Through comparing forecast with observation, a forecasting failure is analyzed. On the basis of the analysis of the numerical forecasts and the meteorology elements, the predicting thoughts of the forecasters then are discussed. It is pointed out that the main physical mechanism inducing the sudden change of the weather system is the southward moving of the northwestly jet over the central Asia at 300 hPa. The changing high-level trough from backward to forward was unfavorable for the upward air motion. With the real time data, the reasons for forecasting failure are analyzed. The unexpected evolvement of the high level trough and the underestimated changing of the low-level path of the cold air as well as the amount of warm air are the main reasons for the forecast failure.