How Japan’s devastating rainstorm came about

by Tomoko Otake

Staff Writer

Torrential rain that caused flooding and the evacuations of tens of thousands of people across the Kanto region on Thursday was the result of a mass of humid air unable to escape the area, a pileup of thunderclouds — and possibly climate change, experts said.

The heavy rainfall in Tokyo and surrounding prefectures was caused by stationary humid air covering a wide swath of sky in the Kanto region, which was unable to move in any direction. It was hemmed in to the west by a chunk of cold air over the Sea of Japan, where Typhoon Etau fizzled out, and a block of humid air to the east over the Pacific Ocean, where Typhoon Kilo was swirling, according to Kunihiro Naito, a forecaster at Weathernews Inc., a Chiba-based weather information company.

“Usually, autumn typhoons pass quickly after making landfall in Japan, and strong rain clouds normally move eastward along with the typhoons,” Naito said. “This time, however, after Typhoon Etau lost its strength and turned into a tropical cyclone in the Sea of Japan, it stayed there, while humid air kept flowing in from the south. This resulted in the formation of a rain zone in Kanto.”

The downpour that afflicted almost half of Kanto had pretty much the same factors behind it as the torrential rain that struck the city of Hiroshima in August 2014, which triggered massive floods and landslides and killed 74 people.

In meteorological terms, what happened to Hiroshima is known as “back building,” whereby thunderclouds typically pile up in a narrow band about 10 km wide, causing intense rainfall in a very small area.

This week’s rainstorm was a larger version of what happened in Hiroshima, in that it involved much larger amounts of thunderclouds creating a thicker, longer band of rain, 100 km in width and 1,000 km in length, Naito said.

While such sudden pileups of thunderclouds are not new, the growing intensity of downpours in recent years might be linked to climate change, Naito said.

“Ocean temperatures around Japan have been rising in recent years, producing vapor and making air conditions unstable. That makes ‘back building’ of sorts easier to happen,” Naito said.

Kei Yoshimura, a hydrologist at the University of Tokyo’s Atmosphere and Ocean Research Institute, said the magnitude of the Kinugawa River flooding on Thursday was “something that takes place only once in 100 years.”

He added that it was too early to conclude the flooding was linked to climate change, saying the heavy rainfall happened to occur along that river.

“It’s my view that the effect of climate change on this particular incident is zero, or unknown at this point,” he said.