CONVECTIVE FORECAST
VALID 06Z TUE 30/03 - 06Z WED 31/03 2004
ISSUED: 29/03 21:15Z
FORECASTER: HAKLANDER

General thunderstorms are forecast across SW'RN France, the Bay of Biskay, Spain, W'RN and CNTRL Mediterranean, the Ionean, Adriatic, and Aegean Seas and coasts.

SYNOPSIS

A blocking upper high across Germany and Poland is perturbed on its SW’RN flank by a VORTMAX across NE’RN Spain. SE of this ULL, a strong SW’LY jet of 50-55 m/s at 300 hPa lies across Tunesia, Sicily, and the SRN Aegean Sea, where it turns WNW’LY. An embedded 60 m/s jet streak across Tunesia has its left exit region near Sicily throughout the FCST period.

DISCUSSION

...Sicily, S’RN Italy and surrounding waters...
Monday’s GFS/12Z forecast soundings for Sicily indicate that only 100-200 J/kg MUCAPE should be present during Tuesday. However, large vertical wind shear is forecast across the area, with about 30 m/s deep layer shear, and 110-140 degrees wind veering with height in the lowest 2 km, implying significant SREH. Monday’s Meteosat IR loops already showed some high-sheared thunderstorms upstream, across E’RN Algeria and Tunesia. These possibly supercellular storms seemed to exhibit rightward motion as a result of cell growth on their right upshear flank. Although GFS/12Z forecast MUCAPEs of upto 700+ J/kg where these cells were active, this slightly stabilizing air mass should be advected into Sicily. Supercells cannot be ruled out, but seem less probable than on Monday, when more CAPE was available. If supercells occur, large hail with 2+ cm diameter cannot be ruled out with WBZ heights of about 2.5 km AGL. However, large hail does not seem very likely with only 100-200 J/kg MUCAPE. Owing to the large vertical wind shear and very low relative humidities at midlevels, downward momentum transport and/or evaporational cooling might produce severe wind gusts in the vicinity of any storm that forms. With strongly veering winds with height and less than 15 m/s wind shear in the lowest 3 km, squall line formation seems unlikely. Key factor in determining the risk of severe convective events remains the forecast amount of CAPE, which is too low to warrant a SLGT at the moment.

...Spain, W’RN Mediterranean, Bay of Biskay, and SW’RN France...
Not only high clouds, but also a persistent boundary layer cloud cover was present across much of the Iberian Peninsula on Monday, limiting the diurnal formation of CAPE. This cloud cover is expected to persist across many areas on Tuesday, but especially parts of N’RN Spain might see a bit more sunshine, which could trigger a few thunderstorms. This could also happen farther north, across the Pyrenees, SW’RN France and the Bay of Biskay. With at most a few hundred J/kg of CAPE and deep vertical wind shear remaining below 20 m/s, no severe convective events are forecast.

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