Storm Forecast
Valid: Mon 21 Nov 2016 06:00 to Tue 22 Nov 2016 06:00 UTC
Issued: Mon 21 Nov 2016 05:12
Forecaster: VAN DER VELDE

A level 1 was issued for NW Iberian Peninsula and W France mainly for severe convective wind gusts.
A level 1 was issued for NE Iberian Peninsula and S France mainly for a chance of local excessive convective precipitation.

SYNOPSIS / DISCUSSION

A depression is deepening over the Celtic Sea to approx. 980 hPa while moving towards Brittany and the southern UK, causing high wind speeds from NW Spain to W France and coasts of the UK. The cold front of this low traverses central Spain and France, curling back to UK. Behind the front, cool unstable airmass is transported southward to Portugal at the west side of the low, and over Bay of Biscay on the south side of the low pressure core, advecting over 300 J/kg MLCAPE into W France. The collocation with winds over 25 m/s at 1-3 km height implies a threat of severe wind gusts produced during convection, in particular in W France. Since 0-1 km shear is over 15 m/s and the region is near the jet axis where deep layer shear has a gradient from 15 to 40 m/s, tornadogenesis may occur as additional threat.

At the cold front over Spain and southern France, slight and mostly elevated instability can cause some convection with slight chances of isolated excessive precipitation over S France and NE corner of the Iberian Peninsula where orographic lifting of large moisture transport vectors is a factor in the second half of the period. Given 800-1200 J/kg SBCAPE in WRF models along the coast of Catalonia, in contrast to slight MLCAPE in GFS, a level 1 for convective precipitation seems warranted.