The Japan Meteorological Agency has urged residents to evacuate parts of the southern island of Kyushu, ahead of a large and powerful typhoon expected to bring up to 500 millimetres of rain when it makes landfall.
Nanmadol, classified as a super typhoon by the US Navy's Joint Typhoon Warning Center, has the potential to be the most destructive tropical storm to strike Japan in decades.
Japan's weather agency said it may issue a "special warning" for Kagoshima prefecture and other parts of Kyushu, Japan's southernmost main island, as early as Saturday evening with the possibility of high waves and heavy rains in the regions.
It would be the first such alert for any prefecture north of the Okinawa island chain, domestic media reported.
"Unprecedented" storms and rainfall could strike the area, JMA official Ryuta Kurora said at a televised press conference, urging residents there to evacuate before it gets dark.
Southern Kyushu could receive 500mm of rain on Sunday, while the central Tokai region could see 300mm, the agency forecast.
Nanmadol, the 14th typhoon of the season, was near Japan's southern Minami-Daito Island heading northwest at 20km/h on Saturday afternoon.
Winds at the centre of the storm are blowing at 200km/h, gusting up to 270km/h, according to the JMA.
The storm, equivalent in strength to a Class 5 hurricane in the Atlantic Ocean, is forecast to curve east and pass over Tokyo on Tuesday before moving out to sea by Wednesday.
Domestic broadcasters aired footage of strong winds and rain are already lashing down on Japan's southern island chain of Okinawa as the storm approached.