Valid: Sun 02 Nov 2014 06:00 to Mon 03 Nov 2014 06:00 UTC
Issued: Sun 02 Nov 2014 08:23
Forecaster: VAN DER VELDE
A level 1 was issued for France, Belgium, Netherlands and parts of Germany and Denmark mainly for tornado chances.
A longwave trough associated with an Atlantic low pressure system centered south of Iceland migrates into western Europe. Ahead of a cold front warm humid air is advected by a strong southwesterly flow. Various models predict CAPE in the range of 200-800 J/kg, developing over Spain, France and Benelux, with maxima in valleys connected to the Mediterranean moist airmass. Evidence is already visible in the form of thunderstorms over SW France and west of Galicia (Spain) at 06Z. Convection is predicted along the cold front under the upper trough, where significant lift from vorticity advection is present, but also in a prefrontal band over western Spain into southwestern France. The strongest lifting should occur however over Belgium/Netherlands into Denmark during evening and night, well indicated by the acceleration of a PV intrusion feature over that area and the increase of low-level vertical theta-e gradients (potential instability). A linear convective organization or separate bowing segments should be anticipated there.
Severe weather may come in the form of severe wind gusts when linear features develop. Background winds at 1-3 km altitude reach over 20 m/s in the northern half of the level 1 area, so some gusts may reach the severe criterion of 25 m/s. Bow echoes but also tornadogenesis is greatly helped by the strong vertical wind shear at low altitudes: 15 m/s 0-1 km shear from northern France northeastward after 18Z. Deep layer shear is predicted in the range of 25-30 m/s with a maximum storm-relative helicity of >200 m²/s² over the Netherlands from 18Z-00Z. Spain and southern France will only have slightly enhanced deep layer shear of some 15 m/s, which could allow for brief rotation, but with the relatively weak CAPE and equilibrium levels barely reaching -20°C (especially over France) the chances of large hail should be low.