The storm, packing sustained winds of up to 107 kilometers (67 miles) per hour, was moving in waters south of Japan, near the main southwestern island of Shikoku.
It had been downgraded from a typhoon earlier in the week. It was expected to hover eastward, affecting Honshu as well as smaller islands such as Izu by early Sunday.
The storm was not expected to seriously affect Tokyo or make direct landfall.
Weather forecasts warned of a heavy downpour and fierce winds, which may cause rivers to swell and towering waves in coastal areas as well as mudslides.
Yuri Kageyama is on Twitter https://twitter.com/yurikageyama