Estimating the avalanche risk

The daily avalanche bulletin is a key piece of information for route planning however in France the full bulletin does not appear until mid December when ski resorts start to open. The data used to compile the bulletin comes in large part from the ski resort’s weather services. The Massif Central, Vosges and Jura are not covered by an avalanche bulletin at all and this is also the case in a number countries.

With four avalanche deaths already this autumn in the Savoie what do you do when winter comes early or where the avalanche bulletin does not take into account the actual conditions on the ground such as more snow or wind than forecast?

Fresh Snow

When it comes to total snowfall it is not just the quantity but the quality that counts. Even where the avalanche danger is forecast Moderate (2/5) you should consider the risk as at least Considerable (3/5) when the amount of fresh snow exceeds certain levels. Remember that in Switzerland nearly a third of all avalanche fatalities occur at level 2.

The amount of fresh snow needs to be considered in relation to the wind, temperatures and the surface of the old snow. You can apply the following heuristics:

I.  30-60cm of fresh snow deposited with no or low winds and mild temperatures (around 0C), rain turning to snow (often the case as a weather system moves across a region and the temperatures cool). The rain will help destroy any surface hoar crystals which are a very dangerous weak layer.
II.  20-30cm of fresh in mixed conditions, for example strong winds but mild temperatures
III. 10-20cm of fresh snow in unfavourable conditions: wind > 50km/h. Low temperatures (< -8C). Surface hoar, hard snow, ice, graupel, a cold layer on a warm layer. -8C is generally reached 1300m above the zero isotherm.

In these cases the risk should be considered between Considerable (3) and High (4). Use a factor of 12 if you are using the Munter reduction method.

To get an idea of fresh snow, wind speeds and temperatures when planning a trip use the Meteo France automatic weather stations

and the Flowcapt stations (which give more detailed information).

More details on Flowcapt in this thread: and on temperature gradients here:

Warning Signs

I. Wumphs and cracks when crossing a slope, these signs are typical of a Considerable risk level

II. Spontaneous slab avalanches

III. Remote triggering: Typical of a High avalanche risk

Typical Situations

I. A start of winter with little snow – as we had in the Alps and North American in the autumn of 2009. Expect persistent avalanche risk especially on shaded slopes.

II. Fresh snow after a long period of cold, dry weather. For example over the last couple of days we’ve had some excellent cold and clear weather conditions in certain areas. Even in the autumn weak layers: facets, surface hoar, can develop on shaded slopes where the ground is warm and the air temperature remains cold. Even where the risk is low it can rise very quickly to 3 or 4 with fresh snow.

III. A rapid warming, first time the zero isotherm goes above 3000m in the winter typically sees a very dangerous situation on shaded slopes.

Remember that a Considerable risk level (level 3) presents a significant risk for back-country travellers and requires a great deal of knowledge and skill to navigate safely.

For a more detail analysis take a look at the Nivotest method explained in detail in the book Avalanche by Robert Bolognesi.

Posted by davidof on Friday, 12 November, 2010 at 02:50 PM

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