Severe thunderstorms caused flooding and landslides in Tochigi and Gunma prefectures on Friday and Saturday, prompting the highest level of emergency alerts.

The scale of the rainfall overwhelmed local drainage systems, turning residential streets into rivers and triggering landslides that collapsed roads. This event highlights the increasing vulnerability of regional infrastructure to sudden, extreme weather patterns.

Authorities in Sano City and Ashikaga City in Tochigi, along with Ota City in Gunma, issued level-5 "Emergency Safety Assurance" orders. This is the most critical alert level, instructing residents to take immediate action to save their lives.

In Ashikaga City, rainfall reached a record 128 mm within just three hours on July 17 [1]. The deluge led to significant structural damage. As of 6 a.m. Saturday, Tochigi Prefecture reported that 30 buildings were damaged, including one total collapse, 11 instances of floor-up flooding, and 18 instances of floor-down flooding [2].

Gunma Prefecture reported similar devastation in Ota City and the Maebashi area. Officials said that at least 73 buildings were affected, with floor-up flooding in 23 buildings and floor-down flooding in 50 buildings [3].

Emergency services coordinated evacuations as water levels rose rapidly. A total of 19 people were evacuated in Tochigi Prefecture [4], while 91 people were moved to safety in Gunma Prefecture [5].

The storms were driven by a double-typhoon-like system that stalled over the region, concentrating moisture and energy into a narrow corridor. Local authorities said the combination of saturated soil and intense precipitation made the slopes in Tochigi particularly unstable, leading to the road collapses observed in Ashikaga.

“Rainfall reached a record 128 mm within just three hours on July 17.”

The issuance of level-5 alerts in multiple municipalities simultaneously indicates a failure of standard drainage and flood mitigation systems to handle current precipitation peaks. The record-breaking nature of the three-hour rainfall suggests that 'once-in-a-century' weather events are becoming more frequent, necessitating a shift in how Japan manages urban runoff and slope stability in the Kanto region.