Japan’s Meteorological Agency said the most recent deluge was brought by warm, humid air flowing into the country’s east that combined with cold air above to make atmospheric conditions extremely unstable.
The rains are expected to last through late on Friday.
“This case was an extremely unusual situation, even by Japan’s historical weather standards,” Chiba Governor Toshihito Kumagai told reporters on Friday morning.
“I have responded to many disasters in the past, but I have never experienced a case like this.”
Chiba, which sits right next to Tokyo, has logged more than 100mm of rain per hour since Thursday afternoon, authorities say.
This is about three times the average rainfall in August, and the highest volume of precipitation ever recorded in the country.
Authorities would prioritise saving lives and persist with rescue efforts, Kumagai added.
At least eight people have died, according to reports from public broadcaster NHK, Kyodo news agency and Asahi newspaper.
Among those killed was a woman who died inside her car submerged on a flooded road, authorities said. Two others collapsed along the streets and later died.
The Japan Meteorological Agency earlier described the situation as an “unprecedented level of heavy rain”.
Several rail services remain suspended on Friday morning while some major highways are also shut.
More than 100,000 households have been asked to evacuate on Thursday after the rain triggered landslides and power outages.
