2026 年 44 巻 p. 13-38
A short-term extreme heavy snowfall occurred over the Tokachi Plain in Hokkaido on 3–4 February 2025. Because a marine heatwave (MHW) was identified offshore during the event, this study investigates the characteristics of heavy snowfall in Tokachi and the uniqueness of this case through statistical analyses focusing on atmospheric and oceanic conditions. The preceding 1-h, 3-h, 6-h, and 12-h snowfalls were the largest on record and deviated substantially from regression relationships, indicating an anomalous event. An analysis of snowfall cases of 1953–2025 revealed three synoptic patterns: WSCC, including the 2025 case, characterized by cyclones along both the western and southern coasts of Hokkaido (44%); SCC, with a cyclone only along the southern coast (41%); and WCC, with a cyclone only along the western coast (13%). Surface observations from 1998 to 2025 show that a cold-air layer formed by the geographical characteristics of the Tokachi Plain maintained low temperatures while supporting abundant precipitation, leading to short-term heavy snowfall. A cold-air layer was present in all three patterns, and a coastal front was suggested in most cases. This case was distinguished by remarkably cold upper-level air combined with lower-to-middle-tropospheric warming and moistening upstream due to the MHW. WSCC cases were subdivided into two groups depending on which cyclone, either western or southern, was more developed, resulting in differing southern cyclone positions and snowfall mechanisms. Remarkably heavy snowfall occurred only in the group with a more developed western cyclone, including this case, which also exhibited the most unstable atmospheric conditions among these cases.
