Storm Forecast
Valid: Tue 15 Mar 2011 06:00 to Wed 16 Mar 2011 06:00 UTC
Issued: Tue 15 Mar 2011 04:08
Forecaster: VAN DER VELDE

A level 1 was issued for parts of northeastern Spain and southern France for excessive convective rainfall.

SYNOPSIS

High pressure is present over Scandinavia, while a large low pressure area covers the Iberian Peninsula and western Mediterranean Sea area. Both on the Atlantic side and the Mediterranean side low pressure centers are found. The latter is injected by a fresh anomaly of potential vorticity at mid levels, and higher instability values are predicted by the GFS model than in previous days. During the evening and night, a surge of warm air from Algeria/Tunisia enters the central Mediterranean, and elevated (not ground-based) convection is likely to develop but not likely to profit from enhanced shear values near the surface.

DISCUSSION

...parts of northeastern Spain and southern France...

The GFS model (18Z run) develops a zone of up to 300 J/kg MLCAPE in the (early) morning hours between the Balearic islands and the coast of northeastern Spain, but a cap seems present due to warm air above the surface level, with steep lapse rates above (as visible in Mallorca 00Z sounding). Near the coast, strong convergence in low levels should be able to release the CAPE, along with further destabilization by strong QG lifitng in mid levels.
Already at time of writing active thunderstorms have formed north of the Balearic islands. This zone will eventually affect the Spanish coast, and instabilty will be present during the afternoon in Catalonia deep inland as well. Considering the convective nature, long persistence of onshore moist, unstable airmass advection and intense large scale lifting, a level 1 is warranted for locally excessive convective (and stratiform) rain. During the evening it will affect mostly southern France.