It’s time for cleaning and recession on Friday October 18, after the damage caused the day before in many municipalities in the Center-East by intense and unexpected rains. In all, three minor injuries and more than a thousand evacuees have been recorded since Thursday.
The red “flood” or “rain-flood” vigilance has been lifted in the six affected departments (Rhône, Loire, Haute-Loire, Ardèche, Lozère and Alpes-Maritimes), but 10 departments in the southern half remain affected by vigilance orange, indicated Météo-France this Friday morning.
An exceptional rainy episode “due to its intensity and duration”, specifies the meteorological service: in Ardèche, the volume of cumulative rain reached up to 700 mm in the Cévennes, i.e. double what had been announced by forecasting models. At issue: the functioning of computer weather forecasts, unsuitable for such extreme rain episodes.
Heavy rains yet well anticipated
To understand how the weather models could have been so wrong, we need to look behind the scenes. Currently, weather forecasts are based on simulations “produced using supercomputers […] capable of performing up to several billion billion operations per second” based on enormous quantities of data linked to past and present weather events, explains engineer Alexis Vandevoorde for The Weather Channel. The result of these complex mathematical calculations then passes “through the expertise of meteorologists” who transform this data into understandable forecasts, continues the specialist in weather data production.
This is what happened for the rain episode of the last two days: the context “was conducive to the triggering of a “classic” episode of heavy rain for the season, and the “anticipated accumulations were expected to reach 300 mm on the Cévennes in 48 hours”, explains meteorologist Régis Crepet this Friday for the same specialized media. However, the level of rain increased much faster than expected, forcing meteorologists to constantly extend their forecasts and “giving the impression of chasing the event which, somewhere, escaped the calculators”.
For hydrology professor Giuliano di Baldassare, interviewed by L’Express, the precision promised by these forecast models can thus prove misleading for predicting “extreme weather events such as floods or storms over small areas”. For this expert, co-author of a study on the subject in 2013, “instead of being approximately right, we are now precisely wrong”.
One of the most intense episodes in 20 years
This is the main problem with this rainy episode in the Center-East: its “extreme” and unpredictable character, classifying this meteorological event “among the most intense of the last 20 years” in the southern regions, says meteorologist Regis Crepet . An area that is nevertheless regularly affected by these phenomena.
To explain it, the specialist cites in particular a factor having aggravated the large-scale episode already expected: “The arrival of a cold front through Aquitaine this Thursday […] towards the Cévennes”. This fresh wind would have caused “instability with violent storms which cause intense [pluvieuses] very important hours. In this context, the rain accumulations increase rapidly for hours,” remarks Régis Crepet.
Finally, the expert highlights one last meteorological phenomenon which could have amplified the rains of the last two days: “We can think that the hot and humid air rising from the Mediterranean [vers les terres] contains more water vapor” than what was modeled. The greater this is, the more intense the rains will be. However, as explained a note from Météo-Francea high sea temperature “promotes strong evaporation” of water vapor. For the meteorologist, “we can see a concrete effect of global warming.”