Japan's Meteorological Agency says the heavy downpour in the country was set off by an influx of extremely humid air.
The agency says sea water temperatures near Japan have been high. Rain clouds are more likely to develop in these conditions.
At the same time, typhoon Prapiroon moved northeast over the Sea of Japan. It left behind extremely damp air and a huge band of dense, vaporous clouds.
These clouds fed humid air into the active seasonal front in the affected areas -- which has led to the record-breaking rainfall.
Yasushi Kajihara of the Meteorological Agency says, "Usually, the potential of heavy rain decreases after a storm moves away, but this time was different. The potential remained high. There's been a huge and unprecedented amount of vapor in the air over the past several days, along the Pacific coast of western Japan. I believe an influx of extremely humid air is the main factor behind the persistent record-breaking downpours."