A computational procedure to model instationary creep movements in alpine snowpacks is presented. The model allows the study of large volumetric and shear strains (new snowfall), temperature dependent material behaviour (sudden warming) and progressive creep fracture. These processes are all important factors in natural avalanche release. A viscoelastic material model for snow is proposed in which the constitutive properties are characterized on planes of various orientations within the material. This model is conceptually simple because it does not employ complicated tensorial arguments. It is, however, capable of modelling the highly nonlinear and complex material behaviour of snow including the influence of material microstructure. The model requires only a few parameters which can be determined from simple uniaxial or triaxial experiments. The new constitutive model and numerical procedure is verified using two field and laboratory experiments.