Heavy snow warnings in Japan: from the two-week outlook to the short-duration heavy snow bulletin

Heavy snow in Japan rarely arrives without notice. The Japan Meteorological Agency (JMA) warns in stages, from a two-week outlook down to bulletins issued while the snow is falling. This guide explains those stages, what changed in 2026, and what to do at each step. JMA's English pages cover some of these products; for the others, the names below are our translations of the Japanese.

The stages, from two weeks out to now

JMA issues Heavy Snow Emergency Warnings when snow depth seen once every 50 years is expected over a prefecture-wide area and warning-class snowfall is expected to continue for around a day or longer. The Advisory and Warning thresholds are set separately for each municipality.

What changed in 2026, and what did not

On May 29, 2026, JMA started a new system that attaches alert-level numbers to warnings for heavy rain, flooding, landslides and storm surges (see "Japan's five alert levels explained"). Heavy snow and snowstorm warnings did not change. JMA notes that these warnings do not correspond to alert level 5 or level 3.

One snow product was renamed. From May 2026, the information formerly called "information on remarkable heavy snow" is issued as the short-duration heavy snow bulletin (our translations). JMA says it is issued roughly when 20 to 25 cm of snow falls in about 3 hours, or 30 to 40 cm in about 6 hours, and warning-class snow is expected to continue (the values differ by observation point). JMA asks people in the area to avoid nonessential trips, and people elsewhere not to travel into it.

Why the Sea of Japan side gets buried

In winter, a convergence zone called the JPCZ (Japan-sea Polar air mass Convergence Zone) can form over the Sea of Japan, about 1,000 km across. When the snow clouds along it keep feeding the same area, record snow can fall in a short time. JMA cites February 2018, when Fukui City recorded 147 cm of snow, more than 1,500 vehicles were stranded on National Route 8, and the Self-Defense Forces were dispatched (Japanese).

On the Pacific side, including Tokyo, heavy snow usually comes from low-pressure systems passing south of Honshu. JMA explains that rain-or-snow forecasts for these systems are often difficult. A few centimeters can stop trains and roads in areas that rarely see snow.

Snowstorms and whiteouts

A Snowstorm Warning is not "heavy snow plus strong wind." It covers serious danger from wind carrying snow, including loss of visibility. Even without new snowfall, strong wind can lift snow off the ground and cause a whiteout. In Hokkaido in March 2013, clear weather turned into a sudden snowstorm after 15:00; 9 people died and 929 vehicles were stranded (Sapporo Regional Headquarters, JMA; Japanese). If you are caught in your car, see "Stuck in your car in a snowstorm."

What to do at each stage (our suggestions)

For heater safety, see "Space heater and kerosene heater safety," and for outages, "Power outage preparedness." A Japanese version is available: "大雪の情報の読み方" (in Japanese).