Record 485mm Rainfall Triggers Level 5 Landslide Warnings in Tokyo's Izu Islands

While historical averages and seasonal forecasts suggested a standard transition into early September for the Izu archipelago, a stationary autumn rain front fueled by Tropical Cyclone Krovanh shattered those expectations, dumping a record 485 millimeters (19.1 inches) of rain on Niijima in just 24 hours. The unprecedented deluge prompted the Japan Meteorological Agency (JMA) to issue Level 5 emergency landslide warnings—the highest alert on its five-tier scale—for the villages of Niijima and Toshima, as well as the town of Oshima. The sudden escalation caught many off guard, transforming a typical late-summer weather pattern into a severe localized emergency that threatened to destabilize the islands' steep volcanic terrain.[1][2]

"Level 5 indicates a life-threatening situation in which a disaster may already have occurred, and immediate protective action is required," the JMA stated in its emergency bulletin, underscoring the severity of the threat. In Niijima, a volcanic island located approximately 160 kilometers south of the capital, the 24-hour rainfall total completely shattered the previous all-time record of 333.5 millimeters, which was set in July 2021. The sheer volume of water overwhelmed local drainage systems and saturated the ground to a point where catastrophic slope failures became an imminent possibility.[1][5]

The extreme weather was not confined to Niijima alone, as neighboring islands in the chain absorbed similarly historic deluges. The main observation station on Oshima recorded 581.5 millimeters (22.9 inches) of rain, marking its highest September total on record and pushing the island's infrastructure to its limits. Toshima and Kozushima also saw their all-time 24-hour rainfall records broken, registering 384 millimeters and 375.5 millimeters respectively. The widespread nature of the record-breaking rain highlighted the intensity of the stationary front parked over the region.[1]

Addressing the public during an emergency press conference, Takuya Hosomi, director of the JMA's Forecast Division, urged residents across the affected islands to prioritize their immediate safety by moving to sturdy buildings situated away from cliffs and ravines. The volcanic soil that characterizes the Izu Islands is notoriously susceptible to sudden, deep-seated slope failures when heavily saturated, making the proximity of residential areas to steep inclines a critical vulnerability during such extreme weather events.[2][4]

The Izu Islands, which are administratively governed by the Tokyo Metropolitan Government, form a chain of volcanic outcroppings known for their rugged topography, dense forests, and steep coastal cliffs. Niijima, primarily composed of a rare volcanic rock known as rhyolite and internationally recognized for its pristine surfing beaches, supports a year-round population of roughly 2,500 people. These residents found themselves directly in the crosshairs of the intensifying rain front, forced to navigate the logistical challenges of sheltering in place on an isolated landmass with limited immediate external support.[4][5]

The destructive impact of the storm system extended well beyond the remote islands, causing severe disruptions and triggering localized emergencies across the broader Kanto region. Heavy rain inundated low-lying areas of metropolitan Tokyo, as well as neighboring Chiba, Kanagawa, and Saitama prefectures, prompting authorities to issue widespread Level 4 landslide and flood alerts. The cascading weather warnings reflected the scale of the atmospheric river feeding the system, which continuously funneled moisture from the Pacific Ocean over eastern Japan.[1][2]

In Fukushima Prefecture to the north, the relentless heavy rain ultimately proved fatal as local infrastructure failed to cope with the sudden influx of water. Emergency services in Iwaki City received urgent reports of submerged vehicles trapped in a rapidly flooding underpass. Rescue crews managed to pull a woman unconscious from one of the submerged cars, but she was later pronounced dead at a local hospital, marking the storm's first confirmed casualty. A second driver involved in the underpass flooding managed to escape their vehicle unharmed.[1][3]

Transportation networks across eastern Japan ground to a halt as the rain intensified and floodwaters encroached on critical rail infrastructure. The East Japan Railway Company was forced to suspend services on regional lines, including portions of the Yamagata Shinkansen, while major commuter arteries in Tokyo operated at significantly reduced capacity. The widespread transit disruptions affected tens of thousands of daily commuters, paralyzing movement in one of the world's most densely populated metropolitan areas as operators prioritized passenger safety over schedule adherence.[1][2]

The JMA noted that the extreme rainfall was heavily exacerbated by warm and moist air from Tropical Cyclone Krovanh, which flowed directly into the stationary frontal system and supercharged its precipitation output. As the front slowly begins to drift north, meteorological authorities continue to warn that the ground across the Izu Islands and the Kanto region remains dangerously saturated. This lingering saturation keeps the threat of delayed landslides exceptionally high, requiring residents to maintain their vigilance even after the heaviest downpours have finally subsided.[1][2]