Downpours threaten flooding across northern Japan into the weekend

Just over two weeks have elapsed since Typhoon Haishen battered portions of Japan and the Korean Peninsula with heavy rain and damaging winds. After a needed respite from tropical activity, moisture from former Tropical Storm Dolphin will bring heavy rain to northern Japan through Saturday.

On late Thursday night 333 mm (13.11 inches) of rain fell in 5 hours at Muroto Misaki. Of this, an incredible 130 mm (5.10 inches) fell within a single hour.

As this moisture shifts to the north, heavy rain will impact northern Honshu and eastern Hokkaido through Saturday.

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A view of former Severe Tropical Storm Dolphin as it spun near the Japanese coast on Wednesday afternoon, local time. (CIRA/RAMMB)

Rainfall through Saturday will average 24-100 mm (1-4 inches) in northern Japan, with an AccuWeather Local StormMax™ of 250 mm (10 inches) possible for areas that end up trapped under persistent, heavy downpours. This is most likely to happen across northeastern Honshu or eastern Hokkaido.

Heavy rain can lead to flash flooding and can increase the potential for dangerous mudslides.

While Dolphin is no longer a tropical storm, it will still bring strong winds across northern Japan. Wind gusts on the level of 65-90 km/h (40-50 mph) will be found along the east coast of Honshu and Hokkaido. An AccuWeather Local StormMax™ of 95 km/h (60 mph) can occur along the immediate coast.

While winds of this magnitude will not cause much in the way of structural damage, there can be sporadic power outages and downed tree branches. Travel impacts are also possible as winds slow some trains and lead to delays at airports.

Seas will continue to be dangerously rough offshore of northern Japan. Seas of 4.5-6.0 meters (15-20 feet) will make boating dangerous much of the weekend.

The threat for heavy rain will come to an end Sunday, though showers will still move across much of Japan.

For the start of next week, high pressure will build across the Korean Peninsula and expand eastward over portions of Japan allowing the area a chance to dry out.

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