Chan-hom weakens, but still remains a threat to Japan

Typhoon Chan-hom is expected to take a swipe at Japan by the beginning of the weekend, following in the footsteps of Severe Tropical Storm Dolphin, which tracked across the region in late September.

A disturbance located in the middle of the West Pacific tropical basin that had been largely stationary late last week became more organized over the weekend. On Monday, the disturbance developed into Tropical Storm Chan-hom.

Satellite loop of Chan-hom churning close to Japan Thursday evening, local time. RAMMB/CIRA

The storm developed at latitude 22 degrees north, a rare occurrence for the month of October. According to Sayaka Mori, a meteorologist for the Japanese broadcaster, NHK, only 10 percent of October tropical storms form north of the latitude 20°N over the Western Pacific.

As Chan-hom neared the northern Ryukyu Islands and southern Japan on Thursday, a 130 km/h (81 mph) wind gust was reported at Onoaida, located on an island well west of the track of the center of the storm.

As of Friday morning, local time, Chan-hom had sustained wind speeds of 130 km/h (81 mph) near the center of the storm and was drifting to the north as a typhoon.

The window for strengthening has already closed as Chan-hom tracks to the north and east, into cooler waters. As of Friday night, local time, sustained wind speeds have already dropped to 111 km/h (69 mph). While the storm has been designated a severe tropical storm, it is still equivalent to a Category 1 hurricane on the Saffir-Simpson Hurricane Wind Scale.

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This storm is expected to gradually lose wind intensity through the weekend as it tracks over the cooler waters just off the coast of southern Japan.

Regardless of decreased wind intensity or even a transition to a nontropical storm, Chan-hom is still expected to bring widespread impacts to parts of southern Japan.

Rain, some of which was heavy, was already falling across southern and central Japan on Thursday and Friday. Eastern Shikoku, Kansai, Chubu, Kanto and southern Tohoku are expected to see the heaviest rain through Sunday.

The risk for flooding and mudslides will increase through the end of the week and into the weekend as rainfall totals can climb as high as 100-200 mm (4-8 inches) near where the storm tracks in southern Japan. An AccuWeather Local StormMax™ of 400 mm (16 inches) will be possible, especially in the rugged terrain from Shikoku to southern Kansai.

The strongest winds expected from Chan-hom have already passed as the storm continues to lose wind intensity, but any gusts that occur along the southern shores of Japan can still lead to downed trees and power lines, localized structural damage and power outages.

This strong onshore flow ahead of the storm will add to the risk for flooding along exposed beaches as winds push ocean water inland.

Chan-hom is forecast to follow a path similar to that of Severe Tropical Storm Dolphin, which tracked near southern Japan nearly two weeks ago. Dolphin brought areas of downpours to the region before quickly transitioning into a nontropical system as it raced away.

On the night of Sept. 17, 333 mm (13.11 inches) of rain fell in five hours at Muroto Misaki. Of this, an incredible 130 mm (5.10 inches) fell within a single hour.

While residents in Japan will be tracking Chan-hom into the weekend, anyone with interests in Vietnam will have to keep an eye on the tropics as AccuWeather meteorologists are monitoring the potential for tropical development in the South China Sea into early next week.

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