Interpreting Avalanche Bulletins

We have had quite a bit of feedback about the Grande Motte avalanche that occurred last Thursday and claimed two British snowboarders. There seems to be some confusion about how to interpret the headline avalanche risk. The Savoie area avalanche bulletin, available in English during the ski season from Henry’s Avalanche Talk and is covered on radio Val d’Isère spoke of the following dangers last Thursday.

Increase in natural avalanche activity and presence of wind-slabs

There has been 5 to 30cm of fresh snow over the last 48 hours accompanied by a few strong winds gusts from the north. New slabs have formed in high mountain areas. Thursday will see a notable rise in temperatures which will humidify the fresh snow and will cause a number of surface sluffs and even some natural avalanches of a more important volume. Regarding slabs, the risk of triggering is high especially above 2300 m. A small extra load such as a single skier is often enough. The slabs are usually small but may cause a large slide, particularly in sectors close to Italy, in the upper Maurienne and in the south of the Upper Tarentaise. The snow over last weekend was considerable and the snow mantle that can slide approaches 1 meter in depth.

Zero Isotherm from 1500 rising to 2300 m.
-10 iso: risiing to 3600.
Wind, mainly from north 10-50km/h.

(note: translated from the French version available from Météo France)

So we have around 30 cm of fresh snow in the sector which is always a warning sign but of course it is this same fresh powder that lead Sam and James and many other skiers and boarders off-piste that day. The late hour and rise in temperatures are less significant for slab type avalanches, remember it was still well below freezing at the altitude where the avalanche occurred but may have been a factor in weakening internal bonds in the snowpack. Time may be a factor for two other reasons:-

1. It probably meant that available powder on what people considered safer slopes that day had already been skied. There is a negative feedback effect (aka powder fever) where having skied some slopes without incident people tend to lose a perspective on danger and push out onto more suspect slopes
2. It makes it harder for rescue services to get organized as it is closer to nightfall (although they still had 3 hours after the slide to operate)

North winds should load south facing slopes, but remember the winds were from the south over the last weekend and gusty winds tend to blow snow in a number of directions. Looking at the slope that avalanched there was also plenty of opportunity for cross loading. What I think Jean-Louis Tuaillon, director of piste security for Tignes summarized as bad luck, is that the slide was unusually large and unexpected on that slope and that the two victims were properly equipped.

Thierry Arnou, who compiles the avalanche bulletin for the area recently gave the following explanation in relation to the headline risk.

“I do not want to give the thousands of people who access the avalanche bulletin the idea that we are capable of predicting exactly where avalanches occur. I prefer to give an idea of which ranges and slope aspects are at risk or have seen slides and then let people extrapolate to see if maybe the same conditions exist on their route. On risk 1 it is only some rare slopes, steep and very localised, for example north facing, that are unstable. It is true you can find a steep couloir, north facing, with rock bands, where you can see the risk is considerable (3) but think of all the slopes on the same day, from the west to the east between 2000 and 3500 meters and angled between 20 to 40 degrees which are more stable - we can’t lump them in as 3 when the risk is very localized. So you have to remember, it is topography and location that make a slope likely to avalanche. On the other hand, a risk of 4, even on medium slopes and not even in known dangerous zones such as convex slopes, can see avalanches in a number of orientations. In the Savoie area bulletin I try to point out the dangers and never what is considered safe - with avalanche forecasting you have to be prudent. For example if the headline risk is 2 (some unstable slopes) there is always the detail: some recent slabs in particular above 2500 m close to the frontier with Italy and on north sector slopes.

I believe the avalanche bulletin should be read just like the weather forecast. It should aid with decision making but no avalanche bulletin will tell you that tomorrow you can do a north face on such and such a peak but what it will tell you is in which mountains the risk is highest and on which slopes and sometimes even giving an altitude range and reasoning, that is already not bad. Of course, for off-piste where many slopes and altitudes can be easily reached by lifts, the temptations and dangers are greater, dangers that can be largely avoided with ski touring and a single descent.”

In short, the devil is in the detail not in the headline risk. You should read the bulletin for the day and ideally have followed the evolution over the previous week as well as being aware of recent and seasonal weather. If you want to boil it down to a simple GO/NO-GO decision use something like the Munter Risk Reduction method but even Munter recognizes that we can’t get it right 100% of the time.

Posted by davidof on Friday, 29 April, 2005 at 11:14 AM

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