CONVECTIVE FORECAST
VALID 06Z WED 04/08 - 06Z THU 05/08 2004
ISSUED: 04/08 00:52Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across eastern and southeastern France, extreme southwestern Germany, northern Italy, western Austria, extreme western Slovenia and Croatia, extreme northeastern Spain.

General thunderstorms are forecast across large parts of Europe except the southern and eastern Mediterranean, southwestern Iberia and northern Russia.

SYNOPSIS

Wednesday at 06Z... at mid and upper levels weak flow prevails over Europe. A weak anticylone is located over eastern Scandinavia with an inverted shortwave trough moving northwestward over southern Scandinavia. A weak to moderate zonal flow is present over the Mediterranean. Several weak vorticity maxima are advected east to northeastward across the western Mediterranean and the Alps. A warm and mostly moist and unstable air-mass covers western Europe.

DISCUSSION

...Slight risk area.......
Surface dew point values are expected to range from around 17 across northeastern France to above 20 degrees across the Po-Valley and the French Mediterranean coast. In places of abundant sunshine - this being dependent on the location of debris of earlier convection - the boundary-layer will likely warm to values around 30C. This will result in approximately 1000 J/kg MLCAPE to form in northeastern France and up to around 2500 J/kg in the south of the slight risk area.

As areas of QG-forced rising motions are expected to affect the area and convective inhibition is expected to be small, scattered to widespread initiation of convection is expected. Given the high CAPE values and low wind shear, pulse-type severe storms are possible, producing large hail and severe winds, this being most likely towards the south where higher CAPE values are expected. Some storms will likely cluster into large convective systems, but their location cannot be exactly forecast in advance. Where this happens a higher threat of wind damage will form. Both these systems as well as with more isolated convection and isolated tornado can not be ruled out.

...Britain, southern Benelux, northern France...
As about 500 J/kg CAPE forms in response to solar heating, some storms are likely to form in the afternoon. Across Belgium and northern France models suggest a nocturnal MCS may form late in the period. Mostly small and isolated large hail and some strong but likely non-severe winds are possible.

...eastern Spain, western Mediterranean...
Around 20 m/s of 0-6 km shear and 1000 - 2500 J/kg CAPE are expected over the indicated region. Approach of a weak trough from central Iberia as is indicated by the GFS model could generate for suffcient ascent to allow for convective initiation. However, storm coverage is expected to be low or very low. Any storm that does form will possibly become supercellular and produce large hail and severe winds. If more confidence is obtained that more than isolated convective initiation will occur, an upgrade will be necessary.

...southern Scandinavia...
Rising motions ahead of the aforementioned inverted trough are expected across the indicated area, inducing convective development. Deep-layer shear in the 15-20 m/s range will likely allow for the formation of a few stronger multicells clusters and perhaps a few rotating updrafts/supercells. These have a chance of producing isolated strong winds and small hail.

...Ukraine...
A east-west oriented frontal zone across the Ukraine remains almost stationary. As warm, moist air is advected westnorthwestward, upward motion of this air aids in convective initiation. Several strongly back-building MCSs are ongoing and expected to continue near the frontal zone. Low-level shear of around 10-15 m/s could occur locally, suggesting that strong gusts and perhaps one or two weak tornadoes could occur.

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