Analysis and forecast of extreme new-snow avalanches: A numerical study of the avalanche cycles of February 1999 in France

Abstract

Snow and weather conditions typical of exceptional cycles of fresh-snow avalanches in thenorthern Alps are investigated using the numerical avalanche-hazard forecasting procedure of Météo-France. Sensitivity tests are performed on the events of February 1999 in the Chamonix France region,and resulting snowpack instability modeled at the massif scale is compared using adapted new indicesand maps. Our results complete conclusions of earlier observation-based studies by providing newinsights into the snow and weather conditions of February 1999. The large avalanches mainly resulted from large and very unstable fresh-snow accumulations. Moreover, the snowpack instability was increased locally by wind transport of light and fresh snow in February. The mechanical weaknesses resulting from the weather conditions prior to February were a key factor in explaining the unusual volumes of these avalanches. This study suggests that the operational numerical SAFRAN/Crocus/ MÉPRA (SCM) chain provides reliable forecasts of extreme new-snow avalanche situations at the massifscale, but that local-scale simulations are still needed to improve the efficiency of risk mitigation andcivil protection policies.