Synoptic- and Meso-scale Features of the Heavy Wet Snow Accretion Event along the Okhotsk Sea Coast on December 22 – 23, 2022

Heavy wet snow accretion occurred along the coast of the Okhotsk Sea, collapsing a transmission tower near Monbetsu City and causing a power outage in the area, on December 22–23, 2022. This study investigated the meteorological conditions that caused heavy wet snow accretion in this area, with a particular focus on three factors responsible for wet snow accretion: strong winds, snowfall, and temperatures slightly above 0 °C. An analysis of the station observations from the Japan Meteorological Agency shows that this case occurred on the most favorable day for the wet snow accretion in Hokkaido since 1976. A duration of favorable temperatures for wet snow accretion for this case was longer than historical events by 30 %. A numerical simulation using the Weather Research and Forecasting model, with a horizontal resolution of 1.667 km, demonstrated that the formation of torrential wet snowfall and strong winds were associated with multiple extratropical cyclones. On the evening of December 22, a cyclone moving northward off the eastern coast of Japan, together with another stagnant cyclone located over the northern Japan Sea, formed a large cyclonic circulation. The cold conveyor belt, a cold airstream located poleward of the warm front, associated with the northward-moving cyclone, caused strong easterly winds along the coast of the Okhotsk Sea and carried a large amount of moisture there, reinforcing snowfall from stratiform clouds through depositional growth. A backward trajectory analysis showed that temperatures slightly above 0 °C were maintained through the balance between heating from the sea surface and cooling caused by snow melting. The norward-moving cyclone tracks resembled other historical events at Monbetsu, but the precipitation amounts were the largest in this event. These findings suggest that a combination of synoptic-scale circulations and cloud microphysics plays an essential role in the occurrence of heavy wet snow accretion.