Storm Forecast
Valid: Mon 13 Aug 2018 06:00 to Tue 14 Aug 2018 06:00 UTC
Issued: Sun 12 Aug 2018 22:36
Forecaster: VAN DER VELDE

Levels 1 and 2 were issued for NE Iberian Peninsula, S France, Corsica and N Italy mainly for large hail, severe convective wind gusts, excessive convective rainfall, and waterspouts.

A level 1 was issued across E Belgium/Netherlands, N Germany and S Denmark mainly for local excessive convective rainfall.

A level 1 was issued for W and C Germany mainly for slight chances of large hail and tornadoes.



DISCUSSION

A low pressure system and upper shortwave trough is tracking from the UK to Denmark. A southerly flow to the east of its cold front stretching from E Iberia to Netherlands advects steep mid level lapse rates but with modest MLCAPE of under 500 J/kg across NW European mainland, and up to 3000 J/kg MLCAPE along the Mediterranean north shores. Vertical wind shear is generally weak, except for western/central Germany, where 20 m/s DLS and 10 m/s LLS might support updraft rotation and tornadoes, but models show even more marginal CAPE and initiation potential.

The GFS model indicated an unstable area over Czech Republic and E Germany/W Poland, but ICON and ECMWF have significantly drier conditions in the lower levels. With the drought, evapotranspiration is unlikely to contribute much moisture.

Benelux, NW Germany to Denmark will have strong forcing as a marked PV feature (shortwave trough) comes through, for the rest there are only weak areas of lift along the cold front over France and NE Iberia. Despite the good linear forcing by the Benelux trough, slow storm motion will benefit intense local rainfall more than wind gusts.

High CAPE in the Mediterranean advects into Catalonia and S France. With borderline moderate shear (10 m/s 0-6 km) there should be intense updrafts and enough organization for large hail. In addition, dry mid levels create extreme delta-ThetaE over 25K. This can create high negative buoyancy in downdrafts with chances of microburst (severe wind gusts).
Excessive rain could result if storms stick around. Also, some waterspouts may be sighted. At night the cold front will push through the N Mediterranean. The cap may be stronger but models do predict convective activity from Balearic Islands to Corsica and Italy.