An extreme dry-slab snow avalanche occurred on January 10, 2021, at Mt. Nodanishoji, Gifu, Japan, during a heavy snowfall. The avalanche ran down a horizontal distance of approximately 2,800 m and damaged trees and infrastructures. This was estimated to be the second largest recorded avalanche in Japan. However, physical snowpack properties and their vertical profiles and spatial distribution, which caused the avalanche, were not addressed in the release area immediately following the avalanche, mainly due to unsafe and lousy weather conditions. Based on a snow depth distribution observed by an unmanned aerial vehicle and a numerical snowpack simulation in the avalanche release area, the spatial distributions of the mechanical snowpack stability and slab mass and their temporal evolution were estimated in this study. The procedure was validated by comparing the calculation results with the observed snowpit and spatial snow depth data. The results indicated that two heavy snowfall events, approximately 3 and 10 days before the avalanche onset, generated two different weak layers made of precipitation particles and associated slabs above the weak layers. The older weak layer was only generated on the northward slope due to its low temperature, whereas the newer layer was predominant over the avalanche release area. The procedure employed in this study is expected to be applied to other avalanche cases in the future.

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