Typhoon 15 weakened into a tropical depression on the morning of Aug. 12 [1] after taking an unusual reverse track across the Japanese archipelago.
The transition of the storm does not signal an end to the danger. Atmospheric instability continues to pose a significant risk of flash floods and landslides, particularly as the system moves toward the Tokyo metropolitan area.
Landfall occurred in Ibaraki Prefecture on the night of Aug. 11 [2]. Strong winds during the landfall caused injuries to three people in the region [3]. The storm's atypical trajectory has left a broad area of instability across several prefectures, including Kanagawa, Chiba, and the Tohoku region, specifically Iwate and Miyagi [4].
Forecasters expect severe weather to persist through the night of Aug. 12. In Tokyo, Kanagawa, and Chiba, officials anticipate "waterfall-like" rain reaching 50 mm per hour [5]. These conditions are likely to trigger "guerrilla thunderstorms," which are sudden, localized bursts of intense precipitation that can overwhelm urban drainage systems [6].
The impact extends to western Japan, where Hyogo, Okayama, and Kagawa are monitoring the system. A level-4 surge warning was issued for northern Hyogo Prefecture [7]. Meanwhile, the coast of Hiroshima is expected to see maximum wave heights of one meter [8].
Tenki.jp (FNN) said that the storm transitioned to a tropical depression on Wednesday morning [1]. While some earlier forecasts suggested the transition would occur by the afternoon off the coast of San-in, the system weakened earlier than anticipated [1, 9].
Local authorities continue to urge residents to remain vigilant against sudden flooding and landslides as the depression moves through the region [6].
“Tokyo, Kanagawa, and Chiba are expecting 50 mm of rain per hour.”
The transition from a typhoon to a tropical depression often reduces wind speeds, but it does not eliminate precipitation risks. The 'reverse-track' nature of Typhoon 15 created a volatile atmospheric environment, meaning that even without a central eye, the remaining moisture and instability can produce extreme, localized rainfall that is harder to predict than a standard storm path.
