A field method for avalanche danger-level verification

Abstract

Even though the danger-level verification indicated in a bulletin should be a priority aim of avalanche-forecast servies, there are no easily applicable verification methods available today. The main difficulty lies in the fact that avalanche observation is no longer sufficient. Therefore, it is necessary to verify the actual condition of the snow-pack stability, particularly concerning low danger levels. This work introduces a procedure for a 'posteriori' field verification of danger levels both in space and time (24-72 hours). The method is based on the following elements: avalanche-activity survey, observation of cross-country skiers' activity, snow profiles and 'Rutschblock' tests. These elements, relating both to time and the examination zone, are combined to provide an objective danger degree according to the European avalanche-danger scale. The method was used experimentally in the winter of 1993-94 in the Dolomites and subsequently, in the winter of 1996-96 in the Catalan Pyrenees. As far as 24 hour forcasts are concerned, the method has shown a forecast reliability of 93% in the Dolomites and 76% in the Catalan Pyrenees, while 48 hour forecasts have given value sof 89% and 64%, respectively. The lower degree of forecast reliability in the Catalan Pyrenees is accounted for by the unusual weather conditions of winter 1995-96, which was very snowy and characterized by few foreseeable avalanche conditions. The practical applicatioon of the proposed verification emthod has given encouraging results, thus allowing experts to find the main errors in order to improve future forecasts. However, simpler survey producers are necessary in order to operate on a regional scale. The method is suitable for further development relating to verification of both degree of danger and danger localization.