Based on the daily NCEP/NCAR reanalysis data,an extraordinary storm occurred in Jiangsu province on June 24—25 of 2011 is analyzed from three aspects:circulation situation,nephogram compared with vapor flux,and change of the water vapor channel.The course can be divided into three stages,the first one is caused by trough,the last by typhoon,and the second one mainly results from the interaction of them.The vapor flux divergence,vertical velocity,divergence and vertical helicity have good indication for the forecasting of the coming 9-12 h rainfall intensity.It is discovered that for this northbound and offshore typhoon (Meari),when vertical helical degree at middle and low layers increases from negative to positive,rainstorm is prone to occur in 12 h;when it decreases greatly,especially changes from positive to negative,the rainfall intensity of the coming 12 h would decrease rapidly,giving about 2 h of advanced time.Analysis of the K index indicates that the precipitation type of June 24 and June 25 respectively belong to convective and systematic one.