The torrential rain wreaking havoc in Kyushu smashed all records for single-day maximum rainfall in numerous locations on the main southern island, phenomena a Kyoto University expert on rainfall linked to global warming.

“We cannot explain why rains in recent years are so severe without mentioning the fallout from global warming,” said Eiichi Nakakita, a professor of hydrometeorology at Kyoto University.

Nakakita called for the creation of a system to intercept rainfall in the entire region along a river and stressed the importance of individual evacuation behavior.

“Rainfall amounts have reached the level where we can no longer protect ourselves with riverbanks alone,” he said, citing the growing volume of water vapors resulting from rising seawater and air temperatures for contributing to the recent spell of severe rainstorms.

Records for 24-hour precipitation were broken at 19 locations between July 3 and 6 p.m. on July 7, as the seasonal rain front that landed there earlier this month continued to pound the area.

When the Japan Meteorological Agency issued its first heavy rain alert for the region on July 4, 24-hour maximum rainfall hit record highs at seven sites, all in Kumamoto Prefecture.

Rainfall measured 489.5 millimeters in Yunomae, 463.5 mm in Asagiri and 410.5 mm in Hitoyoshi, municipalities along the Kumagawa river, which overflowed extensively.

The river burst its banks at one site and overflowed at 11 others, inundating an estimated 1,060 hectares along the waterway, with the rain front crawling north after that.

A second emergency alert was issued on July 6 for Fukuoka and other prefectures. Torrential rain continued to sweep the area the following day, with record 24-hour rainfall seen in five prefectures, including Fukuoka, Saga and Oita.

Omuta, Fukuoka Prefecture was hit with 446.5 mm of rain, topping the 373.5 mm for all of July recorded in a regular year.

Meteorological experts attributed the recent torrential downpours to a linear rainband, a group of cumulonimbus clouds stretching dozens of kilometers.

“Since around July 3, Kyushu has had weather conditions which led to the formation of a linear rainband as groups of cumulonimbus clouds emerged in succession and lined up downwind,” said Seiji Tashiro, a senior researcher at the nonprofit Crisis & Environment Management Policy Institute.

“The rainband triggered heavy rains in Kumamoto Prefecture on July 4 and then in the north of Kyushu between July 6 and July 7,” added Tashiro, who formerly headed the agency’s Shimonoseki Meteorological Office in Yamaguchi Prefecture.

In general, a high-pressure system in the Pacific Ocean increases its strength toward the end of the rainy season in Japan.

As a result, the seasonal rain front tends to be trapped between the Pacific high and a high-pressure system in mainland China, remaining over Kyushu and nearby.

As temperatures rise toward summer, water vapors have been constantly supplied to the rain front from warm winds from the south, resulting in heavy downpours.

This explains the torrential rain that flooded in many parts of northern Kyushu in 2017 and further flooding from intense rainfall in western Japan in 2018, which both took place in early July toward the end of the rainy season.

But the way rain fell in the disasters of 2017, 2018 and this year varied significantly, according to Haruhiko Yamamoto, a professor of environmental disaster management at Yamaguchi University.

In 2017, heavy downpours of more than 100 mm an hour fell in a small area over a relatively short period.

In contrast, during the disaster the following year, rain totaling 30 mm per hour or so continued for two to three days.

The amount that has fallen in this year's torrential rains to date lies in the middle of the two previous deluges, Yamamoto said.

“This year, the rain front moved north and south in Kyushu, repeatedly unleashing downpours and stopping over a broader area of the region,” he said, adding, “rainfall amounts stood at about 60-80 mm per hour.”

Kazuhisa Tsuboki, a professor of meteorology at Nagoya University, noted that this year’s rain front stretches into mainland China, with a long stream of a huge volume of water vapors.

“Water vapors were carried into Kyushu from a location farther away than usual,” Tsuboki said.