Wide parts of western and eastern Japan are expected to continue experiencing oppressive heat on Tuesday.
Precautions are advised against heatstroke.
The Japan Meteorological Agency says a high pressure system will prevail over mainly western and eastern Japan, pushing up temperatures.
Daytime highs are expected to reach a dangerous level of 38 degrees Celsuis in Kurume City in Fukuoka Prefecture and Hita City in Oita Prefecture.
The mercury is likely to hit 37 degrees in the cities of Osaka, Kyoto, Saga, Kumamoto and Takamatsu in Kagawa Prefecture, and 36 degrees in Nagoya and Hiroshima cities.
The Meteorological Agency and the Environment Ministry have issued heatstroke alerts for 29 of Japan’s 47 prefectures, mainly in western Japan.
People are advised to avoid going out or exercising outdoors, and to adequately consume fluids and salt. Special attention is needed for infants and the elderly, who are at higher risk of developing heatstroke.
Meanwhile, weather officials say atmospheric conditions are extremely unstable from the Tohoku region to western Japan, with localized rain clouds developing over some areas.
Rainfall in the 24 hours through Wednesday morning could reach up to 100 millimeters in the Kanto-koshin region, and 80 millimeters in the Tohoku region.
The Meteorological Agency is warning of flooding in low-lying areas, swollen rivers and mudslides.
It is also urging caution against lightning strikes, hail, and violent gusts, including tornadoes.
People are advised to seek shelter in sturdy buildings when there are signs of thunderheads approaching.