Mediterranean episode: cumulative rain, intensity… what to expect in Hérault and Gard overnight from Friday to Saturday

First perceived as very threatening, the episode announced for the night of Friday to Saturday will be intense in places but more measured than feared, illustrating the difficulty facing forecasters. It should only affect eastern Hérault and Gard.

September, October, November: the privileged season of Mediterranean episodes opened with orange then red on 7 September; and slipped towards a phenomenon as of this Friday that the meteorological models first gave as critical, before showing themselves less “alarmists”. It will rain, heavily in places, on a green vigilance map this Thursday but with all the fragility of analysis that prevails in autumn on the shores of the Mediterranean.

French Weather Document
French Weather Document

1 – Up to 100 mm of rain

“The probability of having a rain-flood phenomenon is 70% for Friday and Saturday”, says Florence Vaysse, Meteo France referent for Languedoc and Roussillon. 70 to 100 mm of precipitation, temporarily intense and mixed with thunderstorms, will center on eastern Hérault and the plains of Gard overnight from Friday to Saturday. They will hit Montpellier after midnight, Nîmes a few hours later, before evacuating towards Paca and western Corsica.

“There are not too many signals of stationarity”one of the main dangers, observes Alix Roumagnac, president of Predict, in Castelnau-le-Lez, a subsidiary of the national institute of meteorology whose teams went into three shifts to follow the event. “It’s something that should flowadds Florence Vaysse, reducing the risk of flooding. But that’s for today…”underlines the former forecaster, pending possible developments today.

2 – Instability and complexity

Less active than what the models anticipated at the beginning of the week, this disturbance will keep its share of uncertainty until the last moment. This is the lot of autumnal phenomena in the Mediterranean, unstable, “with predictability more difficult than in winter or summer, as they are multifactorialinsists Florence Vaysse. Different things come into play, the temperature in the lower layers of the atmosphere, the direction and strength of the winds, etc. And when we have thunderstorms, it is necessarily on small spatial and temporal scales that the models have difficulty seeing.

Another more distant factor intervenes in these phenomena which cannot be reduced to their Mediterranean character. At the same time, the hurricane season in the Atlantic exaggerates their characteristic instability, with an impact “three to four days later”.

3 – Warm sea and dry soils

The Mediterranean is still warm, “in positive anomaly compared to the averages of the season”, notes the referent of Météo France, evoking 21 and 22° of temperature in the Gulf of Lion. a postman “who can bring fuel to this episode”, analyzes Alix Roumagnac at the end of a summer when the sea will have reached up to 28°, exceptional. But the engineer delays: “The tramontana and the mistral of the past few days have been able to push the warm waters further offshore.”

Another element of risk to take into account, linked to the weather of the past months, is the dryness of the soils measured by an indicator, the Soil Wetness Index, and therefore their ability to absorb part of the water, limit runoff. “The rains that have passed over the region have improved the situation a little”observes Alix Roumagnac, but the situation is very heterogeneous. “The soils are very dry in the Aude and the POs, where it has sometimes not rained for a monthcontinues Florence Vaysse. It is quite correct where the heaviest precipitation is expected.

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