Storm Forecast
Valid: Wed 28 Nov 2007 06:00 to Thu 29 Nov 2007 06:00 UTC
Issued: Wed 28 Nov 2007 05:00
Forecaster: GATZEN

SYNOPTIC / DISCUSSION

European trough moves eastward , while a weak ridge spreads across central Europe ahead of northern Atlantic short-wave trough crossing British Isles during the day. To the south, a cut-off low is centered over northern Algeria. A strong upper jet streak is present at the south-eastern flank of this cut-off spreading eastward during the period. Associated low surface pressure is located just north of the African coast, yielding easterly winds over most of the Mediterranean. Rather moist low-level air mass is present over the south Mediterranean Sea in the range of the low that will remain during the day. Rather steep mid-level lapse rates are also present, but given also cool boundary-layer, instability is not too good over most places, especially to the south of Italy/Sicily, where rather warm mid-level air mass is present. It seems that best instability release is present east of Sardinia, where intense thunderstorms have developed during the night hours.

On Wednesday, latest models indicate continuing height falls spreading eastward into Sicily, and it seems that current cap will weaken significantly. Additionally, low-level convergence is expected to increase south of Italy along a weak convergence line that moves northward. As a consequence, expect that convective activity will spread eastward during the day, affecting a rather large region from Balearic Island to southern Adriatic. Local flash floods will likely pose a threat given slowly propagating cells and intense convection/MCS development. Especially in the western/central areas of the affected region waterspouts may develop given weak vertical wind shear and rather strong low-level buoyancy. South of Italy/Sicily, strong deep layer vertical wind shear around 30 m/s and also strong low-level shear in the range of a strong southerly low-level jet reaching 15 to 20 m/s at the 700 hPa level will likely support new MCS development during the period, spreading northward into southern Italy, and Sicily. LEWP may be capable of producing strong, or isolated severe wind gusts. Given favorable veering profiles, embedded mesocyclones are also not ruled out especially at the southern flank of the developing convective systems. Isolated large hail may occur. Tornadoes are forecast in the Sicily region, where low-level buoyancy may improve late in the period. Threat seems to warrant at least a level 1 at this time.