Storm Forecast
Valid: Sat 05 May 2007 06:00 to Sun 06 May 2007 06:00 UTC
Issued: Sat 05 May 2007 06:22
Forecaster: DAHL

SYNOPSIS

Main feature this period is an upper long-wave trough extending well into the western Mediterranean, which will make slow eastward progress during the day. Weak cyclonic low-level flow is accompanying the feature, supporting advection of warm/moist air masses into the central Mediterranean and the Balkan States. Meandering and moderately intense northern branch of the upper frontal zone, and its accompanying surface lows are affecting the British Isles and Scandinavia. Potentially severe convective evolution is expected to develop mainly over S Italy along and ahead of the cold front associated with the Mediterranea upper trough.

DISCUSSION

... southern Italy ... Adriatic Sea ... central Balkans ...

South Italian 00Z ascents reveal the presence of an EML which will continue to advect northwards during the day. Mixed-layer boundary-layer moisture will likely be on the order of 10 g/kg over S Italy, the Ionian/Adriatic Sea, and the S Balkans by afternoon. This may result in reasonably strongly capped MLCAPEs of up to 1000 J/kg over S Italy, though this will depend on the degree of insolation, which may be obstructed by mid/high level cloudiness.

Deep-layer shear is expected to be on the order of 25 to 30 m/s in the warm sector air mass. 0-1 km shear of up to 15 m/s will also be present, but it seems that it will be slightly offset to the east of where the CAPE is present, though GFS does simulate appreciable juxtaposition towards late Saturday night, mainly over the Ionian Sea.

Main negative today appears to be a lack of deep and intense large-scale ascent. GFS simulates some small bullseyes in the vorticity-advection fields between 18 and 00Z over S Italy and the central Balkans ahead of a small peripheral perturbation. If this solution verifies, these features will be potential foci for convective evolution. Strong warm advection will persist over Ionian and S Adriatic Sea, which is where the LLS is maximized. It seems likely that storms developing in this regime will tend to be elevated, though some of these may well become surface-based and profit from quite favorable LL wind profiles for low-level rotation.

Greatest severe threat seems to unfold over southern Italy, mainly associated with large hail and damaging wind gusts. Weak LLS and strong capping may limit the tornado threat, though local, unsampled modifications of the shear and thermodynamic fields over the mountainous terrain may be substantial, so that at least a small threat for a brief tornado or two also exists. Storms may build northeastwards and spread into the central Balkans in the late evening hours. Farther E over the Ionian/Adratic Sea, stronger LLS exists but elevated nature of the storms may limit the severe threat.

Models are not quite in agreement on the spatial extent of the convection, and BOLAM fails to dimulate convective initiation altogether, while NMM supports the GFS solution with appreciable precipitation over Italy and the Balkans. It seems that bulk of activity will develop in the early evening hours and persist though the night.

Otherwise, organized severe threat should be quite low, though 00Z sounding from the NW Balkans/NE Italy, as well as from the W Mediterranean Sea reveal some potential for waterspouts.