Based on analysis of potential vorticity(PV)on isentropic surface and cross section before and after the typhoon landing,study is performed of the PV structure and the evolution of the landing storm interacting with midlatit ude weather systems.Evidence suggests that 1)ty phoon-related PV is a deep high PV column with a high center both in the upper and low troposphere;2)mid-level high PV advection growing with height and shears on the isentropic surface can be taken as an index for the falloutarea of the rainstorm inside the storm and on its surroundings;3)midlatit ude system has effect on the typhoon by transporting cold air into the circulation,whereby the typhoon gets decay ed quickly;4)analysis of isentropy and, particularly,its PV,is an important means for diagnosing and predicting typhoon activity and rainstorm falout region.