Abstract:In order to further understand the severe weather originating in the p eriphery of typhoons and improve the capability of severe weather potential fore cast and warning, the synoptic background, water vapor, CAPE, WI, vertical veloc ity and sea breezes are analyzed on a severe convection weather occurring in out er periphery of Typhoon Rananim in August 2004 based on the Doppler radar, autom atic weather station and NCAR/NCEP reanalyzed data etc. The continual heat wave in South China resulted from the subsidence from subtropical high and the periph ery of Typhoon Rananim, consequently, internal energy is accumulated，and strong instable state is established. When the subtropical high weakened and the subsidence disappeared a s a result of northwestward moving of Typhoon, the severe convection is triggere d by the weak trough and sea breeze and a dry line. The stabi lity outsides the typhoon is of fluctuant characteristics. The convection develo ped on an instable tongue with higher temperature and higher humidity that joint with the end of an exterior spiral cloud band of Typhoon Rananim. Wind Index is more available in forecasting thunderstorm wind gusts than other indexes in Sou th China.