Typhoon Khanun was on course Monday to dump prolonged, heavy rain on one of Japan's main islands this week, forecasters said, forcing commemorations of the Nagasaki atomic bombing to be held inside.
The typhoon last week reportedly killed at least two people, injured more than 100 and cut off power for several hundred thousand people in the southern Okinawa region before barrelling towards Taiwan.
The typhoon "could bring significant rainfall in wide regions," an agency official told the briefing, adding that "heavy rainfall will increase the risk of disasters".
The storm forced Nagasaki, one of the main cities on Kyushu, to scale down its annual commemoration ceremony of the 1945 bombing scheduled for Wednesday.
The ceremony is traditionally held outdoors at the city's Peace Park and is attended by government ministers, officials and thousands of guests including survivors of the bombing.
The typhoon was located east of the island of Amami Oshima on Monday afternoon, some 350 kilometres (220 miles) south of Kyushu, gusting up to 144 kilometres (90 miles) per hour, the agency said.
The slow-moving nature of the typhoon could mean rainfalls could become prolonged, increasing risks of disasters such as flooding and landslides, the agency said.