A heavy downpour dumped more than 100 millimeters of rain per hour over parts of typhoon-ravaged Chiba Prefecture on Saturday morning.
The Meteorological Agency says the combination of damp air flowing into a rain front and a low-pressure system is creating unstable atmospheric conditions over areas of eastern Japan and the Tohoku region in the northeast.
In the hour to 8 a.m., 110 millimeters of rain fell near the town of Yokoshibahikari and the cities of Sammu and Sosa in Chiba Prefecture.
The Meteorological Agency issued a warning for unprecedented hourly rainfall for those areas.
The risk of mudslides is increasing in Chiba Prefecture, where Typhoon Faxai left an extensive trail of damage last month. Some communities have been placed on alert.
Weather officials say torrential rain has already peaked in the Kanto region, but that unstable atmospheric conditions are expected to continue and rain is forecast mainly in the Tohoku region.
In areas hit by Typhoon Hagibis last weekend, some of the breached dikes have yet to be repaired, and the ground has loosened.
Authorities are warning that even light rain could cause further landslides and flooding. They're urging people to stay away from swollen rivers and mountain slopes.