Abstract:A synoptic dynamic diagnosis of the heavy rain event in the Huaihe River valley from 4 to 7 July 2003 is carried out with the theory of isentropic potential vorticity (IPV). The results show that the westerly jet on the isentropic surface transported the high potential vorticity to the lower reaches, while the strong NW flow traversing the isobars became a strong sinking flow and transported the high IPV southward from higher layers and higher latitudes. This process kept a high IPV above the rain area and thus counted for the persistency of the heavy rain band. High IPV area is closely associated with heavy rain and thus can be a significant predictor.