Flood warning and effective radial gate operations of Deduru Oya reservoir are important in flash flood controlling and reducing the impact of flash floods. Future rainfall forecast is essential to obtain runoff forecast into the reservoir and for the hydrological modelling. A limited number of studies have been conducted on weather forecasting in Sri Lanka. This study proposes a weather forecasting model for the basin using Weather Research and Forecast (WRF) model. WRF model was optimized by doing a series of simulations with multiple microphysics and cumulus physics parametrization schemes. Optimum physics combinations were identified considering two extreme rainfall events on 2 November 2015 and 16 May 2016, and the results were validated for the extreme rainfall event on 1 September 2017. WSM3 and Betts-Miller-Janjic cumulus schemes were found to be the optimum parameterization schemes for microphysics and cloud microphysics, respectively. The developed model can be used for weather forecasting in the region.