Cold Spell in Japan from late January 2018
5 February 2018
Tokyo Climate Center, Japan Meteorological Agency
http://ds.data.jma.go.jp/tcc/tcc/index.html

Summary An extremely cold spell lasting several days hit Japan and surrounding areas around 23rd January. Thiswas caused by the movement of a significantly cold lower-troposphere air mass over the country due toa stronger-than-normal northwesterly monsoon from eastern Siberia, where the extraordinary cold airmass accumulated in association with the meandering of the polar front jet stream over the area.
 A tendency for cold air to move over Japan and surrounding areas is expected over the next two weeks.
Be sure to check the latest weather forecasts and information for updates.

1. Weather conditions
An extremely cold spell lasting several days hit Japan and surrounding areas around 23rd January. Cold air has
prevailed throughout the country since then, especially in eastern and western parts. A record-low temperature of-9.8°C was observed in Saitama City just north of Tokyo.
2. Cause
The polar front jet stream over northern Eurasia exhibited clear meandering from mid-January onward (Figure 1(a)) with southward meandering over eastern Siberia. These conditions caused extremely cold air to accumulate in
the lower troposphere over eastern Siberia (Figure 2 (a)). Temperatures over this area are generally at their lowest at
this time of year, but the air mass that accumulated there in January 2018 was much colder than usual. The
area-averaged 850-hPa temperature over eastern Siberia in this event was significantly lower than the corresponding
historical values (Figure 2 (a) and Table 1).
The mid-tropospheric trough over eastern Siberia subsequently shifted slightly eastward to cover Japan and again
persisted (Figure 1 (b)). The northwesterly monsoon was enhanced (Figure 3 (b)) in association with upper-level jet
meandering, which caused continuous flowing of the low-level cold air mass over Japan from eastern Siberia (Figure2 (b)). The 850-hPa temperature over Wajima in Japan’s Ishikawa Prefecture at 12 UTC on 24 January was thelowest on record for 12UTC-observation since April 1957.
3. Outlook
A tendency for cold air to move over Japan and surrounding areas is expected over the next two weeks. Heavysnow is expected along the Sea of the Japan side of the country. Be sure to check the latest weather forecasts andinformation for updates.
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Figure 1. 5-day mean 500-hPa
geopotential height [m] (contours)
and anomaly (shading) ((a) 20 -24 Jan. 2018, and (b) 23 -- 27 Jan.
2018)
Contour and shading intervals are
60 m. Anomalies are deviations
from the 1981 – 2010 average.

Figure 2. 5-day mean 850-hPa
temperature [°C] (contours) andanomaly (shading) ((a) 20 -- 24
Jan. 2018, and (b) 23 -- 27 Jan.
2018)
Contour and shading intervals are 4and 2°C, respectively. Anomalies
are deviations from the 1981 – 2010average. Red boxes indicate areas
referred to in Table 1.

Figure 3. 5-day mean sea levelpressure [hPa] (contours) andanomaly (shading) ((a) 20 -- 24 Jan.
2018 and (b) 23 -- 27 Jan. 2018)
Contour and shading intervals are 4
and 2 hPa, respectively. Anomalies are
deviations from the 1981 – 2010average.
Table 1. Top six coldest events of 5-day mean 850-hPa temperature averaged over eastern Siberia [45° -- 65°N, 90° -- 135°E](red boxes in Figure 2). Data are based on based on JRA-55 (Kobayashi et al. 2015) from January 1958 onward. Anomalies are
deviations from the 1981 – 2010 average.
Rank
1234
55 (tied)

Period (5 days)
11 Feb. 1969 – 15 Feb. 1969
18 Jan. 1969 – 22 Jan. 1969
26 Jan. 1979 – 30 Jan. 1979
30 Dec. 1958 – 3 Jan. 1959
20 Jan. 2018 – 24 Jan. 2018
1 Feb. 2001 – 5 Feb. 2001

Mean temperature (°C)
-31.6
-30.6
-30.1
-29.0
-28.9
-28.9

2

Anomaly (°C)
-13.3
-11.0
-10.7
-9.7
-9.4
-9.9

Locations of Wajima (Ishikawa Prefecture) and Saitama City (Saitama Prefecture) in Japan
Detailed information on regional division is provided on JMA’s web site at
http://www.data.jma.go.jp/gmd/cpd/longfcst/en/tourist.html.

References
Kobayashi, S., Y. Ota, Y. Harada, A. Ebita, M. Moriya, H. Onoda, K. Onogi, H. Kamahori, C. Kobayashi, H. Endo, K. Miyaoka and K.
Takahashi, 2015: The JRA-55 Reanalysis: General specifications and basic characteristics. J. Meteor. Soc. Japan, 93, 5 -- 48.

JMA website
Analysis Charts
5-day Mean Figures of Atmospheric Circulation: http://ds.data.jma.go.jp/gmd/tcc/tcc/products/clisys/figures/db_hist_pen_tcc.html
Animation Maps (Northern Hemisphere): http://ds.data.jma.go.jp/gmd/tcc/tcc/products/clisys/anim/anim_nh.html
OutlookMonthly Outlook for Japan: http://www.jma.go.jp/en/longfcst/000_1_00.html

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