A torrent falling on Annonay (Ardèche), submerged cars, firefighters in diving suits: six departments face the rain on Thursday, October 17 “exceptional” up to 650 millimeters in certain areas of the Ardèche. A phenomenon that will probably be repeated with climate change, recalls Aurélien Ribes, Météo-France researcher at the National Center for Meteorological Research.
Is this episode of heavy rainfall notable?
The strongest accumulations are observed at this stage in Ardèche, in the Cevennes relief. They locally reached between 600 and 700 mm in forty-eight hours, of which about 500 mm in twenty-four hours, which is exceptional. There are very few episodes of intense rainfall exceeding 500 mm in a day, even in the Cévennes region, the region that most frequently experiences this type of phenomenon.
What is also notable is the extent of the affected territories: thirty-three departments were placed under orange watch for rains and floods and six under red watch on Thursday night. [dix-huit restaient concernés par une vigilance orange vendredi matin]. In certain departments of the northern half of France, between 30 and 50 mm of precipitation were expected, on soils already saturated with water, due to the passage of Storm Kirk last week.
What is the cause of this phenomenon?
As usually happens in this type of phenomenon, a depression coming from the Atlantic brought a mass of warm and humid air from the south. In the Cévennes and in the Alpes-Maritimes, this gave rise to what we call a “Mediterranean episode”. As it touched the reliefs that border the Mediterranean, the air rose and cooled. Therefore, it began to condense, forming water droplets, and triggered a phenomenon of convection, that is, very rapid upward movements up to about 10,000 meters of altitude, which cause storms. The relief facilitates the maintenance of storm cells in the same place, which can cause very significant accumulations of precipitation.
In just a few months, we are experiencing a sequence of several notable episodes of heavy rainfall. Before Kirk, there were also exceptional floods in the Aspe Valley, in the Pyrénées-Atlantiques, in September, and the flood that devastated Bérarde (Isère) at the end of June. Part of Central Europe was also hit hard in September. This series is challenging, but is consistent with the expected clinical picture of climate change.
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