Japan is facing a combination of extreme heat and atmospheric instability, with temperatures expected to exceed 35 °C in several regions this Thursday [1].

These diverging weather patterns pose simultaneous risks of heat-related illness and flash flooding, complicating public safety efforts across the archipelago.

Forecasters said that the Kanto region will experience unstable atmospheric conditions throughout Thursday morning and afternoon [1]. This instability may lead to localized heavy rain and thunderstorms, particularly in the plains of the region [1, 2]. The volatility is attributed to an approaching low-pressure system, which consists of the remnants of Typhoon No. 9 and an associated front [1, 2].

While Kanto deals with storm risks, western to northern Japan is bracing for a severe heatwave. This includes Hokkaido, Tohoku, Hokuriku, and parts of Kyushu [1, 2]. Maximum temperatures in these areas are forecast to rise above 35 °C [1].

Nighttime relief will be limited for many residents. Minimum temperatures are expected to remain at or above 25 °C [1]. This threshold results in what is classified as tropical nights, where high humidity and heat persist well after sunset [1].

The combination of the remnants of a typhoon and a stagnant high-pressure system is driving these extremes. The atmospheric destabilization in the east occurs while the western and northern sectors remain trapped under intense summer heat [1, 2].

“Maximum temperatures will exceed 35 °C in western to northern Japan.”

The simultaneous occurrence of a heatwave in the north and west and thunderstorm instability in the Kanto region demonstrates the complex interaction between tropical remnants and seasonal fronts. The persistence of tropical nights—where temperatures do not drop below 25 °C—increases the risk of heatstroke by preventing the human body from recovering during sleep.