A strong rainstorm occurred in the Jianghuai River Valley from 8th to 9th July 2003.The rainstorm case has been analyzed using the NCEP/NCAR reanalyzed data(4 times per day)including wind,humidity and height fields.The results show that the heavy rain wasmainly associated with themerging ofmesoscale vortices.The strengthening of the lower level convergence and upper level divergence was associated with the vortex-merging enhanced the updraft,moisture transfer andmoist helicity,resulting in heavy precipitation.The allocation of lower level convergence and upper level divergence was favorable to themaintenance of strong updrafts,and themoisture and energy carried by the updrafts was in turn favorable to the genesis and development of convective cloud cluster.The warmmoist flow from the Bay of Bengal and the South China Sea provided the rainstorm sufficientmoisture.