Storm Forecast
Valid: Mon 28 Apr 2008 06:00 to Tue 29 Apr 2008 06:00 UTC
Issued: Sun 27 Apr 2008 20:09
Forecaster: GROENEMEIJER

SYNOPSIS

Several vorticity maxima rotate around a trough situated over the British Isles. On monday at 06 UTC, one is expected over Brittany. A lower-tropospheric cold front, that moves eastward into Europe is expected along a line from NW Germany to Luxemburg via the central Pyrenees to central Portugal. A westerly jet over northern Iberia moves eastward during the forecast period. During the night an intense jet streak rotates into the Bay of Biscay. An associated deepening surface low is forecast to pass to the southwest of Ireland.

DISCUSSION

North and central Germany...

Ahead of the aforementioned cold front, a few 100s of J/kg CAPE will likely form during the afternoon as temperatures rise to values around 20C and the dew point rises to near 12 C. A few thunderstorms forming multicell clusters will likely develop in the afternoon. Low values of CAPE and only moderate wind shear suggest that the severe weather threat will be low, so that a threat level is not issued. A few stronger gusts and some mostly small hail will be possible though.

Northwest France and Belgium...

The vorticity maximum initially over Birttany should move northeastward during the day. The air-mass on the cool side of the jet should be rather rather moist with abundant convection developing during the late morning in response to both the advancing trough and diurnal heating. Deep-layer shear will locally be rather strong, but appears to be confined mostly to the highest levels that the storms will reach. Hence, the chance that bow-echoes or mesocyclones will form appears to be small at the moment. Rather widespread mostly small (<2cm) hail is however possible. If low-level winds become stronger than currently foreseen, an upgrade to level 1 may be required.

NE Spain...

The westerly jet over northern Iberia should have its exit region and associated forcing for upward motion over Catalonia during mid-afternoon. Numerical guidance from the GFS model currently suggests that this will lead mostly to fields of stratiform rain rather than convective precipitation. Based on past experience of GFSs behaviour in this area with westerly flow, the development of isolated to scattered convective storms appears possible as well. In that case, storms would develop in a strongly shear environment and could become severe. Currently, the overall threat is deemed to low for a level 1 category.
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