Storm Forecast
Valid: Fri 23 Feb 2018 06:00 to Sat 24 Feb 2018 06:00 UTC
Issued: Thu 22 Feb 2018 16:30
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for S Italy, Malta, the Ionian Sea and SW Greece mainly for severe convective wind gusts and excessive convective precipitation, and to a lesser degree for large hail and tornadoes.

SYNOPSIS

A blocking anticyclone is centered over the North Sea at 500 hPa and over N Scandinavia at the surface. Mid-level cut-off lows are placed over NW Russia and the W Mediterranean Sea. This configuration yields a broad flow of dry and cold continental air from the NE over most of Europe.
In contrast, rather low surface pressure and milder, moister air create unsettled conditions over the Mediterranean region.

DISCUSSION

... Malta, S Italy, Ionian Sea, SW Greece ...

The cut-off low over the W Mediterranean is accompanied by a surface cyclone near Sicily. The SW-erly flow at its forward flank advects an elevated mixed layer (EML) from Libya atop a moist maritime airmass over the Ionian Sea. CAPE on the order of 500 to 1000 J/kg is predicted to materialize. The EML will initially create a stout cap.
Wind profiles in the warm sector look sufficient for storm organization, though vertical wind shear (10-15 m/s) and storm-relative helicity (100-200 m^2/^2) are mostly concentrated across the lowest kilometer, owing to a SE-erly low-level jet, and hardly increase at higher levels anymore.
Main foci for convective initiation will be (1) orographic lift of the low-level jet in Sicily and Calabria, and (2) the cold front which moves eastward and crosses Malta, Sicily and the Ionian Sea. By afternoon the cold front will be overrun by a strong short-wave trough at mid-levels, and confidence increases that the created lift will be strong enough to break the cap also over offshore areas (away from orographic lift).
The formation of an MCS appears quite likely. Such a system may produce all kinds of severe weather. Flash floods are the main hazard over Sicily (morning to afternoon) and Calabria (afternoon to evening), which are exposed to upslope flow with embedded thundery rain for several hours. The main body of the MCS could bring quite widespread severe wind gusts over the Ionian Sea. Tail-end storms at its southern flank, or discrete storms in general, may turn multi- or supercellular with a risk of large hail and one or two tornadoes.
CAPE and shear decrease overnight along the storms' further path across the Ionian Sea, but synoptic lift remains strong, and the mature MCS may impact SW Greece (Peloponnes and possibly Crete) late at night with a gradually decreasing, but still ongoing severe weather risk.

... Aegean Sea, E Mediterranean Sea, Turkey, Near East ...

Moist and mild air allows a few hundred J/kg of CAPE under weak vertical wind shear. Scattered and rather low-topped thundery shower are possible throughout the forecast period over sea surfaces and mostly in the afternoon over land. Severe weather is unlikely, though an isolated heavy rain event or a waterspout are not excluded.