Japanese weather officials say downpours will bring more than 30 millimeters of rain per hour to Pacific coastal areas of central and eastern Japan overnight Tuesday and into Wednesday.
The Meteorological Agency says warm moist air flowing toward a stationary seasonal rain front brought torrential rain to Kyushu in the southwest on Tuesday morning.
Developed rainclouds are gradually moving eastward.
The Tokai region encompassing Nagoya and Kanto, which includes Tokyo, can expect heavy rain accompanied by thunder through Wednesday morning.
Some parts of Tokai and the Izu Islands in the Pacific south of Tokyo could have up to 120 millimeters of rainfall, and southern Kanto up to 100 millimeters, in the 24-hour period through Wednesday evening.
Weather officials are urging people to be on the lookout for landslides, flooding in low-lying areas and swollen rivers.
People are also advised to be aware of the danger posed by possible lightning strikes and sudden gusts of wind.