The paper investigated a heavy rainfall forecast method of application and interpretation NWF in accordance with its forecasting fields. An objective fuzzy cluster typing technique for heavy rainfall history data was proposed, in which a small probability turned into large by using condition probability in favor of trying out the heavy rainfall indexes. The method summed up 25 parameter indexes to diagnose heavy rainfall according to historical samples for same period which include pressure, temperature, humidity, wind and interrelated physical quantities in the 3D space of atmosphere, and developed the mechanism and the atmosphere structure feature such as high temperature, high humidity and convective instability described by idiographic physical factors. In addition, the study explored some rules of forecasting heavy rainfall area by means of specifically physical quantities. A rainstorm forecast tool was set up, of which the result of NWP operation in existence can be regarded as the start-up condition for reducing rainstorm false rate of forecast.