Storm Forecast
Valid: Sat 15 Dec 2007 06:00 to Sun 16 Dec 2007 06:00 UTC
Issued: Fri 14 Dec 2007 23:23
Forecaster: TUSCHY

SYNOPSIS

A major high pressure area is still centered over Sweden/Norway and model guidance denotes that this will be the case for the next few days. Downstream of this feature, cold air spreads southward, covering eastern Europe and also many parts of the central Mediterranean. An interesting side note is that the 850hPa temperatures are colder over Italy than over the southern Barents Sea, which is for sure a nice indicator how disturbed the current pattern is.
The cold-core feature over Italy is forecast to move westwards, but the speed of this system should ease somewhat betimes.
As a consequence of increasing baroclinity,
the environment is favorable for surface depressions and GFS indeed has a broad surface pressure channel south of Sicily.

Not many changes compared to yesterday regarding the thunderstorm coverage and areas. We decided to issue a broad thunderstorm area as thermodynamic stays excellent with steep low-/mid-level lapse rates.
We wanted to include a fetch along the lee side of any island and also along Italy as airmass stays quite dry and some time for mixing is needed, but yesterday's IR loop showed an increase in convection over the whole Adriatic Sea during the night hours, as onshore flow relaxed and upper-level low moved overhead. We therefore included all areas into a thunderstorm area.
The main threat will be a waterspout risk as 0-3km CAPE is enhanced and overall background flow quite weak.

....Sicily and southern Italy...

LL shear next to the surface pressure channel is likely somewhat enhanced, but it already starts with many uncertainties about strength and placement of this feature and we therefore issued no higher probabilities for Sicily right now.
The main attention will be the impressive helicity values and a quite low wet-bulb-zero layer and hence hail could become increasingly likely over those areas during the evening and night hours.
Instability signals are slim and best shear looks like to be in advance of better instability and we therefore did not issue a level-1.

Please note that not all regions in such a broad thunderstorm area will see thunderstorms, but it is hard to pinpoint any specific area of concentrated thunderstorms that far out.