Forecast Update
Valid: Tue 15 Sep 2009 18:00 to Wed 16 Sep 2009 06:00 UTC
Issued: Tue 15 Sep 2009 18:08
Forecaster: TUSCHY

A level 2 was issued for the Tyrrhenian Sea, Sardinia, Corsica and parts of Italy mainly for large hail, severe wind gusts, tornadoes and excessive rainfall.

A level 1 was issued for most parts of the western Mediterranean mainly for excessive rainfall and to a lesser extent for tornadoes.

SYNOPSIS

Please refer to the main outlook.

DISCUSSION

... Level 2 ...

The main reson for this update was a significant expansion of the level 2.

A training band of showers and thunderstorms was present west of Sardinia during the day and finally evolved into an huge convective cluster, which now affects Corsica and thereafter ought to reach the coast of NW-Italy during the early night hours. Latest data suggests a gradual weakening of this cluster but the atmosphere is supportive for re-development over the Ligurian Sea. The level 1 was expanded northwards to highlight the flash flood risk. Embedded thunderstorms can occur well inland over NW-Italy and increase the rainfall amounts.

The risk for severe flooding gradually increases along the west-central coast of Italy during the upcoming hours. An healthy looking cluster of storms is currently developing east of Sardinia and will affect central Italy during the following hours. Dewpoints along the coast are in the upper tens / lower twenties and the 12 UTC sounding already reveals a favorable sheared and unstable environment. Next to the flood risk, large hail, severe wind gusts and tornadoes are all possible.

Further south, over the central and southern Tyrrhenian Sea, numerous organized looking thunderstorms develop rapidly in a very favorable environment for supercells. Sounding data from Tunisia and Sicily were not transmitted but strong shear and high to extreme CAPE values are forecast (supported by explosive thunderstorm development). This activity will continue to increase during the next few hours with an enhanced flood, severe wind gust, large to very large hail and tornado risk over parts of south-central Italy and Sicily.

Scattered thunderstorms over central Tunisia indicate increasing chances for a few thunderstorms as far south as the east coast of Tunisia. Coverage of those storms is more isolated in nature despite the very favorable environment for organized thunderstorms. Warm LL air mass below 3km is still present, which could also limit thunderstorm coverage and history showed that those thunderstorms tend to weaken rapidly offshore due to missing support of the warm surface. Unseasonable warm SSTs next to some mid-level forcing however could support further initiation offshore with the main risk being excessive rain, severe wind gusts and large hail.

No major changes were done for the rest of the outlook.