By using the products of ECMWF,T213,T106,and various physical parameter fields,the large-scale conditions for mesoscale system occurring and the statistical occurrence probability of mesoscale systems are discussed.The contrast analysis is conducted between the severe convective weather models that have been summed up by the Wuhan Central Meteorological Office and the circulation patterns from numerical weather prediction(NWP) products.When the NWP forecast circulation according with the severe convection weather model,the mesoscale weather would be resulted from the large scale conditions.The diagnostic analysis is made of the relative physical factors of the numerical forecasting products.If the results show that the mesoscale circulation is favorable to the occurrence of mesoscale weather,then the raining areas and the precipitation characteristics of the severe convection weather system are diagnosed and analyzed.Meanwhile,the AREM mesoscale model products are used to diagnose and analyze the weather process of 21 July 2002,and good results are received.