Abstract: The effects of synoptic scale circumstances on the occurrence and growth of the mesoscale systems in the process of torrential rain are studied by using the synoptic and dynamic analyses methods and using a global spectral model T42L12 and a mesoscale model MM4 experiments, and from which several forecast clues of torrential rain over the middle and lower reaches of Yangtze River and Yellow River are presented. It is helpful to the operational forecast for correlative meteorological services