Storm Forecast
Valid: Thu 01 May 2008 06:00 to Fri 02 May 2008 06:00 UTC
Issued: Wed 30 Apr 2008 19:08
Forecaster: TUSCHY

SYNOPSIS

Large mid - / upper - level cyclone over the NE Atlantic slowly develops SW- ward as a huge cut - off complex and this retrograde movement continues during the next 24 hours. Downstream of this feature, very weak pressure gradients at all levels prevail with about 5 hPa pressure difference from SW Europe to Belarus.
As a result, an extensive forecast area will see mainly diurnal driven thunderstorm activity. Cold mid-levels and more or less favorable diabatic warming result in steep lapse rates both at low / mid - levels with widespread low -end instability release. Only pulsating thunderstorms are expected with small hail / strong wind gusts as the main risk. Locally, higher coverage of thunderstorms could lead to an upscale growth into small clusters of thunderstorms but environment does not support any enhanced severe weather threat. Weak background flow could support an localized flash flood risk with those clusters.

There are a few spots where stronger instability release / at least somewhat stronger DLS could support an isolated large hail risk, which includes N-France ( DLS up to 20m/s ) and N / NE Germany / N -Algeria ( better instability release ). We decided not to include those regions in a marginal level - 1 area, as expected coverage of reports and seesaw changes of best instability fields from run to run yield too many uncertainties. A level area was issued for the Ukraine / Slovenia / parts of Croatia , where confidence is higher that hail could locally reach our criterion. The other regions could see a conditional upgrade later on if data continues to hint at an isolated large hail threat.
In all those areas we talk about a potential large hail size between 2 and 3cm and therefore a marginal level - 1 situation.

Thunderstorm coverage should rapidly decrease over NW / central Euopre after sunset, but should keep going until 06Z over parts of E / SE Europe. No severe thunderstorms expected during the night hours.

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