Photo/Illutration Torrential rains flood an area in Hitoyoshi, Kumamoto Prefecture, on July 4, 2020. (Asahi Shimbun file photo)

Meteorologists are urging the public to be on guard in early July, which has become an annual season for heavy rainfalls, landslides and flooding.

Since 2017, at least one deadly disaster caused by torrential rains has occurred each year during the period from July 1 to 10.

A total of 430 people were killed or went missing in those disasters, according to the Fire and Disaster Management Agency’s data.

Weather experts say there is no reason to think that this year will be drier.

“Heavy rains occur every year, but in recent years in particular, downpours that cause disasters have occurred frequently in early July,” Masaki Sato, a professor of meteorology at the University of Tokyo, said at Meteorological Society of Japan symposium held in May.

On July 3, 2021, a torrential rainfall triggered a deadly landslide in Atami, Shizuoka Prefecture.

A week later, a special heavy rain warning was issued in Kagoshima Prefecture in Kyushu and other areas. More than 200 buildings were flooded.

In July 2018, torrential rains hit western Japan, flooding rivers and causing mudslides.

The special heavy rain warning is issued for supposedly rare rainfalls that occur once every several dozen years.

Since 2013, when the warning was established, it has been issued 20 times, including nine times in early July.

With the exception of 2019, the warning has been issued in early July every year over the past five years.

The period is called the late “tsuyu” season, when conditions are right for heavy rains.

Rainy areas generally remain in southern China around June and move north around July, making the atmosphere over Japan unstable.

Furthermore, the large amount of water vapor flowing into Japan can lead to the formation of rows of thunderstorm clouds.

Under these conditions, long linear rainbands that continue to dump rain over wide areas can occur in the same areas.

Linear rainbands have formed in early July in each of the past five years. They develop quickly, and their locations and times can change by minimal alterations in the conditions.

Linear rainbands are more difficult to predict than typhoons, and residents may be caught off-guard by the downpours brought on by the rainbands.

That is why experts are advising the public to frequently check weather information.

On the evening of July 3, 2020, the Japan Meteorological Agency forecast up to 200 millimeters of rain over 24 hours in Kumamoto Prefecture. But in several locations in the prefecture, more than 400 mm were recorded over that period.

The special heavy rain warning was issued in the predawn hours of July 4.

The rain had already flooded the Kumagawa river, and more than 60 people who could not flee died.

The head of the JMA said at a news conference held later, “We were incapable” of predicting such a disaster.

In June this year, the agency started simulating occurrences of linear rainbands from six hours to half a day before their expected formation.

The JMA also provides a broad time frame for such occurrences and shares the information with the public.

Some local governments have decided to spread evacuation information more quickly through these simulations.

On the agency’s website, individuals can see rain forecasts for several hours later based on radar observations. They can also check a map detailing the risks of mudslides and floods in their neighborhoods.

Shingo Shimizu, a senior researcher at the National Research Institute for Earth Science and Disaster Resilience, urged people to keep abreast of the latest information.

“In this time of the season, weather forecasts may change at a fraction of the time,” he said.

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