Abstract: A basic fact prevailing in the forecasting practice in Zhejiang Province of process rainfall due to tropical cyclones is that there is always a historic case that is most similar to the current state in the post event check. Its process rainfall and distribution features are so close to the reality that comes after the current state that they generally meet the requirements of forecasting services. It is, however, very difficult to locate it. A method^[1] of using an operator DE within the whole space of samples in search of a historic case to best match the current state proves to be unsatisfactory. In this paper,starting from occurrence probabilities of historic samples, a number of reference states are set up for precipitation due to tropical cyclones. In operational utilization, a pattern dete rmination is done by comparing the current state with the reference state, followed by a screening of the pattern isolated for a historic case that bears the most similarity. Results of such determination and screening are given for five of the tropical cyclones occurring in 1990 and discussions presented.