Factors behind the heavy snowfall and low temperatures in Japan frommid-December 2020 onward and weather outlook
15 January 2021
Tokyo Climate Center, Japan Meteorological Agency
https://ds.data.jma.go.jp/tcc/tcc

SummaryFrom mid-December 2020 onward, a series of heavy snowfall events hit Japan,
especially the Sea of Japan side of the country. The snowfall amounts were recordbreaking in some places.
One of the factors for these events is considered to be the southward meandering ofboth the polar front and subtropical jet streams around Japan, resulting in continuous
cold air flow into the country. The meandering of these polar front and subtropical jet
streams was likely to be linked to a blocking high over western Siberia and the ongoing
La Niña condition, respectively.

1. Climate conditions
From mid-December 2020 onward, cold air from Siberia repeatedly flowed into the vicinityof Japan, associated with intensified East Asian winter monsoon. This cold air mass, suppliedwith a huge amount of moisture from the warmer-than-normal the Sea of Japan, caused
heavy snowfall events mainly on the Sea of Japan side from northern to western Japan(Figures 1-1, 1-2, 1-3 and 1-4). In particular, previous records of 72-hour snowfall amount
were broken at 19 observation stations in the Tohoku and Hokuriku region of the country(Figure 1-2). Even though it’s still the middle of this winter, the maximum snow depths havealready exceeded the climatological normal for winter in many places, mainly on the Sea ofJapan side of the Japan’s main island, and some places such as plains in Hokuriku region haveexperienced very high maximum snow depths more than double the normal (Figures 1-3 and1-4).
Associated with cold-air inflow over Japan, temperatures were below normal nationwide
in this period (Figures 1-5 and 1-6). In particular, the average temperatures in early January
were the lowest in the last 36 years in northern Japan and in the last 35 years in western Japan(Figure 1-5).
2. Factors
The primary factors contributing the climate conditions detailed above are illustrated inFigure 2-1.
One of the factors behind this heavy snowfall and low temperatures is thought to be thesouthward meandering of both the polar front jet stream (PFJ) and the subtropical jet stream(STJ) around Japan, resulting in continuous cold air flow into the vicinity of Japan. In
association with the meandering of the PFJ, the polar vortex in the troposphere over the Arcticregion split into two parts and one of them moved southward to the north of Japan, bringing
with it intense cold air mass from the polar region to the vicinity of Japan. This repeated cold
air outbreak hitting mainly northern Japan was so intense that the area-averaged temperature
at 700hPa for about one month from 14 December 2020 was the second-lowest since 1958
(Figure 2-2).
The southward meandering of the PFJ around Japan was likely to be influenced by ablocking high over western Siberia and Rossby wave packet propagation from Europe.
The southward meandering of the STJ around Japan was likely to be caused by theenhanced convective activity over the Maritime Continent associated with the present La Niña
condition.
3. Outlook
In many areas, the current snow depth has already exceeded the maximum depths of anormal winter. Please pay attention to the latest weather information.

Figure 1-1. Cumulative snowfall [cm] averaged over snowy areas (the areas are designatedunder the Act on Special Measures for Heavy Snowfall Areas) (1 November 2020 - 14 January2021).

Figure 1-2. Maximum 72-hour snowfall [cm] during the period (14 December 2020 - 11January 2021).

Figure 1-3. Maximum snow depths [cm] for the period (14 December 2020 - 11 January 2021).

Figure 1-4. Maximum snow depth ratio during the period (14 December 2020 - 11 January2021) to the normal of the annual maximum [%].
The base period for the normal is 1981 – 2010.

Figure 1-5. Time-series representations of 5-day running mean temperature anomalies [°C]from November 2020 to January 2021.
The base period for the normal is 1981 – 2010.

Figure 1-6. Distribution map of mean temperature anomaly [°C] (average from 14 December
2020 - 11 January 2021)
The base period for the normal is 1981 – 2010.

Figure 2-1. Characteristics of the atmospheric circulation that have brought heavy snowfall
and low temperatures in Japan from mid-December 2020 onward

Figure 2-2. (Left) 700hPa-temperature (black line) and its anomaly (color) [°C] averaged overthe period from 14 December 2020 to 11 January 2021. (Right) Area-averaged 700- hPa(blue)and 850hPa (red) temperatures over northern Japan for the period from 14 December to 11
January from 1958/59 to 2020/21.
Area-averages for northern Japan are calculated within the red box in the left figure. The anomaly
is the difference from the mean for the period 1980/81 - 2009/10.

Figure A1. Climatologicalregions of JapanThe JMA defines seven regional
divisions for climate monitoring
and forecast (the Sea of Japansides and the Pacific sides ofnorthern,eastern,western
Japan, and Okinawa/Amami).