STORM FORECAST
VALID Thu 06 Apr 06:00 - Fri 07 Apr 06:00 2006 (UTC)
ISSUED: 05 Apr 21:06 (UTC)
FORECASTER: GATZEN

A threat level 1 is forecast across central Italy

SYNOPSIS

East of high geopotential over northern Atlantic ... long-wave trough has formed over western and central Europe ... with an axis from Baltic Sea region to Iberian Peninsula. During the period ... strong arctic trough moves into northern North Sea ... while old trough center to the east disappears. Associated trough axis accelerates northeastward ... reaching from Baltic States to Adriatic Sea and further to Tunisia by the end of the period. At lower levels ... cold airmass enters northern Mediterranean ... while temperate airmass over Iberian Peninsula spreads into southwestern Mediterranean Sea. Warm airmass is advected into southern Mediterranean ahead of the trough axis ... and should reach Aegean Sea at the end of the period.

DISCUSSION

...Central Italy ...
At the northwestern flank of Mediterranean WAA regime ... frontal boundary is expected over Italy. As the upper trough axis/vort-max approaches ... weak cyclogenesis is expected over central Italy. Expect that low-level airmass will be quite moist underneath the inversion of African airmass ... sufficient CAPE may build in the range of the frontal boundary as mid-levels start to cool. Due to QG forcing ... thunderstorms may develop along and N of the frontal boundary ... where capping inversion should be weak. Thunderstorms that form will likely organize given 20+ m/s DLS ... and multicells and supercells are expected ... capable of producing isolated large hail and severe wind gusts. A tornado or two is not ruled out near the frontal boundary where low-level moisture and helicity may be sufficient for low-level rotation. Allover threat should be rather low, though, and chance for initiation is not clear ATTM.

...Southern Mediterranean ...
In the WAA regime over southern/eastern Mediterranean ... very steep lapse rates originating from the Sahara desert will be present above the boundary layer. Latest Cagliary sounding indicates a deep EML between 950 and 650 hPa. At lower levels ... a huge inversion in present ... that is expected to suppress surface-based convection during the period. However ... rather strong QG forcing is forecast in the range of the approaching trough axis ... and thunderstorms are not completely ruled out. Thunderstorms that form will likely be supercells given favorable and strong vertical wind shear. An update may be necessary if it will turn out that thunderstorms will be more likely.

...Iberian Peninsula and southwestern Mediterranean ...
Temperate maritime airmass has entered Iberian Peninsula ahead of weakening short-wave trough/cut-off low. This airmass is characterized by rich low-level moisture and neutral to unstable low-level lapse rates ... and diurnal heating will easily lead to instability during the day. Due to weak CIN ... showers and thunderstorms are forecast to develop over a broad region ... spreading eastward into western Mediterranean Sea ... where rather cold boundary layer should gradually limit the potential for deep convection. Given weak deep layer vertical wind shear ... single cells should be most likely. Thread of severe convection should be low. However ... given steep low-level lapse rates and localy rich boundary layer moisture ... chance for an isolated tornado may be enhanced.

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