The WRF-single-moment-6-class (WSM6) microphysics scheme has been one of the options of microphysical process in the WRF model since August 2004. This scheme predicts the mixing ratios for water vapor, cloud water, cloud ice, snow, rain, and graupel. The characteristics of the cold rain process in the WSM6 scheme follow the revised ice microphysics process (Hong et al ), whereas the warm rain processes are primarily based on the works of Kessler and the auto conversion process from the Tropoli and Cotton. The daily forecasts at NCAR have shown that the WSM6 scheme works successfully in predicting mesoscale convective systems, but it sometimes overestimates the peak intensity and underestimates the areas of anvil clouds. We attempt to improve such existing deficiencies in the WSM6 scheme by incorporating the prediction of number concentrations for warm rain species. This new method uses the approach proposed by Cohard and Pinty (2000) to determine the autoconversion rates and allows a more sophisticated coupling between cloud field and number concentrations of warm species. Double-moment prediction for the warm species in WSM6 scheme will allow more flexibility of the size distribution enabling the mean diameter to evolve in contrast to the one-moment scheme. The effects of the double-moment warm rainfall physics in the WSM6 scheme will be discussed in the conference.