| Summary: | This study showed the frequency of local-scale heavy winter snowfall in Hokkaido, Japan, its historical change, and its response to global warming using self-organizing maps (SOM) of synoptic-scale sea level pressure anomaly. Heavy snowfall days were here defined as days on which the snowfall exceeded 10 mm in water equivalent. It was shown that the SOMs can be grouped into three categories for heavy snowfall days: 1) a passage of extratropical cyclones to the south of Hokkaido, 2) a pressure pattern between the Siberian high and the Aleutian low, and 3) a low-pressure anomaly just to the east of Hokkaido. Groups 1 and 2 were associated with heavy snowfall in Hiroo (located in southeastern Hokkaido) and in Iwamizawa (western Hokkaido), respectively, and heavy snowfall in Sapporo (western Hokkaido) was related to group 3. The large-ensemble historical simulation reproduced the observed increasing trend in group 2, and future projections revealed that group 2 was related to a negative phase of the western Pacific pattern and that the frequency of this group would increase in the future. Heavy snowfall days associated with SOM group 2 would also increase as a result of the increase in water vapor and preferable weather patterns in a globally warming climate, in contrast to the decrease of heavy snowfall days at other sites associated with SOM group 1.
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