Gusts of more than 150 km / h surprised Corsica in the early morning and at least five people died, including two killed by falling trees on their bungalows.

In the Météo-France bulletin of 6:00 a.m. Thursday, the island was still on yellow vigilance («be attentive»), announcing «powerful storms (…) at sea near Corsica, with strong gusts of wind» , which may «very temporarily affect the west and north coasts».

It was not until 8:35 a.m., observing the unexpected shift of the storm towards the land and the «extreme» intensity of the gusts, that the two departments switched to orange vigilance to thunderstorms («be very vigilant»). At this level, a notch lower than the red of «absolute vigilance», the inhabitants are invited to take shelter in permanent buildings, while the civil security services and the health authorities are put on alert.

«We were a little surprised by the values ​​of the gusts, quite exceptional values», with records at more than 200 km / h of wind in places, recognized Christophe Morel, head of the forecast office at Météo-France, during a press briefing on Thursday afternoon.

Some simulations produced by «AROME», the in-house model running on a supercomputer in Toulouse, «suggested a storm close to that which was observed», said forecaster François Gourand. But other simulations, «which seemed more likely, placed it more at sea».

«Multicellular stormy systems occur when a certain mayonnaise takes hold: managing to predict the combination of these elements is very complicated,» he explained.

“Very sophisticated models like AROME are able to predict them better and better, but not always”, he added, affirming that meteorologists are “very often in these situations where the models do not make it possible to decide”.

If we had to alert as soon as an extreme scenario appears in the forecasts, «we would be far too vigilant and the system would become useless», justified Christophe Morel. «There is a balance to be found, to alert sufficiently and not to over-alert», he underlined, believing that «here, we did not have enough elements» in advance.

– False alarm rate –

«We must hear the criticisms», admitted Philippe Arbogast, researcher at Météo-France, while reaffirming that «we must not alert as soon as an extreme situation appears in the simulations».

In 2021, 14% of alerts turned out to be a false alarm while the non-detection rate stood at 1.7%. In other words, «seven departments have experienced a phenomenon for which an orange Vigilance would have been appropriate when it was not triggered», according to the report of Météo-France, which set up this system in 2001.

«In all objectivity, we can speak of a missed out on an extreme situation», recognizes Pascal Scaviner, head of the forecast service at the Weather Channel, a private competitor of Météo-France, which has not issued a heightened alert either. .

As often for thunderstorms, «there were very significant disparities» between the numerical models and «there was no certainty», he explains to AFP.

«A few factors» — the very hot Mediterranean, temperatures at altitude 8°C lower than normal, warm surface air,… — «lead to think that we could have severe phenomena, but these were minority scenarios, of the order of less than 20% probability», explains the forecaster.

«That said, there may be a concern for communicating the different scenarios,» he wonders. «In the United States, it has become customary to speak in terms of probabilities», he recalls: «if you are told that there is a 20% probability that a phenomenon with severe consequences will produce, it’s still that it’s not zero».

«In certain sectors of activity, for example golf, even a 5% scenario is taken into account,» he says. And «if I announce a risk of hail, even low, you may drive in your car».

«Perhaps three levels of alerts do not appeal to people enough», wonders the forecaster again, who pleads for communication «more in advance on the different scenarios». «We haven’t changed for twenty years,» he regrets.