what is a Cevennes episode?

Severe thunderstorms and record rains hit the Gard on Tuesday. In the town of Saint-Dionisy, about fifteen kilometers from Nîmes, 244mm of rain fell in three hours, well beyond the previous record, of 216.4 mm in Saint-Martial in September 2020. The department has was briefly placed on red vigilance by Météo-France on Tuesday, between 11:30 am and 2 pm. Severe bad weather described by the authorities and meteorologists as a “Cevennes episode”, which is “not exceptional for the season”. We take stock of this very particular phenomenon.

How do Cevennes episodes occur?

The Cévennes, or more broadly Mediterranean episodes, specifically affect the South-East of France. They usually take place several times in early autumn, between September and October, after the Mediterranean Sea has become loaded with heat during the summer, causing significant evaporations.

These hot and humid air masses evaporating from the sea then go up towards the north and strike against the mountain range of the Cévennes. They then rise along the relief and cool down, generating instability. Results: clouds form and all the accumulated humidity eventually falls in the form of heavy precipitation, linked to thunderstorms. The equivalent of several months of rain falls in a few hours or a few days, resulting in sometimes devastating floods.

Meteo France also specifies that the term “Cévenol” is “often misused to characterize any episode bringing torrential rains on the southern regions”. “While it is true that the Cévennes massif is renowned for the intensity of the episodes that affect it (hence the qualifier), heavily rainy situations affect the entire Mediterranean arc and are therefore far from being exclusively” cévenoles “, underlines the organization.

How long do they last?

Cevennes episodes generally last 24 to 72 hours. They can be spread over four days in the rarest cases. Regarding the last episode in Gard, the situation calmed down on Tuesday afternoon and in the evening in Gard. But it could deteriorate again this Wednesday. Météo France, however, showed little alarmist in his 10 o’clock bulletin, while the department is in orange vigilance for thunderstorms and rain-floods, with Ardèche, Drôme, Isère and Hérault.

How to anticipate them?

The Minister of the Sea Annick Girardin had validated last year a project to study water vapor in the Mediterranean, making it possible to better prevent Cevennes episodes. This project, developed by the space center of the University of Montpellier with the National Institute of Geographic and Forest Information (IGN), the engineering school ENSTA Bretagne, the Port of Sète and Météo France, provided for to embed sensors on boats leaving from Sète (Hérault) and relay their information via a nanosatellite. The objective was to collect data on the quantity of water vapor present, an essential analysis to understand the meteorological phenomenon.

Source: Le Progrès : info et actu nationale et régionale – Rhône, Loire, Ain, Haute-Loire et Jura | Le Progrès by www.leprogres.fr.

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