The Indian summer monsoon rainfall climatology during the monsoon onset phase in different climate zones is simulated using the regional model ARW for the decade 2000-2009, using NNRP global reanalysis. Comparisons with IMD & NNRP rainfall show that the model could simulate the onset phase in terms of progression and distribution of rainfall in most zones (except over the northeast) with good correlations and low error metrics, and improves the onset time by 1-3 days in zones 4 and 7 over NNRP.