Use of Seasonal Meteorological Metrics to Forecast for Deep Slab Avalanches in Southwest Montana

Deep slab avalanches and avalanches that fail on old persistent weak layers are often more destructive than new snow avalanches, yet are relatively less understood and harder to predict. Newly formed instabilities normally gain strength over time, and the rate of strengthening lessens over time eventually leading to a generally constant strength, independent of meteorological conditions. Determining when a weak layer generally stops gaining strength after it is buried could be a clue to how much stress, resulting from meteorological inputs, is needed later in the season to cause an avalanche on this layer. Previous studies have found correlation between deep slabs and meteorological trends over several days preceding deep slabs, and suggest that deep slabs response to load lags up to 5 days. This study will examine seasonal meteorological metrics on years with deep slab avalanches and years without deep slab avalanches. Seasonal metrics may better reflect the development of weak layers over the course of a season and allow for more precise prediction of ranges of meteorological inputs that precede deep slab avalanches by up to seven days. Avalanche records and meteorological data from Big Sky ski resort and Bridger Bowl ski area will be used. Deep slab avalanches will be identified through specified criteria, and old persistent weak layers will be identified using weather patterns and records of snowpack observations. Seasonal meteorological metrics associated with weak layer development will be derived to test differences in metrics between years with deep slab avalanche activity and years without deep slab avalanche activity. Rank sum tests and classification trees will be used to determine which seasonal meteorological metrics are associated with deep slab avalanche activity, and if the magnitude of seasonal meteorological metrics is associated with the magnitude of meteorological inputs directly preceding days with deep slab activity. The results of this study will help determine seasonal meteorological metrics that can be used to more accurately predict the range of daily meteorological inputs likely to precede a deep slab avalanche.