Abstract:This paper analyzes the cause of the extremely heavy rainfall in northeastern Zhejiang induced by Typhoon Chan Hom (1509) from 10 to 11 July 2015 by using conventional observation, AWS data, satellite and NECP reanalysis data. The results show that under the effect of the special terrain in northeast of Zhejiang, strong intensity and wild impaction of Chan Hom meeting with the long time northeast airflow contributed for this heavy rain process. During the early stage of the Chan Hom impact, this area was in a state of convective instability. Besides, the abundant water vapor, low level convergence and upper level divergence provided a beneficial condition for the heavy rainfall. In the rainstorm areas, the MPV1 changed from negative to positive at low level, leading to an increase of vertical vorticity. The long time maintenance of the high θse energy region at 925 hPa also benefited the updraft. The long time transportation of water vapor was not only favorable for Chan Hom’s maintenance, but also beneficial to the formation of the heavy rainfall. There was a positive correlation of 6-12 h in advance for the increasing and decreasing of the heavy rainfall with the vapor transportation, surface convergence zone and θse at 925 hPa. And the special topography enhanced the heavy rainfall supported by the northeast airflow because of the effects of the upslope and terrain convergence, which were formed by the trumpet shaped terrain of Hangzhou Bay, Siming Mountain, elongated northeast southwest Xiangshan Harbor and its southern mountains.