Abstract: The earliest torrential rain event over South China in 1996 is simulated, and a series of sensitivity experiments are carried out using a nested grid version of PSU/NCAR’s mesoscale model (MM5) with a fine mesh grid size of 20 km. It is shown that the MM5 model reproduces successfully the development of the torrential rain and mesoscale system; latent heat release acts a vital role in the development of the torrential rain event by a mechanism similar to CISK; the low-level southwest jet transports potential unstable energy to the torrential rain region, and interacts with torrential rain to maintain and strengthen itself; the store of the low-level potential unstable energy over the South China Sea and the South China region at initial time (12 h before the occurrence of the torrential rain) is a crucial condition for the formation of the torrential rain