Refinements of empirical models to forecast the shear strength of persistent weak snow layers PART A: Layers of faceted crystals

This paper presents an adjustment for the normal load for persistent weak layers in the Columbia Mountains of Canada and an improved empirical model to forecast the shear strength of layers of faceted crystals for indexing the stability of the snowpack. This model is based on manual snowprofile observations, as was a previous model; however, the measured shear strength is adjusted for the normal load by a constant factor of ϕ = 0.21. The study includes the analysis of more field data for the rate of change of shear strength, which is assessed for daily and average loading rates. The Forecasting Model predicts the shear strength of layers of faceted crystals with an accuracy of 77–79% depending on whether the shear strength change between snowprofile observations is based on daily or average loading rates. A companion paper, Part B, develops a forecasting model for layers of buried surface hoar.