The onset of the Indian summer monsoon (ISM) in 2012 was investigated using the nonhydrostatic icosahedral atmospheric model (NICAM), a global cloud-system resolving model. We focused on the effect of tropical disturbances on ISM onset and considered the potential extension of onset predictability. A series of NICAM experiments was performed under various initial conditions for the period 10 May to 10 June, 2012. NICAM showed promising performance by realistically simulating ISM onset based on the initial conditions two weeks before the onset. ISM onset in both observations and simulations was accompanied by northward-migrating tropical disturbances over the Bay of Bengal and the Arabian Sea. These disturbances were generated by the eastward propagating disturbance along the equatorial Indian Ocean. As indicated by a comparison of NICAM experimental results with those obtained by the Japan Meteorological Agency Operational Ensemble Prediction System, we suggest that the better reproducing the tropical disturbance enhances the potential to extend the predictability of the transition phase in the Asian summer monsoon.