Japanese weather officials forecast rain and thunder storms will hit wider parts of western Japan from Thursday night through Saturday.
The Meteorological Agency expects a seasonal rain front and a low-pressure system to approach the region on Friday and later.
Warm and humid air may flow into the front and the low-pressure system, creating an unstable atmospheric condition.
The weather officials say torrential rain accompanied by lightning is expected to hit the southern part of the Kyushu region from Thursday night, the northern part from Friday noon, and the Shikoku and Kinki regions from Friday night.
The officials warn that heavy localized downpours of more than 50 millimeters per hour are likely in some places.
They say the rain front is expected to linger along the Pacific coasts of wider regions from western to eastern Japan until Saturday.
The agency is calling on people to be cautious about mudslides, flooding in low-lying areas, overflowing rivers as well as lightning strikes, and sudden gusts of wind including tornadoes.