Weather: what are “derechos”, these mega-storms which risk multiplying in France ?

Plus violents que les tempêtes, ces orages peuvent s’étendre sur “plus de 400km” et causer d’énormes dégâts. Illustration UNSPLASH – NOAA

A recent study published on April 3, 2024 reports that nearly 38 mega-storms "derechos" were identified between 2000 and 2023 in France. What are these phenomena??

This April 3, the journal Weather and Climate Dynamics published a first climatological study on superstorms "derechos" in France, reports Le Monde.

What is it and why "derechos"?

Also called "mesoscale convective system" according to one of the authors of the study interviewed by Sciences Avenir, these superstorms are "  widespread, long-lasting windstorms that are associated with a band of fast-moving showers or thunderstorms", as defined by the National Weather Service.

More violent than storms, these storms can extend over "more than 400km" and cause enormous damage.

"Derecho" means straight ahead in Spanish and these super storms move in a straight line, unlike tornadoes or cyclones which move in a circular manner.

How are they formed?

According to Lucas Fery, researcher at the Climate and Environmental Sciences Laboratory at the University of Paris – Saclay and first author of the study, four ingredients are needed: SLIM for Shear, Lift, Instability and Moisture, which in French gives: shear, thrust, instability and humidity.

Warm, humid air and atmospheric instability are essential ingredients in the formation of these super storms.

Should we be worried?

Rather widespread in the United States, particularly in Oklahoma, mega-storms can affect any area of ​​France. According to this study, they mainly affect the northeast and east of the country, mainly during the summer.

If these phenomena are relatively rare in France, they are not new, reports Lucas Ferry, author of the study. A "derecho" had struck Corsica in August 2022 causing the death of 5 people

Concerning an intensification of these phenomena due to climate change, the author of the study explains that it is impossible to predict it on the sole basis of this study. However, he adds to SciencesAvenir, that according to other studies, these stormy events could be more intense in the future.