Valid: Fri 04 May 2012 06:00 to Sat 05 May 2012 06:00 UTC
Issued: Thu 03 May 2012 22:14
A level 1 was issued for parts of E-Europe mainly for isolated large hail/strong to severe wind gusts. A sporadic tornado event can't be ruled out.
A level 1 was issued for parts of NE France, Belgium, Luxembourg and far W/NW-Germany mainly for isolated large hail and strong wind gusts.
A level 1 was issued for parts of SW/S France and far N/NE Spain mainly for isolated large hail and strong wind gusts.
A level 1 was issued for NE Algeria and N Tunisia mainly for isolated large hail and strong wind gusts.
An extensive cyclonic vortex placed over N-Europe and an upper low over the Bay of Biscay both avouch the presence of a warm and unstable air mass for most of Europe with scattered to widespread thunderstorms likely. Cold and stable conditions exist over far N-Europe.
... From far E-Poland to Hungary to Greece all the way to Belarus, W-Ukraine and W-Romania ...
An air mass with history of producing widespread and partially organized DMC over W and CNTRL Europe during the past 2 days now covers the highlighted area. Thermodynamic conditions again will be supportive for strong to isolated severe thunderstorms over a broad area with 500 - 1000 J/kg MLCAPE available, locally even beyond 1 kJ/kg. Shear remains weak with somewhat stronger DLS along the fringes of that CAPE plume, where CAPE magnitude decreases. Hence, a broad level 1 was issued for isolated large hail and strong to severe wind gusts (downburst events). Betimes, thunderstorms cluster with locally heavy rainfall forecast. Despite unfavorable LL shear, an isolated tornado event can't be ruled out mainly over Hungary, extreme W-Romania and Serbia, where augmented LL CAPE is present. Thunderstorms will last well into the night hours.
... France, Belgium, Luxembourg and NW-Germany ...
Downstream of an upper low atop the Bay of Biscay, some BL moisture recovery occurs over France beneath cool mid-levels. Right now, best LL moisture probably will be found along and just south of a stationary front, which is situated over N-France. A weak occlusion also approaches from the west during the daytime hours and assists in somewhat enhanced LL moisture advection. Therefore a level 1 was issued for that area, as 15 m/s DLS overlap with modest MLAPE of 500-800 J/kg. Strong wind gusts and isolated large hail are forecast. Sideward MCS storm motion vectors forecast along that boundary with 700 hPa winds increasing to 20 m/s over far W/NW Germany point to an augmented chance for small eastward moving lines of thunderstorms with a locally enhanced strong wind gust risk. Therefore the level 1 was expanded all the way to NW Germany, despite decreasing CAPE. Storms gradually decay after sunset.
... N/NE Spain, SW/S France and NE Algeria/N Tunisia ...
Similar conditions with even stronger DLS (20 m/s) exist over far SW France, where isolated large hail and strong wind gusts are likely. The level 1 was expanded far east along the Mediterranean coast, where marginal CAPE and aforementioned shear overlap.
Similar set-up also exists over NE-Algeria and N-Tunisia. Isolated daytime driven storms with large hail and strong to severe wind gusts are possible. Wind gust risk may be enhanced compared to the other level areas due to a deep and well mixed subcloud layer and 15 m/s 0-3 km shear.
... Rest of the thunderstorm areas ...
Scattered to widespread thunderstorm development is expected. Local topography and/or mesoscale features like outflow boundaries from nighttime activity all play a role in storm coverage and intensity. An isolated large hail event is possible, but weak to non existent shear and marginal CAPE keep severe probabilities too slim for a level 1.