Temperatures are soaring in the Kanto-Koshin region including Tokyo, with the mercury already topping 35 degrees Celsius in some cities.
The Japan Meteorological Agency is forecasting mostly cloudy and sunny weather for the region on Saturday as it is in the grip of a high-pressure system.
By 11 a.m., the temperature in Ebina City, Kanagawa Prefecture, hit 36.8 degrees, and that in central Tokyo 35.8 degrees.
The mercury is expected to rise further during the day. The expected highs for the cities of Kumagaya and Saitama in Saitama Prefecture, Maebashi City in Gunma Prefecture, and Tsuchiura City in Ibaraki Prefecture are 37 degrees.
The temperature is projected to hit 36 degrees in central Tokyo and Kofu City in Yamanashi Prefecture, and 35 degrees in the cities of Yokohama, Chiba and Utsunomiya.
Humidity is also expected to be high. The agency and the Environment Ministry have issued heatstroke alerts for the prefectures of Ibaraki, Tochigi, Saitama, Tokyo, Chiba and Kanagawa.
In the afternoon, the region can expect fairly unstable atmospheric conditions, which could trigger heavy thundershowers late into the night.
The agency is warning of mudslides, flooding in low-lying areas and swollen rivers.