CONVECTIVE FORECAST
VALID 14Z THU 28/08 - 06Z FRI 29/08 2003
ISSUED: 28/08 14:57Z
FORECASTER: GROENEMEIJER

There is a moderate risk of severe thunderstorms forecast across eastern and southeastern France

There is a slight risk of severe thunderstorms forecast across an area surrounding the moderate risk area including central and south-central France Luxembourg, southern Germany, western Austria, Switzerland, northern Italy and extreme northeastern Spain

General thunderstorms are forecast across Much of France, central and southern Germany, the southern Benelux countries, much of central Europe and the northwestern Balkans, northern and central Italy, northeastern and northwestern Spain and northern Portugal as well as the central North Sea and extreme southern Norway.

SYNOPSIS

An upper low is situated over Scandinavia. On its southern flank a strong westerly jet is present. A trough is moving eastward over eastern Europe. The air-mass is rather stable there so that general thunderstorm formation is not anticipated. Another low pressure system is located over the eastern Atlantic, approximately 300 km northwest of Galicia. This system will be slowly moving eastward and will later start to merge with a sharp trough located from the Scandinavian low pressure area to the the southern British Isles. A westsouthwesterly mid and upper jet streak is has formed over the western Mediterranean and is intesifying and propagating northeastward.

DISCUSSION

...France, Germany, Switzerland...
A moist latently unstable air-mass is flowing northward over southeastern France as is indicated by the Nimes LFME and Lyon LFLL soundings at 12Z. It is expected that in the next few hours latent instability will build over much of eastern France with MLCAPES on the order of 300-600 J/kg over northeastern France to 600-1500 J/kg over east-central France and 1500-3000 J/kg in the southeast. Scattered storms are initiating over much of France, with the highest coverage over central and eastern parts.

In northeast and eastern France...deep-layer wind shear is on the order of 40-50 kts, which will allow for formation of supercells capable of producing large hail and damaging winds. Cyclogenesis is taking place over north-central France at the moment. It is expected that storms will tend to cluster in one or more precipitation systems this evening near the low center. South of the low centr lower tropospheric winds will become quite strong (~40kt @ 850 hPa) and convection will likely organise into one or more linear systems containing bow-shaped elements and possibly embedded supercells. They will be capable of producing very strong winds which will likely be the main threat. Hail and possibly one or two tornadoes may also occur as they move northeastward into Germany. There, lower instability is in place and the storms will gradually weaken. Strong storms may continue to initiate over east central France during the whole night, while the activity in the northeast should diminish in the second half of the night.

Southeastern France will likely see some scattered storms forming in the next few hours. Given that deep-layerwind shear is expected to increase to 60-70 kt and low-level shear will also be quite strong (~20 kt) while MLCAPE is in the 1500-3000 J/kg range, expect that storms may become supercells and severe bow echoes capable of producing large or very large hail (> 5 cm), damaging winds and one or two tornadoes.

...northern Italy...
About 1000 - 2500 J/kg MLCAPE is in place over large parts of northern Italy. Scattered storms are forming. Given that deep-layer shear will strongly increase during the remainder of the forecast period that stroms may become supercells capable of producing large hail and damaging winds. Storms will likely be continuing this evening and during the night as forcing for upward vertical motion over the area is expected to intensify. Severe storms are expected to continue into the next forecast period.

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