The last few days of 2013 have seen quite a few avalanche accidents, in part due to the number of holiday makers in the mountains, in part due to the current conditions. There is a persistent weak layer of depth-hoar in the snowpack and we have not seen quite such a dangerous situation for a number of years.
A reminder of the situation in the Northern Alps. The snowfall in November was fairly poor and was followed by a number of weeks of fine weather that formed a layer of depth hoar / facets on most slopes except for south where the snow has thawed then transformed into a spring snowpack. Since then it has snowed on the fragile layer. The snowpack has a fairl unstable structure and slab avalanches are probable as has been shown by recent incidents.
In the Haute-Savoie, a number of episodes of rain to 1800 meters has stabilized the snowpack to some extent at lower altitudes. In the High Mountains (>3000m) the snow has been hardened by the wind and is safer than mid altitudes. The current danger is not classic windslab that you find in the lee of ridgels or behind convex roll-overs but quite simply slabs that can be triggered by one or a number of skiers, walkers, snowshoers. These slabs are possible pretty much everywhere.
In the last few days there have been a number of avalanches but steep slopes have also been skied without triggering slides. Do not draw the conclusion that the snowpack is not that dangerous because the new snow is not that deep or cohesive enough to provoke an avalanche. And what may be true today, slopes that are not yet ready to go, may become so after the next snowfall.
The weak layer of depth hoar will persist. In light of the storm cycles to come one can expect a considerable danger of avalanches in the near future. Large, deep slabs are possible even far from ridgelines. Remote triggering is possible. Slabs on slopes that have already been skied are possible.
Statistically thin snowpacks such as at present are the most dangerous. The situation may change later in the winter, for example if it snows heavily deeply burying the weak layer or more unlikely, heavy rain at altitude
A Summary of recent avalanche activity
20th December: Crête de la Pendine, Put St Vincent. 100 meter wide skier triggered avalanche at 2200 meters altitude.
21st December: Skier triggered slabs on the Petit Van, Chamrousse. Fatal avalanche in the Combe de Charbonniere, Alpe d'Huez. Les Contamines Montjoie, slab triggered by two snowboarders at 2100 meters altitude on a North sector slope. Slab measured 50-70 meters wide and was 5-40cm deep.
22nd December: Aravis, skier triggered avalanche at the exit of the Tchadar couloir on the Tardevant. Skier taken 60 meters. Fatal avalanche in the vallon de Chavière above Pralognan.
23rd December: Ski tourer triggered avalanche at the col de Peygu above Cervières in the Hautes-Alpes. One skier suffered a thigh injury requiring rescue and hospitalization.
24th December: Remote triggered avalanche at the Col du Sabot above Vaujany.
On the 26th December. A lot of activity in the Chablais and Aravis right after the snowfall. Slabs triggered by pisteurs, even without explosives. St St Gervais on the Pavillion du Mont Joly three Swiss nationals triggered a slide on a north slope at 2000m. At La Clusaz a number of of slides 60cm deep and sometimes taking the entire snowpack. A large 200m wide avalanche at 1800m.
On the 27th: At Megève : two slabs triggered at 1950 meters on a north sector slope of 60-80cm depth. Two skiers killed in La Clusaz by a slab at 2400 meters altitude on a North-West slope. The slab was 100 meters wide and 80-100cm deep.
On the 29th December at Les Les Contamines : A slab at 2150 meters on a north facing slope. 40 meters wide and 30-50cm deep. At Chatel. A 30 meter wide slab at 1900 meters on a west facing slope. Another at 50 meter wide slab on a North-East facing slope again at 1900 meters. This was 30-40cm deep. Above Petit-Châtel in the Barbossine sector. Triggered by two off piste skiers both escaped without injury. Avalanche risk 4/5.
On the 30th December. At Megève a 20-30 meter wide slab on a West facing slope. 50-60cm deep. The slope has already been avalanched controlled with explosive and skied afterwards. At Flaine, a 100cm deep slab at 2100 meters on a North sector slope. At Dérochoir a slab at 2200 m on a North sector slope (crête du Dérochoir). It was 70-80cm deep. Two skiers were caught, one injured. Some big powder slides on SE Faces at La Giettaz in the Aravis.
At la Plagne a skier was taken 300meters by an avlanche at the Dos des Frasses. He triggered his ABS and stayed on the surface of the slide suffering just a dislocated shoulder. He was rescued by the PGHM.
Meteo France, Chamonix