Shear strength is an important parameter for interpreting the stability of simulated snow covers. In the SNOWPACK model, snow shear strength is estimated as a function of snow density using expressions for different grain types. In the model, shear strength changes discontinuously as grain type changes from rounded to faceted grains (and vice versa), but in nature, shear strength changes take place more gradually. An experiment on the growth of depth hoar allows a new parameterization of continuous changes in shear strength. A parameter is induced in the expression for shear strength as a function of water vapor transport. It ranges from 0 (rounded grains) to 1 (depth hoar) depending on the metamorphic stage of the snow. The parameterization is incorporated into the SNOWPACK model to calculate the progressive change in shear strength during snow metamorphism. The calculated shear strength using the improved SNOWPACK model agreed well with that measured in cold-room experiments using artificial snow. This model, which can calculate gradually changing shear strength, is expected to improve the accuracy of avalanche forecasting. © 2009 Elsevier B.V. All rights reserved.