The kids are asking for it and the riders are dreading it: after a very cautious breakthrough this week, the snow will look even more real this weekend, on Île-de-France. Enough to satisfy football fans and take photos of our most beautiful monuments covered in white? Not sure.
If a particular collision occurs, it occurs between two air masses. The first, continental, was the one that gave Parisians the creeps for several days. An extraordinary “early winter episode”, according to Gilles Matricon, forecaster for Météo Consult and La Chaîne Météo. “The nights from Friday to Saturday were very cold, with temperatures dropping to around -5°C, in Pontoise (Val-d’Oise) for example,” explained the weather expert. Since 2013, we haven’t had a late November like this. »
However a disturbance comes from the Atlantic which promises a little warmth in the coming days. It is the contact with the cold air that will cause a curtain of snow to fall on the axis from Hauts-de-France to Limousin, passing through the Paris Basin, part of the Center Val-de-Loire and Champagne. “This episode will start in Yvelines, between 7pm and 8pm, then expand to the rest of Paris, between 9pm and 2 or 3am,” continued Gilles Matricon.
Maybe just for snowmen
However, this mild weather should come at a cost: the snow that so enchants us may not last long. Gilles Matricon dampens the hopes of little balaclava wearers dreaming of snowmen.
“It might last a little at higher altitudes, on the Ile-de-France plateau, with 2 to 3 cm on Pontoise or 2 to 4 cm on Vexin. But in Paris itself, the snowflakes will not stick to the ground. Above 150 m, snow may continue a little on Sunday morning, but from the afternoon with temperatures of 6 or 7°C, there will be nothing at all.”
Olivier Proust, weather forecaster at Météo France, offered a less clear opinion: “The temperature suggests that it will remain at ground level. Tomorrow, there may still be some beautiful areas. For those looking for photogenic spots, you should bet on the western tip of Yvelines, Vexin, the hills of Taverny, Montmorency, even the Longchamp racecourse and the Bois de Boulogne.”
He estimates 2 to 7 cm in the west of Paris and the hills of Normandy, 1 to 5 cm in the east of the region and up to 3 cm in the heart of the capital, otherwise it would only be traced “by systematic traffic and road maintenance”.
“A lot of things have to fall in a short time”
Complex weather and atmospheric physics make snow cover projections difficult. “It depends on the soil, air temperature, wind speed and temperature, and infrared radiation (the invisible component of the light that surrounds us) », says Christian Ghiaus, professor of thermal sciences at INSA Lyon. “It is this balance of phenomena that causes the flakes to heat up or cool down. »
The result: “In a residence, one building may be covered with snow, while another building is not,” notes Olivier Proust. The same applies to roads. What plays the most role is the temperature of the road surface and the intensity of the snow. In order to stick to a surface, much of it must fall in a short time, 1 or 2 cm per hour. When it starts to persist, it’s gone! »
During the last major snow episode in the region, nearly a year ago, the coat was far from uniform. “In Montmartre there was 10 cm and nothing in the Seine,” recalls Gilles Matricon. This sometimes happens at temperatures of half a degree and at a height of 50 m. »
Without worrying too much, forecasters also warned of the risk of icy conditions. “Ahead of Brie and Seine-et-Marne, temperatures will drop overnight and these light snowfalls could produce more severe frosts,” announced Gilles Matricon.
