Abstract: The thermodynamic conditions of atmosphere resulting in tcrrential rain in Guangdong as well as the relationship between terrential rain and specific low-level jet stream are analysed in this paper. It is found that some physical quantities fluctuate sharply 18 to 30 hours before the onset of terrential rain. The phenomena is not obvious in other time. The above mentioned quantities are combined into L index to predict the location of intense rain. When terrential rain is predicted to occur, the index can foretell the rainfall domain in the coming 24 hours or so, with reference to the position of low-level jet. The average forecast accuracy has been above 80 percent for the past seven years. This method can also be applied to the forecasting of heavy rain location to some degree.