In April, don’t uncover yourself by a thread, says the proverb. But that was true at a time that neither those under twenty nor global warming know about. Because from Saturday, shorts and tank tops will be out in part of France, with peaks of 30°C locally. A sudden heat peak, while the atmosphere for several days has been mainly one of showers, even floods.
But what explains this phenomenon? Could there be a link between intense precipitation and heat? Will we one day be able to return to the famous “seasonal normals”? 20 minutes takes stock with Tristan Amm, forecaster at Météo-France.
How can we explain the sudden “hot spell” predicted for Saturday?
With the temperature hovering around 15°C when this article was written, we could easily say that talking about “heat” is an abuse. However, “we can qualify what awaits us as a heat episode, since the threshold defined at 25 degrees will be largely exceeded in many regions,” clarifies Tristan Amm from the outset. Indeed, Météo-France forecasts maximums between 26 and 28°C in the Paris Basin and Champagne, around 30°C in the Center or in the Landes and up to 32°C locally in the South-West.
“It’s due to a depression that wanders in the Atlantic” without reaching our side, explains the forecaster. “Around a depression, winds rotate counterclockwise. So if you look on a map, you will have very warm air from North Africa which will rise through the south of France,” he pictures. If the weather should quickly deteriorate with a return of rain, this heat peak will be “remarkable for its intensity and its precocity”, notes the forecaster, who mentions possible “monthly record minimums in the Basque Country”, which are coming normally much later in the month.
Can we make a link between this heat and the very rainy month of March?
France has just come out of a month of March that was “remarkable in terms of rainfall”, with a level of precipitation not seen since 2006. But does rain call for heat? It’s rather the opposite effect, for Tristan Amm, who recalls that the month was also very mild. “There is a link between the temperature of the air mass and the water it can store,” he explains. In other words, by evaporation effect, “the warmer an air mass is, the more water it can capture and therefore have a significant precipitating potential”.
This link has already been highlighted by IPCC reports, which warn in particular of the violence of the Cévennes episodes, also linked to high temperatures. “Each degree gained means +7% additional water in the air, or +2 to 4% precipitation,” notes agrometeorologist Serge Zaka on X.
The month of March was the 26th consecutive month above seasonal norms. Can we still “return to normal”?
If we have to be precise, April 2023 was only very slightly above average. But to find traces of a month spent below seasonal norms, you have to go back to January 2022. The National Assembly only had 8 RN deputies, Neymar was still playing for PSG and you had probably just received a third dose of anti-vaccine. Covid. Another world.
“Even among forecasters, we often say to ourselves that we are always either above or just at the normal level,” confides Tristan Amm. An observation which can be explained by the method of calculating these normals. “They are established over 30 years, it is important to have solid references” which would not make sense with an average over 10 years, he explains. But these averages also make it possible to see that “the climate is changing rapidly”. “It is not impossible to find a month colder than normal, to have cold episodes, but it is becoming less and less likely, and it will be one month among many others which are on the opposite trend “, he says.