A cold air mass at high altitudes and warm, damp air are causing extremely unstable atmospheric conditions in western and eastern Japan.
Heavy rain is expected to continue through Tuesday in some places. The Japan Meteorological Agency is urging people to be on the alert for mudslides, swollen rivers, and flooding in low-lying areas.
The agency says cold air is flowing in over Japan at high altitudes, and warm, damp air is also moving in, circling the edge of a high pressure system in the Pacific.
Forecasters say this is triggering extremely unstable conditions mainly on the Pacific coast of western and eastern Japan, and causing localized rain clouds to develop.
Rainfall of 66 millimeters per hour was observed in the hour to 6 p.m. on Monday, in the town of Tokigawa in Saitama Prefecture, near Tokyo.
A video posted to NHK’s Scoop Box website shows flashes of lightning streaking across the sky from left to right in front of Higashi-Omiya station in Saitama City.
The woman who shot the video says she was surprised to see so much lightning. She says the rain started long before the loud rumbles of thunder, and it was very scary.
Through Tuesday, atmospheric conditions are likely to remain extremely unstable, mainly in western and eastern Japan. Extremely heavy thundershowers are forecast in some areas.
Forecasters are also calling on people to be alert for lightning strikes, tornadoes, gusts, and hail.
They say people should get to a safe place if they notice signs of developed cumulonimbus clouds approaching, such as sudden bursts of cold wind.