Abstract：Based on the routine observational data, a diagnostic analysis is performed of the severe storm on 18—19 July 2008 in Weifang, Shandong Province. It is shown that the heavy storm is caused by the interaction between a weak cool front in the westerlies and the warm and wet air along the path of typhoon owing to the powerfulness and stability of the subtropical high and atmospheric blocking in Baikal. The southeast warm and wet air along the path of typhoon is the main vapour source of the rainstorm. The heavy rain occurs in the powerful updraft condition with convergence at low level and divergence at high level. The convergence layer is thick, which is the cause of heavy rain. The 925hPa layer is an important layer with respect to the event because of its role in supplying plentiful vapour. Concentrated period of time of positive vorticity center corresponds to the period of heavy rain, which is propitious to the development of convection. It occurred in the north of the very stiff and dense section of moist isentropes and in the low troposphere of strong convective instability (MPV1<0). The region of positive MPV2 at low level has a very good correlation with the location of rainstorm. The distribution of MPV2 at low level may provide a good indicator for the forecast of rainstorm region.