The French secretary of state for sports, Thierry Braillard, is in the Hautes-Alpes today to participate in a round table on avalanche safety. One of the topics is whether the risk level 3 is specific enough to described the dangers faced by skiers. The Hautes-Alpes have been at the epicentre of avalanche incidents in France this winter with 13 fatalities.
However it is interesting to look at another recent incident involving a group of Italian ski tourers who were caught by a slide in the Combe de Morts on the Swiss side of the Grand St Bernard Pass while climbing. The press and even the Swiss Avalanche Research Institute (SLF) initially reported the risk as 2 out of 5 (The SLF revised their report later).
This is the avalanche bulletin issued at 08h00
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SLF
Avalanche bulletin for 21 February issued at 8am 21.2.2015
Risk Level: 2 MODERATE
Fresh, wind blown snow
Danger areas
All slope aspects above 1800 meters
Dangers
Accumulations of wind blown snow will form with the fresh snow and wind. They will lie on a surface of unstable snow especially on shaded slopes. An avalanche can be triggered by a single backcountry traveller. In the afternoon the risk level of 3 - Considerable will probably be reached. Tours and off piste trips will need to take a prudent choice with their routes.
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The accident occurred at 2300 m, at 13h30. It killed four of the ski tourers. The second major incident in Switzerland this winter. The Combe de Morts is generally north facing. That is, a shaded slope where you could expect to encounter persistent weak layers buried in the snowpack. Timing was crucial. A short ski tour in the morning might be relatively safe but the conditions were forecast to be different in the afternoon. A slab big enough to bury a skier can form in a hour in the right conditions. Arrive late at the start or experience delays during the tour due to equipment, route finding or the abilities of the group can change the risk exposure significantly. Werner Munther suggests that going up one point on the avalanche risk scale doubles your exposure. If we use his reduction method we go from a socially acceptable risk of 1 to 2 in the afternoon.
Even if you didn't take the 8am bulletin and relied on an earlier forecast of risk level of 2 you need to look at the conditions as you find them not as they were forecast, is there snow transport, did it snow more than forecast, is the zero isotherm as forecast? All these might affect your route choice when you are actually on the ground. Backcountry travellers should certainly be looking beyond the headline risk number.
Of course we don't know what decision making process the Italians took or if they even consulted the bulletin.