Japanese weather officials say atmospheric conditions are extremely unstable in areas from western through northern Japan.
They are warning that lightning strikes, tornadoes, strong gusts of wind, hail and localized downpours may occur in western and northern Japan later on Friday and in eastern Japan from Friday until early on Saturday.
The Meteorological Agency noted that atmospheric conditions are extremely unstable over vast areas. It said that is because warm and humid air is flowing into a cold front that extends southward from northern Japan's Hokkaido. It added that there is another stationary front over the southern part of Kyushu, which is located in southwestern Japan.
There was intermittent heavy rain in Kyushu and Shikoku, western Japan, on Friday morning. On Kagoshima Prefecture's Yakushima Island, 41 millimeters of rain was recorded during a one-hour period up to 8:30 a.m.
Developed rainclouds have been seen in Hokkaido, in northeastern Japan's Tohoku region and in central Japan's Tokai region. Lightning strikes have also been observed.
Officials are advising people to take shelter in sturdy buildings, if the sky suddenly turns dark or cold winds start to blow. They say these may be signs that thunderheads are approaching.