embedded in the flow is a vort-max that travels across the Ionian Sea in the afternoon. At lower levels, a very moist air mass (mixing ratio around 16 g/kg) is advected westward beneath an elevated mixed layer originating from the Atlas mountains. MLCAPE can reach 3 KJ/kg as indicated by latest models. At the same time, robust capping is not forecast along the warm air advection regime below the strong mid-level flow. Initiation of storms is possible along a convergence zone south of Italy and Sicily, most likely in the afternoon hours when the mid-level vort-max passes by and low-level warm air advection is maximized.

Storms that form may quickly organize given long hodographs in the same region. Strong low-level vertical wind shear will support mesoscale convective systems in particular, whereas deep layer vertical wind shear about 20 m/s will also allow for supercells. Main threat will be severe wind gusts with the larger systems, along with heavy rain and flooding. Furthermore, any supercell that forms is capable of producing large hail given high CAPE and strong vertical wind shear. Tornadoes are also possible given veering low-level profiles.

Storms will advect northward, while southward backbuilding can be expected if a well-defined convergence zone evolves. This increases the threat of excessive precipitation across southern Italy during the evening hours and overnight.

Tyrrhenian Sea and surroundings

Close to the mid-level vortex, vertical wind shear is relatively weak. However, skinny CAPE profiles and moist air result in a threat of excessive precipitation since many storms are forecast along low-level convergence lines given CAPE and weak CIN.