The abnormal severe rainfall resulting from depression circulation of Typhoon Fitow (1323) is analyzed using routine observation data, AWS data, Liangmao Mountain wind tower data, radar data, satellite data and NECP reanalysis data. The results show that the heavy rainstorm occurs during the weakening of Typhoon Fitow. When the weak cold air to the north of Fitow invades into the air column from low layers, it causes the development of the vertical vorticity, the ascending current and the upper outflow and the accumulation of energy frontal zone as well as the convergence expanding, producing the meso and micro scale weather system in this process. The persistently strong easterlies in the north of Zhejiang are responsible for the movement of Typhoon Danas to its east, which provides abundant water vapor and heat energy, making the low level convergence stronger. The convergence of easterly flow and northeast flow in the surface layer is the dynamic mechanism for this extremely heavy rainfall. The strength of the easterlies increases 2 hours earlier than the increase of the rainfall. The water vapor increases and decreases 6 hours earlier than the increase and decrease of the rainfall. The bell shaped Hangzhou Bay which has higher terrain in its west and south has precipitation enhancement effect by the windward slope and topographic convergence. Therefore the predicted rainfall time should be prolonged when cold air intrudes into the typhoon in autumn, and the wind in the Hangzhou Bay is obviously stronger when the prevailing wind directs from east, for it may increase the precipitation in surrounding areas. Improving the usage of the local and PBL observation data of local and PBL has significant effects on the nowcasting of rainstorm.