CONVECTIVE FORECAST - UPDATE
VALID 10Z FRI 29/08 - 06Z SAT 30/08 2003
ISSUED: 29/08 10:32Z
FORECASTER: DAHL/GROENEMEIJER/GATZEN

AN EXTREMELY DANGEROUS SEVERE WEATHER EVENT IS FORECAST!

There is a high risk of severe thunderstorms forecast across parts of northern Italy, Slovenia, western Croatia, extreme western Bosnia

There is a moderate risk of severe thunderstorms forecast across an area surrounding the HIGH RISK area including the remainder of northern Italy, the northern Balkans, Hungary, Slovakia, southern Poland, and the western Ukraine.

There is a slight risk of severe thunderstorms forecast across an area surrounding the MDT RISK area including central Italy, northeastern Spain, southern, central and eastern France, southern Germany, Austria, and parts of Poland, Belarus, the Ukraine, Romania, Serbia and Montenegro, Bosnia.

General thunderstorms are forecast across much of central southern and eastern Europe

SYNOPSIS

European flow regime is controlled by an intense long wave trough over Scandinavia. Southwestern European cut off gets involved and will migrate northeastward. At its southern periphery ... very strong jet streak (>100kt @ 300hPa) indicated by water vapor satellite image will move over southern France, northern Italy, and the Alpine region during the forecast period. Embedded vort maxes are analysed over the Alps and over southwestern France at 6Z.

At the surface ... low pressure system will cross northern Central Europe. Associated cold front will propagate eastward affecting southern Central Europe and the northern Mediterranean.

DISCUSSION

...northern Italy, Slovenia, western Croatia, extreme western Bosnia...
About 70kt deep-layer (bulk) shear should be present over northern Italy today. An unstable air-mass (1500-3000 J/kg) in in place over the central and eastern Po-Valley and the northern Adriatic as is indicated by the 00Z Cuneo-Levaldigi (16133) and 06Z Udine (LIPD) soundings. Strenghtening low-level shear is present over the area as eastsoutheasterly surface winds are overspread by 30-40 knot southsouthwesterly 850 hPa winds. Thermodynamic setup seems to be favourable for tornadic supercells. Scattered storms are starting to initiate over north-central Italy at this moment. These will likely track eastward over the Po-Valley in the next few hours and affect the northern Adriatic region later this afternoon and this evening. The stroms are capable of producing tornadoes, that could be strong (>F2) or violent (>F4). Highest risk will be over the central and eastern Po-Valley, Friuli Venezia Giulia, western Slovenia and northwestern Croatia. Apart from the threat of tornadoes, damaging winds of up to 70-80 knots may occur, in combination with supercells and severe bow-echoes that will become more likely over time as the convection will likely tend to cluster into linear MCS's.

...Northern Balkans, Hungary, Slovakia, southern Poland, western Ukraine....
About 750-1500 J/kg MLCAPE should become available today as abundant solar heating warms the boundary layer. Deep shear will be on the order of 70+ knots. Though low-level veering will be not as pronounced as in the HIGH risk area ... shear will become strong (to around 25-40 knots) during the day in response to differential pressure falls. Orography could aid in creating helical inflows, increasing the supercell threat. Most storms are expected to initiate during late afternoon over Austria, western Hungary and Slovakia. Some of them will likely be supercells and are expected to move northeastward and rapidly organize into severe bow echoes capable of producing very strong winds. More isolated storms are forecast to the southeast over parts of Serbia, Croatia, western Romania and the western Ukraine. Apart from the risk of damaging winds, an isolated tornado cannot be ruled out either. Also there is a risk of large hail. The MCS should become increasingly elevated late in the night/towards early morning and possibly affect E Poland/W Belarus early Saturday morning.

...southern half of Germany ... N Austria ... W Chech Republic ...
Scattered TSTMS are currently developing in weakly unstable/weakly capped airmass beneath DCVA regime over E France ... W and S Germany. Thermodynamic profiles will be rather weak ... but ample deep-layer and low-level shear (70+ knots at 500 hPa and 30 to 40 knots at 850 hPa) is in place and threat of a few rotating updrafts exists ... especially over the complex terrain of western and the southern half of Germany. LCL/LFC heights are quite low ... and a brief tornado or two may occur. Hail/wind threat will generally be limited owing to nearly saturated environments ... but a few large-hail and marginally severe straight-line wind events are possible.

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