The torrential rainfall in the middle and northern parts of Zhejiang Province on 7 June 2013 was diagnosed based on the latest ECMWF ERAInterim global reanalysis data with 0.75°×0.75° resolution. The results show that the torrential rain was caused by two developing eastwardmoving systems-an upperlevel shortwave trough and a lowlevel southwest vortex, and the torrential rainfall area over the middle and northern of Zhejiang are located in the coupling area of the upperlevel and lowlevel jet streams. A large amount of instability energy was stored below 700 hPa before the occurrence of rainstorm, which was triggered by the southwest vortex close to the torrential rainfall area. The analysis of environmental fields by means of physical decomposition of atmospheric variables shows that background ambient was in favor of this regional rainstorm, causing the instantaneous anomaly field to trigger the torrential rainfall at last. The torrential rainfall is obviously located in the anomalous warm convergence shear line at 850 hPa.