Snow and weather conditions associated with avalanche releases in forests: Rare situations with decreasing trends during the last 41 years

Mountain forests play a crucial role in avalanche control by modifying the snow's mechanical properties. However, snow avalanches do occur in forests due to unfavorable combinations of site, snow and weather conditions and these so-called ‘forest avalanches’ can pose hazard to human settlements and infrastructure. While the meteorological contributory factors of avalanche releases in open unforested terrain are relatively well understood, much less is known about snow and weather conditions associated with avalanche releases in forests. In order to identify such critical meteorological situations, we analyzed 21 snow and weather variables of 189 naturally released forest avalanches. By applying a hierarchical clustering method, we distinguished two forest avalanche types: (1) ‘new snow forest avalanches’ which release in periods of heavy snowfall and under stormy and permanently cold conditions and (2) ‘old snow forest avalanches’ which release after periods of high insolation and an increase in air temperature. Snow and weather conditions of new snow forest avalanches are rare and clearly distinguishable by classification trees from those of avalanches released in open unforested terrain at similar elevations. We tested for long-term trends in the occurrence of favorable meteorological conditions for forest avalanches during 41 winters (1970/71–2010/11) applying a logistic regression model. The number of potential forest avalanche days decreased at 11 of 14 snow and weather stations in the Swiss Alps for new snow forest avalanches and at 12 of 14 stations for old snow forest avalanches, independent from elevation and climatic region. These negative trends suggest a further decrease of snow and weather conditions associated with avalanche releases in forests under current climate change and, in combination with the currently observed increase in forest cover density in the Swiss Alps, it is thus likely that avalanche releases in forested terrain will become less frequent. However, such events will also occur in the future and the presented characterization of meteorological conditions could support avalanche warning and forest services in forecasting forest avalanches.

► We analyzed 21 snow and weather variables of 189 naturally released forest avalanches.
► Forest avalanches can be distinguished in two clusters: new and old snow forest avalanches.
► A classification tree model on new snow forest and non-forest avalanches is presented.
► We applied a logistic trend analysis on favorable conditions for forest avalanches.
► Such situations are rare with decreasing trends during the last 41 winters.