Relating storm and weather factors to dry slab avalanche activity at Alta,Utah, and Mammoth Mountain, California, using classification and regression trees

Using classification and regression tree models, we evaluated 31 factors in terms of their importance to explaining avalanche activity indices at two ski areas: Alta, UT and Mammoth Mountain, CA. This study derived new empiri cal factors that combined wind velocity with new snow amount, air temperatu res with time, and total snow depth with time. The analyses created over-fi t tree models in exploring structures inherent in the data to obtain the re lative ranking and scores of various combinations of the 31 factors. Avalan che activity indices included maximum size, number of releases and sum of s izes of released avalanches, Results showed that time lagged conventional f actors describing snowfall and derived wind-drift parameters ranked highest in all tests. Snow drift factors segregated into prominent wind directions showed only moderate importance. Among the non-storm factors, the starting snow depth of a particular year ranked highest showing the importance of i nterannual variability. This was followed by the accumulated vapor pressure difference, which we formulated to better describe the conditioning of old snow with age. The average snow depth increase and other factors followed in importance. (C) 1999 Elsevier Science B.V. All rights reserved.