Abstract:Using ECMWF Ensemble data, the performances of ensemble forecasting about large scale circulation and heavy rain processes in the mid lower reaches of the Yangtze River during the drought and flood transition period in 2011 are examined. The results show that ensemble average forecasting has a good performance on the adjustments of large scale circulation in the whole troposphere in extended range period of validity. The forecasting period is up to 10-15 days. During different forecasting periods, ensemble average forecasting displays stability of forecasting the main affecting systems during the heavy rain processes. With the shortening of the forecasting period, the dispersion of various ensemble members to weather systems gradually decreases. The forecasting of individual ensemble member including ensemble average and control member has not obvious regularity in different period of validity. The forecast of a few ensemble members is in accord with the real weather condition about main impact systems during different forecasting periods. Occurrence of low level stronger winds is closely related to heavy rainfall in mid lower reaches of the Yangtze River. The small probability information of ensemble forecasting is meaningful in the extended range forecasting period. It can provide a reference for extended range heavy rainfall forecast.