CONVECTIVE FORECAST
VALID 06Z SAT 30/08 - 06Z SUN 31/08 2003
ISSUED: 29/08 22:26Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across the S Baltic States ... Belarus ... W Ukraine into the N Balkan States.

General thunderstorms are forecast across central Europe.

SYNOPSIS

Intense large-scale upper long-wave trough is covering western ... central ... eastern and northern Europe. Several vort maxima rotating about its periphery ... the strongest of which will affect France and N Italy on Saturday ... at low levels ... deep polar air is overspreading the northern portions of the forecast area ... while modified subtropical air will dominate the southern parts of Europe as well as extreme E Europe and W Russia ahead of the upper longwave trough. Plume of EML is present across the S central Mediterranean ... covering S Italy ATTM ... expected to advect east across the S Balkan and the Aegean Sea towards Sunday morning.

DISCUSSION

...S Baltic States ... Belarus...
Plume of weakly unstable moist airmass will stretch from the Balkans into NE Europe. GFS 500 hPa vorticity forecasts show numerous ill-defined lobes of vorticity across this region ... which should aid in TSTM formation/redevelopment over Belarus/S Baltic States after mid-day. Shear will be quite strong ... and short-lived lines of multicellular storms/bow echoes may be possibe ... primarily producing severe wind gusts. An isolated supercell or two may also occur ... posing additional threat of large hail.

...Mediterranean...
Abundant MLCAPE will likely be present beneath formidable cap over southern portions of Italy ... the Ionian Sea and Greece. Given no hint of synoptic- or mesoscale forcing for upward motion during the day ... confidence that TSTMS will initiate is pretty low. As low-level WAA increases late in the evening/night ... slight chance of a few elevated TSTMS exists over the E ionian Sea and Greece. However ... Chances appear too low ATTM for a gen thunder area.

The N Balkans don't appear to be affected by such strong capping. Though degree of instability is uncertain ... indications are that very moist air will be present at low levels. Thermodynamic profiles will likely not yield too much CAPE ... as also supported by numerical model guidance ... but strong low-level orographic forcing along with diurnal heating should initiate TSTMS early in the afternoon. Deep-layer shear of 70+ knots will remain in place ... and any TSTM that forms will at least briefly pose the threat of becoming severe ... primary threats being severe straight-line winds and large hail.

...France...Switzerland...S Germany...
Nearly neutral lapse rates are present over France per latest radiosonde data ... with a rather dry BL. Though DCVA-related lift ahead of the French trough will likely initiate numerous TSTMS during the day ... thermodynamic setup is not particularly conductive to severe thunderstorms. Given about 50 knots deep shear ... a few severe-wind events may occur. Also ... some of the storms may fortuitously encounter orographically-induced large low-level SRH ... or simply split in a rather stright-hodograph environments ... and become supercellular. This will enhance the threat of large hail. Weak thermodynamic fields and marginal shear don't support significant organized-severe thunderstorm threat though ... and a SLGT is not necessary ATTM.

S Germany will likely see TSTMS ahead of this trough as well ... though thermodynamic setup is expected to be very similar to that over France ... past cases indicate that S Germany and Switzerland are very prone to local orographic flow modifications ... allowing supercells to form. Given about 70 knots deep-layer shear and fast storm motion ... a few supercells appear likely tomorrow ... and an isolated large hail event or two may occur. However ... probability of rotating storm occurances and areal coverage of supercells will likely be too low for a SLGT ... but an upgrade may be necessary tomorrow.

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