The model reproduces a train of mesoscale convective systems that simultaneously develop along the synoptic-scale Meiyu front and coincide with an arrival of monsoon air at night, consistent with observations. This process is primarily regulated by a nocturnal low-level jet (NLLJ) in the active southwesterly monsoon that forms southern China and extends to central China. The NLLJ mainly penetrates south–north-oriented valleys and strengthens moisture transport by more than 50%. It leads to three preferred regions of elevated conditionally unstable air at its northern terminus, where an enhanced low-level ascent overcomes small convective inhibition triggering new convection. A budget analysis on convective instability further shows that the intrusion of moist monsoon air can generate a large convective available potential energy prior to convection initiation, while free-atmospheric forcings are much weaker. The instability precondition is mostly induced by the NLLJ-related horizontal advection (100–190 J/kg per hour). Although the advection effect declines in convective phase, the instability production by vertical lifting increases to 100–160 J/kg per hour, suggesting that updrafts may exploit near-surface moisture to offset energy depletion for sustaining convection. The convection dissipates in morning with decaying LLJ and moisture. Thus, the multi-scale interactions among the diurnal forcings, mesoscale disturbances, synoptic-scale front, and regional monsoon are essential for the prediction of the morning-peak heavy rainfall corridor.