A 72-hour precipitation in a storm event over the American River Watershed (ARW) in California is maximized using a physically-based numerical model for a regional atmosphere. A storm event over ARW is caused by a high-moisture atmosphere coming from the Pacific Ocean, called «atmospheric river.» Moreover, the path of an atmospheric river affects the amount of precipitation over the ARW. Therefore, the contribution of an atmospheric river to 72-hour precipitation over ARW is maximized by spatially shifting the atmospheric conditions for the numerical atmospheric model. In this study, this methodology is applied to the 1997 January storm event, one of the severest storm events over the ARW. Consequently, the 72-hour precipitation is maximized by shifting the atmospheric conditions southward by 6.0 degrees and westward by 1.0 degree, and the maximized value results in 568 mm. The results show that a historical storm event had the capability to cause higher precipitation, and the presented methodology is effective for the estimation of maximum precipitation over the ARW.