We describe the use of SNOWPACK, a snow cover model, for areas with heavy snowfall. Record-breaking snowfall was recorded over the Sea of Japan and northwest coast of Honshu in the winter of 2005/2006. Avalanche forecasting was conducted at Tsunan, Niigata Prefecture, where the snow depth exceeded 4 m. Measurements from an Automated Meteorological Data Acquisition System (AMEDAS) operated by the Japan Meteorological Agency were used as the model input data. To verify the model output, snow pit observations were carried out at 10-day intervals. Simulated snow profiles were verified by applying a comparative method developed by Lehning et al. [Lehning, M., Fierz, C., Lundy, C., 2001. An objective snow profile comparison method and its application to SNOWPACK. Cold Reg, Sci. Technol. 33, 253-261.] and were in reasonable agreement with the observed results, with an agreement score of 0.74. However, the equations for the stability index (SI) were unsuitable for the study area considered. Dangerous conditions continued for more than two months in the model, but they were caused by unsuitable parameterisation of the shear strength for Japanese snow. Thus, the parameterisation of shear strength was improved. The empirical equations formulated by Yamanoi and Endo [Yamanoi, K., Endo, Y., 2002. Dependence of shear strength of snow cover on density and water content. Seppyo., 64(4), 443-451, (in Japanese with English Abstract)] were the most suitable for simulating SI for this region; therefore, they were incorporated into the SNOWPACK model. The unstable conditions that occurred during heavy snowfall were reproduced. To expand the forecasting area to include the area along R405, a national road, the distributions of meteorological parameters in the study area were estimated for grid points with 10-m spacing using simple lapse rate and interpolation methods.
In an avalanche on 24 December 2005, a car was pushed off the road and into a valley. The snow depth increased by about 70 cm within 24 h; such large and intense snowfall is commonly associated with avalanching and, hence, instability. Thus, the snow stability decreased. This was reproduced by the SNOWPACK model. Furthermore, the stability index maps showed that most slopes were dangerous on that day.
The improved SNOWPACK results were compared with other avalanche events on the Sea of Japan side of Japan. Nine of 11 surface avalanches occurred when the simulated stability index was lower than 2. Two shortcomings responsible for the failure to predict avalanches had to do with the reproduction of the graupel layer and the estimation of the shear strength of layer interfaces.