Forecast Update
Valid: Fri 25 Apr 2008 15:00 to Sat 26 Apr 2008 06:00 UTC
Issued: Fri 25 Apr 2008 15:22
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 24 Apr 2008 16:51 Z.

DISCUSSION

... N - central and NE Italy ...

Latest HI-VIS image reveals widespread initiation over Slovenia and parts of NE Italy but also over parts of Umbria and Tuscany. Those thunderstorms develop in an environment with 30 - 40 m/s DLS which can be also seen in anvil structure.

Regarding the latest surface report we have temperatures of 17 - 19°C and dewpoints around 8°C over NE Italy, increasing to ~ 22 / 10 °C over N - Central Italy. The 12Z sounding of Milano has quite steep lapse rates between 700 and 850hPa and some veering in this layer. The main focus will be a short wave trough, which leaves the eastern Alpine region right now towards the south and this feature should graze the area of interest during the upcoming 3 - 6h. Not much forcing is needed for further initiation and expect scattered thunderstorms to continue to develop over the area of interest.
Strong DLS should ensure a rapid storm speed mainly towards the south and also updraft / downdraft separation. Despite the lack of better LL moisture and therefore more robust instability release, even pulsating thunderstorms would be able to produce hail matching our criterion and hail larger than 3cm would be possible, too, if locally enhanced SRH and those steep lapse rates overlap. Overall environment does not look supportive for extremely severe hail events.

Thunderstorms will diminish over the level-1 area after sunset but isolated thunderstorms are forecast along the S / SE - ward shifting forcing over S - Italy until the morning hours. Rapidly diminishing instability should keep the severe threat very limited.

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