At ski areas, a majority of avalanches fail in storm snow. Using thousands of observations from avalanche control work at Mammoth Mountain, CA, USA, a large coastal ski area, I analyzed important predictors of avalanche activity. New (24 h) precipitation increased avalanche activity, while changing temperatures and different wind patterns had no effect. If slopes remained undisturbed for one day after snowfall, the number and size of avalanches as well as the explosive yield (avalanches per shot) were all significantly reduced. I also examined a smaller dataset of 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 avalanches and larger sizes. Days with propagating ECTs also had significantly greater new snow amounts, with a threshold value of 0.29 m of new snow, very close to the 031 m threshold from Atwater's 10 factors. That new precipitation above a threshold causes greater avalanche activity is not a new finding; the new finding is that ECT propagation (versus non-propagation) also has a similar new snow threshold. Thus, I suggest that ECT propagation is an important tool to predict explosively-triggered avalanches in storm snow. (C) 2012 Elsevier B.V. All rights reserved.
