Storm Snow Avalanches: Characteristics and Forecasting

At ski areas, a majority of avalanches fail in storm snow. We investigate these avalanches using stability tests and avalanche observations from California and Alaska. Collapse amplitudes during fracture, measured using particle tracking, were 1 mm for a failure layer of precipitation particles and 7 mm for a layer of unrimed sectored plates. Stability test results showed little dependence on slope angle, suggesting that both precipitation particles and older faceted crystals (persistent weak layers) fail as described by the anticrack model, with collapse providing energy. Using observations from avalanche control work at Mammoth Mountain, CA USA, a large coastal ski area where 9/10 avalanches fail in storm snow, we examined Extended Column Test (ECT) results and their relation to avalanche activity. ECT propagation was a powerful predictor; days with ECTs that propagated had significantly more and larger avalanches. Since other studies have shown that the ECT is an effective predictor of avalanches involving persistent weak layers, we suggest that the ECT is an effective test to predict both types of avalanches, those that fail in storm snow and those that fail on persistent weak layers.

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