Storm Forecast
Valid: Sun 02 Sep 2012 06:00 to Mon 03 Sep 2012 06:00 UTC
Issued: Sun 02 Sep 2012 05:35
Forecaster: VAN DER VELDE

A level 1 was issued for the central Mediterranean region mainly for isolated large hail, excessive convective rain and tornadoes.

A level 2 was issued for Tunesia, Malta, Sicily, western Italy and eastern Corsica mainly for excessive convective rain, tornadoes, large hail and severe wind gusts.

SYNOPSIS

A cold upper low has cut off and installed itself over the warm western Mediterranean Sea. Warmer very moist low-level air is advected northward across the Tyrrhenian Sea with a cyclonic circulation developing along the frontal boundary between Tunesia, Sardinia and Sicily, moving northward.after 21Z and arriving between Corsica and Italy at 06Z the next morning. It deepening rapidly in the Hirlam model and not so much in the GFS model. It is interesting to note that at this moment several models develop it into a hurricane-like structure (e.g. warm core and eye) on Monday, and the currently unusually high sea surface temperatures would assist such development.

DISCUSSION

...Tunesia, Malta, Sicily, western Italy and eastern Corsica...

While models do not all agree on the positioning and amount of CAPE, some 1000-1500 J/kg may be available on the warm side of the front. In some models this may already have been turned into convection. Precipitable water (PW) is large and column relative humidity as well, leading to PW-SD (saturation deficit) of around 40 kg/m². It will fall out mainly over Sicily and the Italian west coast, with significant chance of excessive convective rain amounts and accompanying problems.
Besides this, the kinematic environment is highly favorable for long lived supercells and MCS maintenance: 20-30 m/s 0-6 km shear and around 300 m²/s² 0-3 km SREH. Expect (very) large hail and severe wind gusts. But also tornado threat is present, particularly along the Italian west coast where during the night and early morning 0-1 km shear increases to 15 m/s or more (depends on the model).

...western Mediterranean and central Mediterranean level 1 areas....

Deep layer shear is larger than 15 m/s in many places, only excluding the western Mediterranean Sea. While storm coverage will likely be more isolated or uncertain, some of the storms in that shear environment could develop rotating updrafts and produce large hail. Tornadoes (mostly spout type) will be possible and very likely to occur over the warm waters of the Mediterranean, where 0-2 km buoyancy is quite strong. The bands of vorticity in the western part with slow storm motion will offer good conditions for spinning up spout-type tornadoes..