TOKYO -- The intense heat wave in Japan shows no signs of abating with the mercury in Kyushu nearing 40 degrees Celsius on Sunday, weather forecasts showed.

Wide areas of western and eastern Japan are expected to experience temperatures exceeding 35 degrees Celsius during the day, triggering heatstroke alerts in 37 prefectures.

With the heat persisting through the night, daytime temperatures on Sunday were forecast to reach dangerously high levels, with Kurume in Fukuoka expecting a maximum of 39 degrees Celcius and Saga and Kumamoto predicting highs of 38.

Tokyo is set to experience 35 degrees Celcius, marking its first extreme heat day in four days, while Nagoya is bracing for its 11th consecutive day of temperatures at 37.

As of 11 am local time on Sunday, the heat stress index, or Wet Bulb Globe Temperature (WBGT), reached the red level in most parts of the country, representing the highest level of heatstroke risk on the country's five-tier system, official data showed.

The WBGT, which measures heatstroke risk, takes into account temperature, humidity, and sunlight intensity as one of the empirical indices representing the heat stress to which an individual is exposed.

Authorities have advised residents to stay indoors in cool environments and plan summer activities that minimize exposure to the heat.

Residents are urged to take heatstroke precautions seriously and adjust their plans accordingly to ensure safety during this severe heat wave.