The Mahaweli development programme contributes to 13% of the national hydroelectric power production and also the major water supply for Dry Zone of Sri Lanka. When considering Mahaweli cascade, the Kotmale reservoir affords a major contribution. The water level of the reservoir directly depends on rainfall pattern of the catchment area of the reservoir. Early prediction of rainfall pattern helps for efficient water management by predicting the water level of the reservoir. The time series analysis is a main statistical tool for predicting metrological phenomena. Therefore, in this study, the rainfall pattern of Kotmale station was modeled and forecasted using time series analysis with R software. The monthly rainfall data from 2003 to 2017 in the Kotmale station was considered. The study revealed that, SARIMA (0, 1, 1) (2, 1, 2)12 as the best fitted model for forecasting the rainfall pattern at the Kotmale station. Using the best fitted model, the rainfall pattern in the Kotmale station was forecasted.