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The precipitation caused by Typhoon Megi is abnormally enormous with the rainfall in part counties in Wenzhou and Lishui breaking the historical records, which caused a severe disaster. Its causes are analyzed by the use of NCEP 0.5°×0.5° reanalysis data, FY2G satellite data, Doppler radar data, the intensive observation data, and so on. The results show that there were three low jet streams: the southeast jet stream between Megi and subtropical high, the eastward jet stream between Megi and Huabei high, and the southwest jet stream connected with southwest monsoon. The shear and convergence zone consisted of three low jet streams was the area of vapor and instability energy convergence and also the trigger zone of mesoscale convection. The extreme precipitation in Wenzhou and Lishui was triggered by the strong development of continual mesoscale convection. Longtime lowlevel jet streams, particularly abundant vapor transportation and cold air are the main influencing mechanism of Megi extreme precipitation. During the cold air influencing, the precipitation echo with tight structure developed strongly. The echo intensity and precipitation efficiency were obviously stronger than those of the typhoon circulation echo with no cold air influence. The cloudtop blackbody brightness temperature (TBB) has good relationship with typhoon precipitation, which has reference value for operational forecasting.