Abstract:By using the observation data, operational model data and NCEP reanalysis data (1°×1°), the characteristics and forecasting difficulties of severe Typhoon Fitow (No.1323) and the cause for the strong wind and heavy rainfall are analyzed synthetically. The results show that Fitow moves westward suddenly and intensifies near the offshore, and these are the major forecasting difficuties in operation and reasons of the big forecasting errors. The subtropical jet over East Asia intensifies rapidly and the subtropical high extends westward, causing Fitow to move westward suddenly. The strong divergence in the right side of the subtropical jet’s entrance is an important dynamic mechanism for Fitow to intensify near the offshore and for the heavy rainfall and strong wind in Fitow’s north side. Danas (No.1324) provides plentiful water vapor transport, bringing the heavy rainfall and causing Fitow’s intensity to sustain near offshore. In addition, Danas makes the subtropical high extend westward. Therefore, the existence of Danas is also an important factor causing Fitow to move westward suddenly. When forecasters failed to pay enough attention to the upper level fields, especically subtropical westerly jet, which is likely the main reason of the bigger forecasting errors of the track, intensity, rainfall and gale of Fitow. Finally, when the tracks of the ensemble prediction systems have large divergence or the different models produce greatly different forecast results, the ammending technique based on multi model ensembles is an effective method to improve the accuracy of typhoon track forecasting.