Heavy rainfall swept through multiple Japanese regions on June 18, 2026, causing localized flooding in cities including Tokyo and Kagoshima [1].
The weather pattern highlights a stark regional imbalance in precipitation. While some areas face flooding, critical water infrastructure in the Kanto region is struggling with a lack of replenishment despite the broader rainy season.
In the Tokara Islands, Takarajima recorded 304.5 mm of rainfall over a 24-hour period [1]. This amount set a new record for the month of June [1]. Other areas affected by the strengthening plum-rain front and associated low-pressure systems included Shibuya in Tokyo, Kofu in Yamanashi, and Kanoya in Kagoshima [1].
However, the Yagisawa Dam, often referred to as the Kanto water reservoir, is experiencing a different trend. Located in the town of Minakami in Gunma prefecture, the dam's storage was nearly full on May 7, 2026 [1]. By June 15, 2026, those storage levels had fallen [1].
Meteorological reports said that the decline at the Yagisawa Dam is due to prolonged dry conditions specifically affecting northern Gunma [1]. This lack of local rain persists even as other parts of the country deal with excessive downpours.
FNN reporters documented the disparity between the urban flooding in Shibuya, where pedestrians walked through ankle-deep water, and the receding water lines at the reservoir [1].
“Takarajima recorded 304.5 mm of rainfall over a 24-hour period”
This situation underscores the volatility of the 'tsuyu' or plum-rain season, where extreme precipitation is often highly localized. The drop in the Yagisawa Dam's storage during a period of national heavy rain demonstrates that regional water security can remain vulnerable even during a wet season if the specific catchment areas for major reservoirs remain dry.
