Modeling strength and stability in storm snow for slab avalanche forecasting

Some avalanche forecasting programs use a ratio of weak layer strength and overburden stress, along with other variables and observations, to help assess the stability of the upper snowpack in their forecast area. During winter storms, the strength–stress balance may change rapidly, and access to start-zone elevation study plots for manual measurements becomes limited. The shear strength of twelve non-persistent weak layers comprised of precipitation particles was measured during the winters of 2006 and 2007 in Glacier National Park, Canada. We present an empirical model to estimate the shear strength of such non-persistent weak layers up to 4 days after burial. The model is based on a backwards stepwise regression of several remotely acquired precipitation and manual snowpack variables as possible predictors of measured shear strength. The approximate age of the weak layer, the overburden stress resulting from overlying layers, and the average air temperature are the significant predictors of the weak layer strength. With an estimate of overburden stress from remotely measured precipitation, the model can generate both the shear strength of the weak layer and a stability index, SN38, at any time during a storm, without any manual inputs. Modeled strengths and the stability index exhibit similar trends as measured values, and most avalanches in the forecast area occurred when the stability index was declining.