The regression analysis indicates that there are asymmetry characteristics of residual distribution in the Meiyu heavy/torrential rain forecasting model. These phenomena are caused by some Ligh Leverage Cases. There is irrationality in the residuals of these high leverage cases statistically,resulting in Least Square (LS) errors in estimating the regression coefficient,consequently errors of heavy/torrential rain forecasting.A regression diagnosis prediction model for heavy/torrential rain forecasting is proposed. The example analysis shows that this new model is superior to general regression prediction models. The further analysis indicates that the irrationality of the Meiyu heavy/torrential rain regression forecasting model is resulted from its mathematical characteristics,not by chance.