Weather: thunderstorms and heavy rain announced in the south of France, what to expect?

Weather: thunderstorms and heavy rain announced in the south of France, what to expect?

Mediterranean episodes occur in a privileged way in autumn, when the sea is warmest, which favors strong evaporation. (©Illustration/Fabien Hisbacq/Actu Occitanie)

Be careful, weather report, take it easy, it won’t last from the beginning of this week! The lights are turning red for this weekend, September 24th and 25th, 2022, over much of southern France.

“A rainy episode is expected to hit Brittany on Friday, while showery and stormy activity will spread to the southeast on Saturday with the likely formation of a Mediterranean episode. Heavy rain could also hit the east of the country on Sunday. On Monday, stormy rains could still persist over the southeast of the country,” Météo France announces this Wednesday.

Ejections of hot, humid and unstable air from the Mediterranean Sea

If its location and chronology remain to be determined, the event could be violent. Either the disturbance extends over the large southern third with heavy rains all over the Mediterranean margin (this would then be a Mediterranean episode), or the heavy accumulations are mainly concentrated in the departments near the Cévennes (if this were a Cevennes episode).

Both are powerful storm systems that bring intense rainfall, the equivalent of several months of rainfall in a matter of hours or days.

Mediterranean episodes are associated with exits of hot, humid and unstable air from the Mediterranean Sea, which can cause violent storms that are sometimes stationary, explains Météo France. They occur in a privileged way in autumn, when the sea is warmest, which promotes strong evaporation.

“We will switch under the influence of the high altitude thalweg”

The term “Cévenol” is often misused to characterize any episode bringing torrential rains to southern regions. While it is true that the Cévennes massif is renowned for the intensity of the episodes that affect it (hence the qualifier), heavy rain situations affect the entire Mediterranean arc and are therefore far from being exclusively “Cévennes”. “, specifies the official service of meteorology and climatology in France.

“We’ll switch to the northwest flow. Rain will arrive in Brittany on Friday,” reports SportRizer meteorologist Yann Amice. But if the rains are light in the northwest of France, they promise to be (very) intense in the south.

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In the southeast, between Perpignan (Pyrénées-Orientales), Montpellier (Hérault) and Carpentras (Vaucluse), we pass under the influence of a high-altitude thalweg (editor’s note: an area of ​​the atmosphere where, at the same level, the atmospheric pressure is low compared to the surroundings, as in a depression, but elongated shape). There will be cold air at high altitudes with warm south-easterly winds at low levels picking up moisture on the Mediterranean which is still warm at this time of year. The dangerous situation should take place from Friday evening.

Yann AmiceMeteorologist

The exact location of these stormy phenomena is very difficult to determine at the moment, “but the cocktail is there to observe the heavy rains. Corsica should also be affected on Sunday. »

Gard, Ardèche, Hérault and Lozère are often the most exposed

The most recent notable Mediterranean episodes date to September and October 2020. On 19 and 20 September 2020, extremely heavy rainfall fell in the Gard Cévennes in the north-west of the department, with accumulations of more than 500 millimeters locally. On 2 and 3 October 2020, the phenomenon was launched in the Alpes-Maritimes.

“Storm Alex triggered an unprecedented Mediterranean episode.” “Exceptional rainfall was recorded in the interior of the Alpes-Maritimes and in the extreme north-east of the Var,” recalls Météo France. Torrential rains caused devastating floods.

While all of metropolitan France can be subject to heavy rain, it is more common in the Mediterranean and mountainous regions.

Gard, Ardèche, then Hérault and Lozère are at the top of the departments where we most often observe episodes bringing more than 200 millimeters of rainfall in one day on average per year.

Mountainous regions are also more often affected by storms than plains: the relief favors the condensation of air mass by imposing lift and tends to block the movement of clouds. As the air rises, it cools, condenses and inevitably causes increased thunderstorm activity. This results in an increase in precipitation on the slopes of reliefs exposed to the wind.

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