By using conventional and nonconventional meteorological data, numerical prediction model, GFS reanalysis data (0.5°×0.5°), the environment features and model errors of extremely heavy rain under northwest flow which happened on 19 June 2016 in northern Jiangxi are analyzed. Compared with the similar situation, some possible causes of the prediction error are found out, such as the deviation of southward northwest dryandcold air, the speed of the subtropical high, the strengthening and convergence of the southwest lowleveljet, shortwave trough from upstream and abnormal moisture conditions. Even more, the prediction of rain belt by ECMWF was northward and much lighter than observation. Finally the forecasting methods of severe heavy rain are given.