Abstract:Errors occur for medium range forecast of heavy rainfall over North China on 20 July 2016 by subjective forecast of National Meteorological Centre and numerical models. The medium range forecast performance of this heavy rainfall event is analyzed based on ECMWF ensemble forecast and T639 ensemble forecast data. The results reveal that: (1) there are obvious changes of location and intensity of rainfall of EC ensemble forecast which is caused by change of forecast for large scale weather system before and after 16 July. Compared to forecast initialized at 20:00 15 July, the ensemble mean circulation on 20 July initialized at 20:00 BT 16 July show that trough at 500 hPa over Hetao Region is deeper whereas downstream ridge is stronger, thus forming the pattern of slowly moving “high pressure to the east and low pressure to the west”. The Yellow River cyclone and mid lower level southwest vortex move northward and become stronger. The forecast heavy rainfall is closer to the observed precipitation accordingly. (2) Ensemble sensitivity analysis shows that sensitivity areas are consistent with the spatial pattern of circulation difference before and after the forecast change. The large scale weather systems leading to the change of forecast of heavy rainfall are also sources of uncertainty of medium range rainfall forecast. (3) The most sensitive circulation system to precipitation forecast is the Yellow River cyclone. The Yellow River cyclone, the ensemble member is westward and northward and intensity is deeper, the corresponding forecast precipitation amount over Bejing Tianjin Hebei Region is larger.