Storm Forecast
Valid: Thu 10 Apr 2008 13:00 to Fri 11 Apr 2008 06:00 UTC
Issued: Thu 10 Apr 2008 13:14
Forecaster: GROENEMEIJER

SYNOPSIS

Thursday at 12:00 UTC... a mid/upper trough with an axis from the English Channel to NW Spain onward to the SW is slowly moving eastward. Downstream of the trough a SWly jet (40-50 m/s at 500 hPa) stretches from Morocco to east-central Spain. An embedded vorticity maximum currently over north central Spain will move into central France during the afternoon hours. A slowly moving frontal zone stretches from the Czech republic, via the NW Alpine slopes into SW France. An initially ill-defined, baroclinic wave over N Spain is expected to travel NE ward into France reaching Luxemburg Friday morning.

...Southern France, Massif Central, Rhone Valley, Jura...

SE of the cold front, ahead of the upper level impulse, strong insolation is taking place across South-central France the Massif Central, N Rhone Valley, where temperatures approaching 20 C and dew points of 11 C should be able to yield 500-1000 J/kg SBCAPE. TCu over the Massif central is locally growing into thunderstorms. linearly organized activity near convergence zone / Pyrenees lee low that has developed over SW France is slowly intensifying.

Model guidance indicates that the surface low will intensify somewhat and detach from the lee of the Pyrenees during the afternoon and early evening, creating a region of backed sfc flow ahead of it with corresponding high SRH and high low-level shear.

Present thinking is that storm coverage will gradually increase along and just south of the front and that these will include a number of supercells capable of producing large hail and damaging winds. A threat of tornadoes will be present also and develop northeastward as low level helical flow intensifies. The forecast low values of CAPE presently preclude the issuance of a level 2, but an upgrade may be issued later on.

During the second half of the evening, the overall convective activity will likely weaken in response to cooling of the boundary layer.

...Andalucia...

Ahead of the primary cold front that will move eastward out of Portugal this afternoon and evening, a very strong 25 m/s low-level flow of moist air with a few 100's of J/kg CAPE will likely persist. Embedded within this flow, small linearly-organized convective elements and more isolated cells are present. The storms will likely retain a threat of damaging winds through the afternoon and early evening. Despite poor directional shear, an isolated short-lived tornado cannot be completely ruled out.

...Central Spain...

Weakening shear from the west and relatively low CAPE, if any, will greatly mitigate severe weather threat despite the occurrence of isolated thunderstorms.

...Catalonia, N Valencia...

The general mesocale environment is characterized by low CAPE, moderately high cloud bases and strong shear. Convection developing this afternoon may develop rotating updrafts, resulting in a threat of some, potentially marginally severe hail and strong to severe winds.

...SE France, Corsica, N Italy, W Slovenia, W Croatia...

Per GFS, some elevated instability should be created and advected NE ward within the SWly warm advection regime over the W Mediterranean. It is expected that during the night, scattered thunderstorms will develop. The presence of very strong shear in the region has prompted the issuance of a level 1 risk for some storms will probably develop rotating updrafts and therefore be able to produce marginally severe hail.