Tropical cyclone Nargis, generated over the Bay of Bengal in late April 2008, caused catastrophic destruction after making landfall in Myanmar. Here, the large-scale environment of cyclogenesis was investigated using reanalysis datasets and a cloud-system resolving model. The reanalysis datasets showed that a westerly wind axis over the Bay of Bengal shifted northward from mid-April to early May. This shift is attributed to a seasonal transition of the Asian summer monsoon and a boreal summer intraseasonal oscillation. The timing of this environmental modulation was consistent with the climatologically early tropical cyclogenesis over the Bay of Bengal. This period was also characterized by high genesis potential, which is an empirical index of the environmental field favorable for tropical cyclogenesis. An analysis of genesis potential showed that the genesis of Nargis was associated with reduced vertical shear and increased lower-tropospheric vorticity. A cloud-system-resolving model successfully reproduced the high probability of tropical cyclogenesis during the observed period of cyclogenesis in late April. The model also simulated the large-scale environment including the northward shift of the westerly wind axis, although the precise location of tropical cyclogenesis was sensitive to initial conditions in the model. The anomaly of sea surface temperature in 2008 had little influence on the simulated probability of tropical cyclogenesis. Therefore, a cloud-system-resolving atmospheric model even without ocean feedback is a promising tool for predicting the probability of tropical cyclogenesis over the Bay of Bengal around the onset of the Asian summer monsoon, which is a favorable environment for tropical cyclogenesis.