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RMS : comments on the floods in Japan

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Intense rainfall over the 24 hours from 12:00UTC  on Wednesday, July 11, has resulted in flash flooding, as rivers burst their banks, and landslides in southwest Japan. The worst affected areas were the Kumamoto and Oita prefectures, Kyushu, where in excess of 100mm (4in) per hour was recorded. Some cities in these regions, including Kumamoto City, received 500mm (1.8ft) of rain in under 24 hours, leading to knee-deep water.

Over 68,000 people were ordered to evacuate from 33,000 households across the prefectures. Officials are reporting 10 fatalities, with 20 others missing. Fatalities confirmed by the Kumamoto prefecture are all elderly people, who were unable to escape the floodwater that destroyed their homes. Hundreds of homes have been reported to be damaged or destroyed, and approximately 22,000 are currently without electricity. Local train service has been disrupted, as well as some roads being blocked off by landslides.

“The rainfall was caused by a stationary front situated across the southwest of the country, which brought warm, moist air from the Pacific Ocean over Japan. As this moist air hit the mountains, intense downpours of rainfall were produced,” said Christine Ziehmann, director, model product management at RMS. “While flooding in this region isn’t rare, the amount of rainfall within the 24 hour period was.”

The Japanese Meteorological Agency (JMA) has reported the rainfall amount to be unprecedented – the heaviest rainfall experienced on record in the Kumamoto and Oita prefectures. Some cities across Kyushu have recorded rainfall in the past few months twice that of the annual average, so grounds were already relatively saturated previous to this event. The terrain in Kyushu is very mountainous and hence susceptible to landslides under saturated ground conditions. Average rainfall in July for Kyushu is around 340mm, which was received by some parts of the island in just four hours on Wednesday.

Forecasts suggest that more rain in these regions is possible due to the stationary front, so alert levels for rivers in the area have been raised in order to help prevent against further overflowing. The JMA predict a further 20cm (8 in) of rainfall through Friday, with a risk of tornadoes developing in some areas.