Severe storms forecast for W France this afternoon and evening – July 18, 2014

A highly robust severe weather setup is shaping up for late this afternoon and evening hours over W France, with threats for torrential excessive rainfall, severe wind gusts as well as large to locally very large hail and some tornadoes.

A pronounced low is located over the Atlantic, west of the Bay of Biscay with a short wave trough being pushed NE-ward into France during the late afternoon and a second short wave in the late evening and night hours. A strong jet streak overspreads into Bay of Biscay and W France. Significant surface backing winds combine with the mid-level jet to produce a strongly sheared environment with 40-60 kt DLS and favourable veering profiles (200-350 m2/s2 SREH3). High instability is in place and with favourable QG forcing is supporting strong organized thunderstorms with high coverage.

500 mb geopotential analysis for Friday, July 18th 12Z. Source: Meteonetwork.it

Regional WRF models forecast 3500-4500 J/kg SBCAPE over S-CNTRL parts of W France and as high as 4000-5000 J/kg SBCAPE in the late afternoon. Dewpoints in the 20-22 °C are forecast ahead of the cold front in W France. As the current dewpoints across the region (11Z) are well in line with regional WRF model forecasts, this adds confidence in the forecast instability levels.

SBCAPE forecast valid for 14Z. Source: Meteociel.fr

SBCAPE forecast valid for 19Z. Source: Meteociel.fr

300 mbar wind forecast valid for 14Z. Source: Meteociel.fr

10 m wind forecast valid for 14Z. Source: Meteociel.fr

Storm initiation is currently already underway over SW shore of the Bay of Biscay, while TCus are firing in CNTRL France as the first of the two shortwaves enters the region. A second wave of thunderstorms is expected during the late evening as the second shortwave takes effect. Further information will be posted as the situation unfolds.

11:45 UTC VIS EUMETSAT image of France. Source: Sat24.com