Winds at more than 200 km / h in places and an already heavy provisional human toll: at least five dead and 20 injured, including four “very serious”, according to the Ministry of the Interior. Very violent storms swept across Corsica on Thursday August 18 in the morning. A few tens of minutes before the sky was unleashed on the inhabitants of the island, the two departments of Haute-Corse and Corse-du-Sud were not, however, on orange vigilance, but only yellow vigilance, like dozens of other territories in France. Faced with questions, Météo France explained during a press briefing why it had triggered this alert so late.
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Storms initially forecast at sea
Météo France initially predicted that the storms would strike at sea, partially affecting the Corsican coast. The prefectures had still been warned from Wednesday noon of episodes that could affect coastal areas with gusts estimated between 80 and 100 km / h, detailed Météo France.
In the 6 a.m. bulletin on Thursday, the island was still on yellow alert (“be careful”) and Météo France then announced “powerful storms (…) at sea near Corsica, with strong gusts of wind”who could “very temporarily affect the west and north coasts”.
Two changes led Météo France to update its vigilance map a little later: the stormy phenomenon went further inland and very strong gusts were observed.
It’s only shortly after 8 a.m.by observing the movement of the storm inland and the intensity “extreme” gusts, that the two departments have swung “immediately” in orange vigilance, meaning: “be vigilant”.
#Corsica An exceptional 224 km/h gust has just been measured at the Marignana station on an exposed part of the west coast, 131 km/h in Ajaccio! The #Corsica goes into #vigilanceOrange immediately ! Be careful, also for other hazards related to #thunderstorms.
— VigiMeteoFrance (@VigiMeteoFrance) August 18, 2022
A rare phenomenon that “surprised” forecasters
“Gust values above 200 km/h were recorded, which is quite exceptional” for a storm, underlined Christophe Morel, head of forecasting at Météo France. “We can say that we were a little surprised” by these values ”which had never been observed before”, he added. The formation of a “squall line” arched allowed structures favorable to these very strong descending winds, explained Météo France.
Passage of the violent squall line, view from the northeast of the #Corsica this morning, with the very rapid arrival of an arcus, followed by 4.6mm of rain at Penta di Casinca, then strong gusts, up to 87km/h, a very modest value compared to others on the island ! #thunderstorms #vigilanceorange pic.twitter.com/TGU9oaovQn
— François Gourand (@FGourand) August 18, 2022
Changing forecasts
Thunderstorms are particularly difficult to predict because they occur on small scales, experts explain. Météo France has models established by the Arome forecasting tool, which works with a supercomputer in Toulouse. These models are updated several times a day, “every three or six hours”, according to François Gourand, forecasting engineer. As advanced by meteorologists on social networks, a model had indeed suggested “a phenomenon close to what has been observed” in Corsica, from Wednesday noon. However, the models that followed predicted storms located rather at sea, insists Météo France.
#Corsica #storm The AROME model had predicted this storm for this morning from yesterday’s scenario at 6 a.m. and especially from that of yesterday at 12 p.m. (but not yet very well set). Here is the scenario published this night at midnight: the affected areas are almost the same! map @meteociel pic.twitter.com/nQnNtFOY8E
— Guillaume Séchet (@Meteovilles) August 18, 2022
François Gourand thus sees “the limits of what weather forecasts can detect.” Other simulations “which seemed more likely placed it more at sea”explained the forecaster.
“If at each very sturdy simulation, we put orange vigilance, we would have all of France in orange vigilance”, added the expert, before specifying that yellow vigilance also alerts to occasionally dangerous or very localized phenomena.
Researcher Philippe Arbogast claims to hear the criticisms leveled against Météo France, but adds: “You have to find a balance between trying not to miss an element and not over-alert”.
The orange alert was lifted shortly after 11 a.m. for Haute-Corse and Corse-du-sud, returned to yellow vigilance. At 4 p.m., vigilance had again changed to orange, announcing a “new rainy-stormy episode over Corsica from [jeudi] evening and until [vendredi] morning”. “Heavy rainfall is expected on the west of the island. Hail and strong electrical activity. Wind gusts less than those of Thursday morning”specifies Météo France.