Abstract: Using the optical fl ow technique, 500hPa height forecasts of two numerical weather prediction (NWP) models ECMWF (EC for short) and T639 during the period of Typhoon MUIFA(1109) are verified and interpreted. The results show that EC has stable errors in 24h, 48h and 72h forecasts. Its 24h forecast intensities are 2-9gpm lower than observations, and angular errors are between 180° and 240°, which are between west and southwest. Its 48h forecast intensities are 8-18gpm lower than observations, and angular errors are also between 180° and 240°. Its 72h forecast intensities are mainly -25 to 26gpm lower than observations, and angular errors are mainly between 170° and 290°. By contrast, the intensity and angular errors of T639 are not so stable. As for displacement errors, the performances of EC and T639 are both stable. The similar disparities between observation fi elds and 24h, 48h and 72h forecast fi elds of the two models also show that EC has better forecasting performances than T639. Finally, an application technique is used to correct the 48h forecast fi elds according to the 24h forecasts errors. The results show that when the distortion errors of 24h forecasts obtained from the optical fl ow fi eld are used to correct the 48h forecasts, the correction fi elds are more similar to the observation fi elds than the forecast fi elds themselves.