Weather radar is one of equipments that can provide real time meteorological data covering large area. A lot of activities in the capital of Thailand, Bangkok depend on the rainfall information in advance. Therefore, forecasting rainfall information is helpful for flood mitigation in the region. The Window Correlation Matching Method (WCMM) is used to determine Z-R relationship for radar calibration and it takes into account the wind effecting to rain measurement and the average reflectivity of different number of pixels in radar image. In this study, the calibration of radar is obtained with fairly satisfactory values for 60 kilometers radar range. Critical Success Index (CSI) is found to be 65 % when translation model is developed to forecast five minute time interval of rainfall over all selected fifteen areas in Bangkok. The present study proposed forecasting algorithm which can provide information of rainfall direction, time duration of rainfall. The new indicator, Critical Success Index by Time (CSIT) has been introduced to determine the accuracy of forecast at a particular location. The forecasted result shows that CSIT is about 67% on average.