Photo/Illutration Torrential rains caused by linear rainbands flooded Hitoyoshi, Kumamoto Prefecture, on July 4, 2020. (Asahi Shimbun file photo)

The Japan Meteorological Agency will start releasing forecast information about linear rainbands, called “senjo kosuitai,” from June that could give the public major flooding warnings. 

“Once linear rainbands occur, they will likely lead to a disaster,” said Naoyuki Hasegawa, head of the agency, at a news conference. “Once the information is released, I want people to prepare for a massive rainfall disaster.”

The new forecast information will divide Japan into 11 wide areas, according to the details that the agency revealed on May 18.

When a chance for heavy rains becomes high, the agency will release forecast information as early as half a day in advance and at least six hours before.

Linear rainbands comprise rows of cumulonimbus clouds stretching dozens to hundreds of kilometers. Regions beneath their wind paths suffer torrential downpours of rain.

The information will be available on the agency’s website.

The new system is expected to help residents evacuate quickly and local officials prepare for a disaster.

The agency examined the new system’s accuracy by using cases from 2019 to 2021 and found that the percentage of accurately forecasted linear rainbands was about one in four times.

Linear rainbands have caused major disasters in recent years.

For example, torrential rains caused outburst flooding in Joso, Ibaraki Prefecture, in September 2015. In July 2020, the Kumagawa river in Kumamoto Prefecture flooded.

Data for water vapor amounts is needed to forecast linear rainbands, but they have been difficult to foresee it due to a lack of observation equipment.

The agency has built an observation network and utilized the Fugaku supercomputer, which was jointly developed by the Riken research institute and Fujitsu Ltd., to forecast linear rainbands.

The accuracy is still developing, however. 

Currently, a forecast is only available for wide areas, such as the northern Kyushu and Chugoku regions.

Missed forecasts are expected, with about two-thirds currently missed, the agency said. 

The agency said it will continue to improve the accuracy and hopes to issue a forecast warning 30 minutes before linear rainbands are formed.

The agency aims to make available the degree of danger on a municipality basis in 2029.

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