Lhe missed Corsica forecasters, who had not introduced the depth of the stormy gusts inflicting a number of deaths on the island on Thursday, illustrates the problem, regardless of the spectacular progress of supercomputers, of translating the chances of weather phenomena into a reputable warning system for the inhabitants.
Gusts of greater than 150 km / h shocked Corsica in the early morning and no less than 5 folks died, together with two killed by falling bushes on their bungalows.
In the Météo-France bulletin of 06:00 Thursday, the island was nonetheless on yellow vigilance (“be attentive”), asserting “powerful storms (…) at sea near Corsica, with strong gusts of wind” , which can “very temporarily affect the west and north coasts”.
It was not till 8:35 a.m., observing the surprising shift of the storm in direction of the land and the “extreme” depth of the gusts, that the two departments switched to orange vigilance to thunderstorms (“be very vigilant”). At this degree, a notch decrease than the crimson of “absolute vigilance”, the inhabitants are invited to take shelter in everlasting buildings, whereas the civil safety providers and the well being authorities are placed on alert.
“We were a little surprised by the values of the gusts, quite exceptional values”, with data at greater than 200 km / h of wind in locations, acknowledged Christophe Morel, head of forecasts at Météo-France, throughout a press briefing on Thursday afternoon.
Some simulations produced by “AROME”, the in-house mannequin operating on a supercomputer in Toulouse, “suggested a storm close to that which was observed”, stated forecaster François Gourand. But different 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 defined.
“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 needed to alert as quickly as an excessive state of affairs seems 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 price
“We must hear the criticisms”, admitted Philippe Arbogast, researcher at Météo-France, whereas 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 whereas the non-detection price was 1.7%. In different phrases, “seven departments have experienced a phenomenon for which an orange Vigilance would have been appropriate when it was not triggered”, based on the report of Météo-France, which arrange this method in 2001.
“In all objectivity, we can speak of a missed out on an extreme situation”, acknowledges Pascal Scaviner, head of the forecast service at the Weather Channel, a non-public competitor of Météo-France, which has not issued a heightened alert both. .
As typically for thunderstorms, “there were very significant disparities” between the numerical fashions and “there was no certainty”, he explains to AFP.
“A few factors” — the extremely popular Mediterranean, temperatures at altitude 8°C decrease than regular, heat floor 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 recollects: “if you are told that there is a 20% probability that a phenomenon with severe consequences will produce, it is still that it is 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 once more, who pleads for communication “more in advance on the different scenarios”. “We haven’t changed for twenty years,” he regrets.
08/18/2022 20:31:31 – Paris (AFP) – © 2022 AFP