During the transition period between the Jianghuai Meiyu and North China rainy seasons in 2016, there was a high-impact rainstorm event, connecting these two rainy seasons. However, both the short and medium range forecasts predicted a rain band that was too far south; this error can have serious implications for flood preparedness. This study discusses the causes of the forecast error using the ECMWF deterministic and ensemble models. The results show that the prediction of a too-weak westerly trough at the 500-hPa level is the dominant factor. Specifically, a weaker predicted 500-hPa westerly trough reduces the strength of the low vortex at 850 hPa; correspondingly, the shear line to the east of the vortex and the Meiyu front are weaker and farther south, and the monsoon is also weaker, which causes the large area of humidity convergence to be shifted southward. Finally, the heavy rainfall is located farther south with weaker intensity. And vice visa, when a stronger westerly trough is predicted, the location of rain band will be northward. This conclusion may be useful for meteorological decision making in medium range and will help to improve capabilities of risk reduction.

在2016年江淮梅雨与华北雨季之间的过渡时期，发生了一个影响很大的暴雨事件，将这两个雨季联系在一起。然而，无论是中短程预报，还是一个偏南的雨带。此错误可能会对防洪工作产生严重影响。本研究使用ECMWF确定性和集成模型讨论了预测误差的原因。结果表明，在500hPa的水平上过弱的西风谷的预测是主要因素。具体而言，较弱的预测500hPa西风谷降低了850hPa处低涡的强度；相应地，涡流东部和梅雨锋的切变线越弱，向南越远，季风也越弱，这导致大面积的湿度会聚向南移动。最后，大雨位于偏南，强度较弱。副签证，当预测到更强的西风谷时，雨带的位置将向北。该结论对于中等范围的气象决策可能有用，并且将有助于提高降低风险的能力。