Torrential, record-breaking rainstorms have deluged southern and central Japan since Friday, killing at least 58 people and damaging more than 4,000 properties. The southwestern-most island of Kyushu, with a population of nearly 13 million, was hardest hit by the initial wave of downpours Friday and Saturday, while heavy rains have since spread into the central mainland.
Japanese Prime Minister Shinzo Abe urged the government to declare an “extreme disaster” to streamline the recovery process in hard-hit areas of the nation. A number of communities were isolated, with roads and other vital infrastructure damaged by heavy rain and landslides.
Meanwhile, meteorologists are eyeing the potential for parts of Japan to receive another foot or more of rainfall next week.
Extreme rain totals
On Friday, 15 inches of rain fell in just six hours, a record, in southwest Kyushu in the Kumamoto prefecture, according to Sayaka Mori, a meteorologist for the broadcasting service NHK World-Japan.
The valley community recorded 19.3 inches in 24 hours, the mountains likely contributing to “orographic uplift,” or extra rising motion, that enhanced heavy downpours. Minamata, closer to the coast, saw 18.7 inches out of the storms on Saturday.
Amakusa, a city in western Kyushu, reported a one-hour rain total of 3.85 inches. Nearby Ashikita picked up 7.5 inches in three hours, also breaking a six-hour record with 12.8 inches.
The torrents caused the Kuma River to overflow, leading to floods and landslides that inundated streets, homes and businesses, and submerged cars. A nursing home was overrun by floodwaters, resulting in the deaths of 14 residents.
In Kyushu, Japan’s third-largest island, 3.6 million people were asked to evacuate and 13 people remain missing.
Meanwhile, landslides have wrought havoc, and the Japanese Meteorological Agency has issued rainfall and ground-loosening warnings for most of southern Japan. The agency warned of a high risk of landslides in Nara prefecture, southwest of Tokyo, with a particular danger in the foothills of Mount Obako.
A recipe for destructive rain
Kyushu is the wettest place in Japan. According to NHK News, Kyushu holds the record for the nation’s greatest one-hour rainfall and annual total — 6.02 inches and 341 inches (28 feet and 5 inches), respectively.
Its position in Japan’s southwest puts it in contact with the warm flow of the Koroshio Current, the Pacific’s “western boundary current.” The current, akin to an even stronger Gulf Stream, transports mild subtropical water northward through the Philippine Sea, adding heat and moisture to the air, fueling potentially drenching downpours.
Most of the Koroshio Current’s flow curves east of Japan because of the Coriolis force, or Earth’s spin, but a portion of it snakes into the East China Sea. This branch, the Tsushima Current, meanders west of the Japanese archipelago. Kyushu is sandwiched between.
Atmospheric moisture is most readily available in the summertime, and often interacts with the Meiyu front — a semi-permanent feature in the East Asian atmosphere. The usually weak boundary divides warm air to the south from a cooler, drier northwest flow emanating from northern China and southeastern Russia. The front sags south in the wintertime, swinging much closer to Japan during the summer as the cold retreats north into the Arctic.
Over the weekend, the Meiyu front was draped directly over Kyushu, with a pair of frontal depressions — waves of low pressure — riding up the boundary. That helped focus heavy downpours and tap into the excessive moisture available.
A weather balloon launched Saturday from Kagoshima, a seaside city on Kyushu, revealed an excessively moist atmosphere typical of what one might expect to find in the middle of a tropical storm. A PWAT, or precipitable water index, of 3.00 inches was measured. PWATs illustrate how much water is present in a column of atmosphere.
That meant that every column of air could theoretically squeeze out three inches of rain. And when that air mass is constantly being “wrung out” and replenished from the south, the stage is set for extreme rainfall.
All told, 4,700 buildings in seven prefectures in southwest and central Japan have been “destroyed, damaged or flooded,” according to Kyodo News.
More rain on the way
On Wednesday, extreme rainfall was once again occurring in parts of Nagano and Gifu, a little over 100 miles west of Tokyo, where the Japan Meteorological Agency had issued a heavy-rain emergency warning.
From Wednesday through the weekend, another 3 to 6 inches of rain appears possible over much of the southern half of Japan, with 6 to 10 inches across Kyushu, southern Shikoku and coastal Wakayama, where enhancement by the coastal mountains occurs. Tokyo looks to reside on the fringe of the heaviest rainfall, with several inches still likely.
It’s not out of the question that a few locales in Kyushu pick up 12 to 18 inches in three days’ time.
Weather models show low pressure meandering northeastward through the Sea of Japan, triggering the waves of heavy rainfall.
An additional foot of rainfall is possible in parts of southern Japan as more stormy weather ensues to kick off the workweek next week.
Japan reeled in the wake of severe flooding following the passage of Super Typhoon Hagibis in October, with a second round of extreme rainfall arriving just two weeks later ahead of Typhoon Bualoi.