Severe storms forecast for France and UK this afternoon and evening – July 19, 2014

A deep, negative tilt trough is pushed into France and the UK this afternoon and evening. High instability builds up ahead with CAPE values reaching 3000+ J/kg in France and 1000-1500 J/kg range in the UK. The left exit region of a strong jet streak crosses the region ahead of the trough, combining with favourable backing sufrace winds to create a strongly sheared environment, favourable for organized thunderstorms. Storm initiation is expected in early afternoon for extreme N France and UK and mid to late afternoon along the cold front in SW-CNTRL-NE France.

500 mb geopotential analysis for Friday, July 20th 00Z. Source: Meteonetwork.it

A strong jet streak is present ahead of the trough, moving across the Iberian peninsula with its left exit placed over France and into the UK. 500 mbar wind speeds reaching 70 kt across eastern Spain and 35-55 kt over France and southern UK.

500 mb wind speed forecast for July 19th 18Z. Source: Wetterzentrale.de

Moderately favourable backing southeasterly winds are present across CNTRL-E France (12Z). The enviroment is strongly sheared with 0-6 km shear reaching approximately 30-50 kt with SREH3 in the 100-200 m2/s2 range. The environment supports multicell and supercell thunderstorms.

Latest surface observations show surface temperatures in the 26-29 °C range with dewpoints in the 17-20 °C range across CNTRL-NE France. The southern UK is currently seeing surface temperatures of 22-23 °C and dewpoints in the 19-21 °C range. UK values agree well with forecasts while dewpoints across France were overestimated by models by about 1-3 °C, so CAPE values in these regions may be overestimated somewhat.

SBCAPE forecast valid for 12Z. Source: Meteociel.fr

SBCAPE forecast valid for 15Z. Source: Meteociel.fr

Main threats with these thunderstorms will be excessive rainfall, severe wind gusts and marginally large to large hail. Storm initiation is expected in two waves. The first wave, with limited coverage, is across extreme N France early in the afternoon (initiation has begun as of 12Z), moving across the English Channel into southern UK. The second wave of initiation is expected along the surface cold front in mid to late afternoon, stretching from SW across CNTRL into NE France and SE UK. Storms will cluster into a MCS, pushing into E France and Benelux in the evening hours.

This region was issued a MDT risk area in Day 1 severe weather outlook for today. Today is the second day in a 4-day active pattern over CNTRL Europe and CNTRL-E Mediterranean/Balkans.