Severe thunderstorms swept across France last Friday, killing one person and injuring another. Two systems were involved, prompting orange weather warnings: the first came from the west via Brittany and hit the north of the country, and the second arrived via Spain and affected south-west France.
More than 30,000 lightning strikes were recorded between midnight on Friday and early Saturday. Eure, north of Paris, was worst hit with 4,326 strikes. Strong winds lashed Normandy – Rouen recorded a 76mph (123km)/h) gust that broke the 64mph record set in 2019. Hail affected several areas, leading to infrastructure and crop damage.
There were further storms over central and north-west France on Saturday night, many with very large hailstones. The largest, found in Orly, Auvergne-Rhône-Alpes, measured 85mm.
Several factors led to the severe weather. A key component was a phenomenon known as goutte froide, in which an isolated, upper-air cold pool detached from the overall circulation, creating significant atmospheric instability.
A thalweg d’altitude (upper-air trough) provided the lift necessary by favouring air ascent. Warm, moist air flowed into the trough and was forced upwards. As the air rose, it started to cool and water vapour began to condense, forming clouds. As this clouds swelled, they created severe thunderstorms.
As air flowed into the trough from every direction, it enabled supercells to develop by introducing the element of spin. Supercells are large-scale, highly organised storms with a rotating updraft that can sustain themselves for several hours and travel hundreds of miles – one of which swept through northern France on Friday.
A recent heat dome over the country helped intensify the storms by providing a greater gradient between the surface and the upper air, and by bringing a greater source of moisture. A heat dome occurs when high pressure persists over a region, trapping warm air and allowing temperatures to rise several degrees above the seasonal average.