Last week’s snow reminded us that winter is just around the corner although looking out over the verdant Belledonne mountain range it still seems a long way off. After an unsettled August, France is basking in an Indian summer, a fairly common occurrence but is this simply the lull before the storm?
Snow in the Northern Alps, the Vosges, Jura and Massif Central is supplied principally by weather fronts arriving from the west to north-west. These cold air currents travel down from the Artic picking up moisture over the Northern Atlantic Ocean. Most of the snow falls in the Pre-Alps, the fronts are pretty much spent by the time they reach the tops of the Savoie valleys on the border. Here, only the Lombard, blowing in from Italy in the east can bring large dumps of snow. Sometimes more than a meter in 24 hours. This can bring traffic chaos and avalanche danger.
In the Southern Alps most of the snow comes from weather systems blowing in off the Mediterranean. The Lombard can also bring spectacular snowfall to the border with Italy. Ski resorts in the Queyras and Isola often have snow depths that belie their southerly latitude.
The western and central Pyrénées also benefit from north-westerly polar weather systems bringing abundant snowfall. The situation can quickly change, a warm southerly fœhn can strip snow of the slopes. In the eastern Pyrénées the largest snowfalls occur when polar air is blocked in the Gulf of Lion by a warm, humid weather system in the Mediterranean. The resultant north-easterlies can bring snow down as far as the coast. These systems also bring snow to Corsica. The island of beauty is in reality a 2,500 meter rock slap-bang in the middle of the Mediterranean Sea. As such it catches some extreme weather. In 1934 avalanches from slopes of Castagniccia at only 700 meters altitude swept through the village of Ortiporio killing 37 and more recently in June 2002 four walkers were surprised by a snowstorm on the Cuscionu plateau, one of them died.
All these weather currents can lead to phenomenal snowfall in one part of the mountains and drought elsewhere. In November last year the Southern Resorts of Montgènevre and Les Deux Alpes had excellent autumn snow as weather systems that usually water the Cote d’Azur pushed high inland before petering out on the high mountains of the Maurienne.It should also be remembered that unlike North America a correct level of snowfall may depend on only two or three big dumps in a season.
Reference: Everything you wanted to know (or nearly) on snow and avalanches, published by the ANENA.