Wednesday evening, Sambreville was the scene of a stormy episode of a rare intensity. In some streets, torrents of water mixed with hail have swept everything away in their path, while a few hundred meters, it was flat calm. Can this striking contrast question: how can such a concentration of rain fall on such a reduced area?
According to Xavier Fettweis, climatologist, the answer is in two words: stationary storms. “We are dealing with thunderstorms that are born and die there by rushing completely. So we will have an area that is possibly flooded and, alongside, nothing at all. »»
Frozen clouds
Unlike more classic thunderstorms, generally pushed by winds and distributing precipitation in large areas, these thunderstorms remain frozen. The absence of air flow prevents them from moving. Result: a brutal and localized accumulation of water.
“We can very well see that the movement speed is very very slow and these thunderstorms are really on site,” says Xavier Fettweis. This type of phenomenon becomes more frequent and more intense with climate change, which changes atmospheric dynamics.
Why was no alert issued by MRI?
While several residents were surprised by the violence of the storm, some question the absence of a weather alert. Amaury Detroz, weather presenter, recalls that the criteria of the Royal Meteorological Institute (MRI) are very strict: “For MRI to issues an alert, the province concerned must be affected at least 25 % of its area by rains, by thunderstorms. If these 25 % are not affected, then there is no alert that is issued. »»
In the case of Sambreville, too few territory was concerned to trigger an official alert, despite the very localized violence of the event.
Bad news, the depression frozen above our heads has not said its last word. New thunderstorms are expected in the coming days.
