Snowy episode: the balance sheet


By Cyril BONNEFOY, meteorologist

A notable snowfall for the season concerned the center-east of France this Thursday, November 14, 2019. Meanwhile, the southeast was undergoing a stormy wave virulent on the French Riviera.

The first snowy episode of the season occurred in east-central France this Thursday, November 14 and the night of 14 to 15. In the morning of Friday 15, the sun was back, and, thanks to slightly positive temperatures, the situation was improving rapidly on the roads.

A rainy-snowy disturbance

This first snow episode of the season in France, concerning all the reliefs of the south-east at low altitude, is due to the passage of a depression in the Mediterranean, circulating from the Gulf of Lion to Corsica and bringing in its wake a blow of wind and thunderstorms on the coast. At the same time, rather cold air covered France, giving rise to a snowy episode when the rainy front ascended to the north, ie to the Rhone corridor.

Several parameters, well anticipated, contributed to the abundance of these snowfalls: the rise of the Mediterranean rains, a phenomenon ofisothermal and one back from east.

Storms and snow

This episode manifested itself stormy wave sweeping the Côte d'Azur from the Bouches du Rhône to the Alpes-Maritimes. The first storms arrived at mid-day on the Bouches-du-Rhone where the storms were violent on the Aubagne region, then shifted towards the Var where the accumulations could reach 50 to 90 mm in 3 hours . Locally accumulations have even reached 87.6 mm at Frejus (83) and 99.6 mm at Aubagne (13). Floods were often observed in these regions. In the Alpes-Maritimes, 67 mm fell in Cannes and 54 mm in Nice. These storms were accompanied by gusts of wind reaching 110 km / h on exposed caps and 80 km / h in Nice.

The snowfall associated with this disturbance was notable for the season, especially on a triangle from Valence (26) to the Loire (42) and Rhône (69), with large disparities. Isothermy has allowed the snow to fall early in the afternoon of Thursday, November 14 on the Drome, especially in the area of ​​Valencia. It was a very heavy snow, waterlogged, wet and sticky, causing falls of tree branches and overhead lines under its weight. At the end of the afternoon and evening, the snow stretched to Lyon, Saint-Etienne and to the Tour du Pin. The alpine valleys (Grenoble, Chambery …) were almost not concerned, the isothermal rather playing in the average valley of the Rhone.

The snow fell, weakening, until around 3am. The thicknesses were sometimes substantial: from 10 to 15 cm in Valencia, 14 cm at the airport of Lyon St Exupery (record for a first fortnight of November), 12 cm at Puy en Velay, 9 cm at Saint Etienne but only 2 cm in Lyon.

The return of is

The meteorological parameters that amplified these snowfalls are therefore linked to the presence of a rather cold layer of air at altitude over France. The passage of the depression on the Côte d'Azur led to a rotating movement of the winds around, which moved this cold air towards the sector of the middle valley of the Rhone. Finally, under the effect of the intensity of the precipitation, the cold air present at altitude has lowered to the level of the plain, allowing the snow to replace the rain. Thus, in Valencia, it was + 4 ° C in the morning, then + 1 ° C at noon and finally 0 ° C in the afternoon. These temperatures did not drop below 0 ° C, explaining why the snow was soaked in water because it was not cold enough for it to turn into powder.

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