Recently, localized heavy rains and typhoons have caused severe damage everywhere in Japan. The Japanese Ministry of Land Infrastructure Transport and Tourism (MLIT) established an urgent action plan against massive rain-related disasters in 2004. The urgent action plan includes prior release. Prior release is defined as releasing stored water from dams before heavy rainfall considering the impact on the water supply purposes of dams such as for irrigation, industry, and domestic water use. The implementation of prior release requires accurate quantitative precipitation forecast (QPF) with enough lead time because a typical Japanese river has just two or three hours of response time between rainfall and inflow into dams. In this chapter, the author provides details of the methodology and result of safe prior release of dam water. Then the chapter proposes a real-time flood control operation method for dams with ensemble weather prediction and real-time monitoring.