The forecast errors and associated surrounding flows of sudden typhoon track changes in the western North Pacific during 2005-2010 are examined in this study. By analyzing the forecasts of National Meteorological Centre of China Meteorological Administration, it is found that the mean 24 h and 48 h forecast errors for the sudden north turning cases are significantly large while the forecasts of sudden west turning cases are close to the average forecasting levels. For the sudden northward change, compared to the average forecast errors, the mean 24 h and 48 h forecast errors are 145.6 km and 317.3 km respectively, increasing by 29.3% and 68.3%. Further composite analysis on the circulations shows that there are clear differences in the westward extension of the low frequency component of the subtropical high and the synoptic scale wind fields associated with the tropical cyclones with sudden track changes.