Japan experienced extreme weather on Monday, with record heat in Oita Prefecture and torrential rain in Nagano Prefecture [1].
These simultaneous events highlight the volatility of Japan's summer climate, where intense heatwaves and localized flash floods often occur within the same 24-hour window.
In Hita City, Oita Prefecture, the temperature reached approximately 40 degrees Celsius [1]. This marks the first "extremely hot day" recorded in the Kyushu region [1]. Temperatures at the observation point had already climbed above 30 degrees Celsius during the morning hours [1]. Other reports placed the peak temperature between 38.3 and 39.9 degrees Celsius [2], [3].
"40 degrees Celsius is unbelievable, it feels like having a fever. I am just surprised," a local resident said [1].
The extreme heat in Kyushu was driven by strong solar radiation and a high-pressure system that brought clear skies to the region [1], [4].
Meanwhile, in Inan City, Nagano Prefecture, the Japan Meteorological Agency issued a record short-duration heavy-rain warning [1], [4]. Approximately 100 millimeters of rain fell within a single hour [1], [4]. The intense precipitation created localized flooding in several areas.
"It was a tremendous sound, like something hitting the ground. What surprised me most was that this area had become a river," a nearby resident said [1].
These convective rainstorms are often triggered by the same atmospheric instability that contributes to extreme heat elsewhere in the country [1], [4].
“40 degrees Celsius is unbelievable, it feels like having a fever.”
The occurrence of a record-breaking 'extremely hot day' in Kyushu alongside a record short-duration heavy-rain event in Nagano demonstrates the increasing frequency of weather extremes in Japan. This pattern of simultaneous heat and flash flooding suggests a highly unstable atmosphere, which complicates disaster preparedness and public health responses during the summer months.
