Storm Forecast
Valid: Tue 27 Oct 2015 06:00 to Wed 28 Oct 2015 06:00 UTC
Issued: Tue 27 Oct 2015 02:47
Forecaster: VAN DER VELDE

A level 1 was issued for northeastern Spain and southern France mainly for chances of tornadoes and marginally large hail.

SYNOPSIS / DISCUSSION

The cold front of a large low south/west of Ireland is moving eastward over the Iberian Peninsula and will enter France after 15Z. North of the Pyrenees the vent d'autan is picking up, advecting Mediterranean moisture (10-12 g/kg) into southwestern France. At midlevels a broad upper trough focuses vorticity advection over Spain at 12Z, then gaining negative tilt as it sweeps over northern Spain into France. This strong forcing has a notable destabilizing effect on GFS temperature fields, with lapse rates going from a neutral 6-6.5 to 7-7.5 degrees per kilometer. Modest MLCAPE of a few hundred J/kg should develop. Largest chances for initiation of convection are in southwestern France, aided by low level convergence between easterly winds and the incoming cold front from the west, and the deeper lifting of the shortwave trough during early evening.
0-6 km deep layer shear of 15-25 m/s is present over the level 1 region, with 0-3 km storm-relative helicity enhanced north of the Pyrenees (on the order of 200 m²/s²) and along the south coast of France (300 m²/s² due to wide looping forecast hodographs in low levels) which favour updraft rotation. 0-1 km shear of 7-12 m/s enhance tornadogenesis chances. The factor that reduces confidence is the marginal CAPE. During evening and night, strong moisture lifting near Montpellier might cause long-lasting convection which could locally cause high rain amounts.