November 29, 2021

Weather Alert: Heavy rains and severe thunderstorms in the southeast

Through Cyrille DUCHESNE, meteorologist

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From October 4, 2021 at 3:00 p.m. to October 4, 2021 at 10:00 p.m.

Situation

This Monday, a depression has widened between the Balearic Islands and Sardinia and goes up towards Provence. This depression brings up in a very dynamic southerly flow unstable air and violent thunderstorms over the south-east of the country. These often stationary thunderstorms can generate very violent precipitation. After the lull at the end of the morning, thunderstorm activity resumes this afternoon and until the end of the evening, giving rise to fears of major floods, in particular in the Bouches-du-Rhône department, due to the accumulations already observed this morning. More generally, the entire Provence Alpes Côte d’Azur region is affected by heavily rainy thunderstorms. Corsica is also swept by brief and powerful thunderstorms.

Observation

This Monday

At 15h, a line of violent thunderstorms accompanied by intense rains persists in the west of the Var. In front of this stormy line, other occasional heavy thunderstorms hit the east of the Var and the Alpes-Maritimes. In Varages, in the Alpes-Maritimes, it fell 45 millimeters between 1 and 2 p.m. Eastern Corsica also continues to experience heavy stormy showers with locally significant rainfall intensities.

A 12 h, the weather is temporarily dry on the Provençal coast. On the other hand, inland, two lines of poorly mobile thunderstorms persist over the center of the Var, the first north of Brignoles, the second north of Vidauban where they cause very heavy localized thunderstorms: for example, in Aiguines located in the far north of the Var department, it has just fallen 60 mm of water under a violent thunderstorm between 10 and 11 a.m. In Corsica, large thunderstorms rise in a turbulent southerly flow all along the eastern coast of the island, between Solenzara and Bastia. They can be accompanied by hail and the formation of waterspouts offshore.

At 9 a.m., the improvement is noticeable in the Bouches-du-Rhône department, while violent downpours are still affecting the west of the Var. Over the last 12 hours, we note up to 173 mm in Marseille (Bouches-du-Rhône), 166 mm in Peypin (Bouches-du-Rhône), 161 mm in Cassis (Bouches-du-Rhône) and 157 mm in Mimet (Bouches-du-Rhône).

At 7 p.m., last night was very rough in the lower Rhone valley and in Provence with powerful lines of thunderstorms coming up from the Mediterranean and causing torrential rains. It thus fell nearly 100 mm on Marseille including 73 mm in the space of 2 hours at the end of the night. Currently, these storms continue with a particularly active cell between Marseille and Toulon.
Since yesterday we have seen 110 mm in Lapalud (Vaucluse), 109 mm in Gréasque (Bouches-du-Rhône) and 94 mm in Orange (Vaucluse).

Evolution

Here is the chronology of this important and lasting episode of bad weather

This afternoon, a new intense thunderstorm area rises from the Mediterranean from 3 p.m. Thunderstorms can be very violent again with hourly intensities of around 40 to 60 mm in the Var and Bouches-du-Rhône, giving rise to fears of significant flooding and flooding in these departments. The other departments on alert are also affected by the passage of heavy thunderstorms, in particular the Alpes-Maritimes and Corsica.

Tonight, the stormy intensity remains marked from Provence to Corsica, but the hourly intensities gradually weaken in the second part of the evening and this press release will be lifted at 10 p.m.

During this bad weather episode we wait

– very intense precipitation with accumulations of the order of 30 to 50 mm in one hour.
– accumulations of 80 to 120 mm rather generalized which will come to be added to what already fell last night in particular on the department of Bouches du Rhône.
– a significant risk of hailstorms
– a high risk of urban runoff, floods and flooding in the sectors most exposed to heavy rains and stationary storms.
– risks of vortex phenomena, in particular in coastal areas (waterspouts and tornadoes).

List of departments concerned