Abstract
On 27 March 2017, a heavy snowfall associated with cyclones caused a surface avalanche in Nasu, Tochigi Prefecture, Japan. Although it is known that large amounts of snowfall in a short time are important for surface avalanches, understanding of snowfall characteristics in mountainous regions during heavy snowfall events is lacking. We conducted a case study of this event and also performed a statistical analysis of snowfall events in Nasu from 1989 to 2017, where we investigated the snowfall characteristics and meteorological conditions of each event.
In the March 2017 event, low-level supercooled water clouds were formed by orographically forced updrafts in mountainous regions in Nasu as moist northerly and easterly flows intensified due to the cycloneʼs approach. Localized snowfall intensification and short-duration heavy snowfalls were produced by the Seeder-Feeder mechanism associated with the low-level clouds and snow from the upper clouds of the cyclone. The statistical analysis revealed that similar heavy snowfall events occur about once every 3 years, but only once every 19 years in March. The surface pressure patterns in heavy snowfall cases in Nasu were about 63% in the typical winter monsoon pattern and about 30% in cyclones. Although snowfall amounts became larger as snowfall duration increased in both patterns, some short-duration heavy snowfalls exceptionally occurred in cases where occluded cyclones passed near the Kanto region.
