Impact forecasting has been applied to estimate flood damage in many different countries based on accurate weather forecasts. This study develops a model forecasting the impact of rainfall on riverside flood risks in the Busan Metropolitan City. The rainfall impact forecasting equations were obtained on a 1km x 1km grid with respect to water levels and critical rainfall thresholds. Critical rainfall was determined based on the end point of river and the discharge-water level relationship between design flood discharge and design flood level by analyzing water-rainfall in 3-hour critical rainfall. Model verification was performed using a heavy rain event with a total of 283.3 mm from July 22 to 24, 2020. According to the rainfall impact forecasting, 60 to 70% of the design flood level (“MEDIUM” alert) was issued on rivers between 10-11pm on July 23. In the rivers where the rainfall impact alert was issued, actual damages were found such as flooded roads and floodplain, which confirmed the appropriateness of the model. The high model accuracy was also validated by comparing the alert standard for water level with forecasted rainfall impact at the water level station in Suyeong River.