Why was the weakening of Typhoon IN-FA (2021) to the east of Taiwan Island not forecasted in the GFS?

Typhoon IN-FA (2021) experienced a weakening process on July 22–23 in a large-scale environment favorable for tropical cyclone (TC) intensification. All operational forecasts and the Global Forecast System (GFS) forecasts predicted a continuous intensification, which deviated significantly from the observation. The analysis of the GFS analysis product shows a coalescence process of Typhoon IN-FA with an intraseasonal monsoon gyre during the period, resulting in an increased outer size of IN-FA and well-organized convection to the east, which prevented transporting the mass and moisture into the inner-core area of IN-FA, thus leading to the weakening. Nevertheless, this essential coalescence process was not captured in the GFS forecasts due to the poor prediction of the monsoon gyre. The analysis shows that the forecasted monsoon gyre on July 20–22 had an eastward location at 72 h and 96 h lead times and a weaker intensity and outer circulation at 24 h and 48 h lead times, leading to the forecasted TC always moving in its north and west, in agreement with numerical simulation results that the monsoon gyre with a weaker outer circulation is not conducive to the coalescence. Thus, the deep convection to the east of IN-FA preventing the inward transportation of mass and moisture did not develop in the GFS forecasts. As a result, the GFS forecasted that IN-FA would continue intensifying in a favorable environment on July 22–23. The findings of this study would prompt forecasters to pay attention to the prediction of the monsoon gyre and its influence on the TC intensity in forecast products available to them.