Southwest Japan torrential rain triggered by weather system pileup over Kyushu

TOKYO -- The torrential rains battering southwest Japan were triggered by a body of warm, humid air sweeping up from the south and smashing into a seasonal rain front extending over central Kyushu and into the Pacific Ocean, according to experts.

The southeastward shift of a trough over the Korean Peninsula accompanied by air of minus 9 degrees Celsius also contributed to record-breaking rainfall in various parts of the country.

Kazuhisa Tsuboki, professor of meteorology at Nagoya University, says the warm, damp air formed numerous cumulonimbus clouds that became linear rainbands bringing heavy rain, leading to localized downpours. "Furthermore, a massive volume of water vapor from a tropical depression in the south is flowing into the seasonal rain front, producing more rain," he said.

According to the Japan Meteorological Agency (JMA), the total amount of moisture flowing into the seasonal rain front is less than when disaster-level torrential rains hit western Japan in July 2018. It is therefore expected that the range of the rain will be narrower than last year's downpours.

However, the JMA forecasts that it will continue to rain at least until July 6, indicating that the total amount of rainfall may exceed that of the July 2018 disaster.

Yasutaka Wakazuki, associate professor of meteorology at Ibaraki University, stated, "Each of these weather phenomena is commonly seen during the rainy season, but the seasonal rain front is hovering over the same place for a long time, and is distinctive in that it has barely moved north or south."

Caution is required in areas in which the ground has become loose due to the rain.

(Japanese original by Mayumi Nobuta, Science & Environment News Department)