Gusts of up to 170 km/h on the Brittany and Normandy coasts, waves of up to 10 meters on the Atlantic coast and heavy rain: Storm Ciaran looks set to be a "weather bomb" when it hits the north-western quarter of France on Wednesday and Thursday. The term, used by forecasters in recent days, raises the specter of the "storms of the century" of 1999, the country's deadliest. On December 26 and 27, storms Lothar and Martin ravaged France, devastating forests and killing 92 people.

With the arrival of Ciaran, Météo-France is anticipating a "very powerful autumn storm." As of Wednesday night, the départements of Finistère, Côtes-d'Armor and Manche have been placed on red alert, and 15 départements in the north-east, from Vendée to Nord, on orange alert for violent winds.

Lauriane Batté, a climatologist at the French meteorological institute, called it a "major storm," but "not on the same scale" as those of 1999 in terms of intensity, severity and, above all, the spread of areas affected. According to the organization's forecasts, Storm Ciaran should be confined to the north-western quarter of France, whereas Lothar and Martin had crossed the whole country, with gusts above 100 km/h over almost the entire territory. After that, the winds are expected to be less violent and to diminish as the weather depression moves inland.

Peak gusts of between 150 km/h and 170 km/h are feared along the Finistère, Côtes-d'Armor and Manche coasts on Wednesday night, whereas the most violent winds came dangerously close to 180 km/h in 1999. Inland, gusts could reach between 130 km/h and 140 km/h over Brittany and the Cotentin region. They will reach 100 km/h to 110 km/h in Pays de la Loire and from Haute-Normandie to the west of Hauts-de-France. Winds could reach 90 km/h or even 100 km/h in the Paris region, compared with over 170 km/h during Lothar.

"It's not the same configuration as in 1999," commented meteorologist and climatologist Robert Vautard, director of the Institut Pierre-Simon-Laplace. "Lothar crossed all of France, deepening with small, highly concentrated eddies that had developed from land. With Ciaran, we're looking at a large low-pressure system stretching from Scandinavia to Spain, with small eddies forming behind it, which should mainly affect the coastline."

But Vautard warned that "extreme conditions are to be expected." These are linked in particular to atmospheric pressure, which is expected to be particularly low as the storm passes over France, with values of between 950 hectopascals (hPa) and 960 hPa. The record low pressure (951.8 hPa) was recorded at the tip of La Hague (Manche) during the storm in February 1989. The lower the atmospheric pressure, the more violent low-pressure phenomena can be. Above all, it's the speed with which the low-pressure system will "deepen" – in other words, drop in pressure – that worries forecasters, making them fear a "weather bomb."

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