Forecast Update
Valid: Thu 22 Oct 2009 15:00 to Fri 23 Oct 2009 06:00 UTC
Issued: Thu 22 Oct 2009 14:59
Forecaster: TUSCHY

A level 2 was issued for parts of the central Mediterranean mainly for large hail, severe wind gusts, excessive rainfall and tornadoes.

A level 1 surrounds the level 2 mainly for severe wind gusts and excessive rainfall.

SYNOPSIS

Please refer to the main outlook.

DISCUSSION

... 14 UTC onwards ...

The overall set-up unfolds as forecast. Latest satellite and radar data show organized and severe thunderstorms, developing over the Tyrrhenian Sea and S-Italy. 12 UTC sounding data of LIBR Brindisi reveals an LCL height of 650m AGL, MUCAPE in excess of 2000 J/kg and 0-3km SRH just shy below 300 m^2/s^2. Surface dewpoints in this area reside in the lower twenties, so a sheared and moist inflow into the approaching thunderstorms ought to yield a favorable environment for rotation at mid-/ low-levels. Numerous severe thunderstorms affect south-central and southern Italy during the afternoon hours and the activity spreads east/northeastwards over the Adriatic Sea.

Rapidly strengthening depression just north of the Balearic Islands is on track with the cold front just having passed the eastern part of those Islands. Prefrontal convergence zone has organized into an active MCS, with new convection firing along the successive cold front. This activity keeps going eastwards with a slight weakening trend possible due to a temporarily lull in shear strength and instability. Nevertheless, severe to damaging wind gusts, isolated tornadoes and large hail are possible over Sardinia during the passage of the cold front.

... 21 UTC onwards over the Tyrrhenian Sea, parts of Sicily and Italy ...

Some minor uncertainties remain after analyzing the latest data as far south the convection is able to evolve. The strengthening depression over the western Mediterranean slowed down somewhat in the past few runs and hence, WAA still affects Sicily and S-Italy until 06 UTC. However, negative tilted upper trough overspreads that LL airmass with impressive forcing and cooler mid-level air. In addition, eastward advancing surface cold front and very moist prefrontal airmass also assist in enough LL forcing, so we therefore decided to went further south compared to the main model solution, also highlighting Sicily and parts of the western/northern Ionian Sea.
Otherwise, a diffluent upper streamline pattern atop of the eastward moving cold front and moderate to high CAPE values all support a rapid and potential explosive development of thunderstorms over the Tyrrhenian Sea, probably as early as 21 UTC. Magnitude of shear and instability and favorable overlap along the front point to the complete facet of severe, including strong tornadoes, damaging wind gusts and very large hail.

The level 2 along the NE coast of the Adriatic Sea was canceled. Numerous more short-lived training MCSs were present, but too short-lived for a serious flash flood risk. A new MCS just evolved over parts of Croatia, so localized heavy rainfall with flooding is still possible but not widespread enough for a level 2.
The level-2 along the eastern Adriatic Sea is still present due to the prolonged period of influx of a very moist and unstable airmass. Repeated re-development of convection-probably in form of backbuilding MCSes- pose a serious flash flood risk.