Valid: Mon 28 Mar 2016 06:00 to Tue 29 Mar 2016 06:00 UTC
Issued: Mon 28 Mar 2016 03:11
Forecaster: VAN DER VELDE
A level 1 was issued for N France, Belgium, Luxemburg and western/central Germany mainly for chances of isolated tornadoes and severe wind gusts.
SYNOPSIS / DISCUSSION
In the cold sector of depression "Jeanne" (North Sea), an unstable and well-sheared airmass passes over the northern half of France, southern England, the Benelux countries and western Germany between 06Z and 18Z. Steep lapse rates of ~ 7,5 K/km with low 5 g/kg mixing ratios cause MLCAPE in GFS ranging between 100-200 J/kg (1000m mixed parcel) and around 250 J/kg in WRF ARW. Cloud tops can easily reach -30°C and lightning is a likely result. Forcing is strongest at the cold front and weaker in the unstable airmass, but probably sufficient for organizing some clustering of cells. Deep layer shear of 20-35 m/s overlaps the unstable airmass between northern Belgium and central France respectively. SREH of 150-250 m²/s² is found mainly between central France and central Germany. Despite the weak instability, the prospective for shallow supercell storms is good, and these can produce tornadoes under favorable 10-15 m/s 0-1 km shear vectors and relatively low (800m) LCL. Severe wind gusts are also likely as low/mid level mean winds reach 25 m/s, which can be transferred to ground in downdrafts. The weak instability precludes a higher threat level.
As the sun goes down instability is reduced and storms should become less abundant, although some potential remains at night over central Germany where the shear conditions will also still allow tornadoes.