Typhoon (TY) Koppu (2015) brought intensive small horizontal-scale rainfall at Baguio, Philippines. This study examines the applicability of the TWRF model (based on the Weather Research and Forecasting model, https://www.mmm.ucar.edu/weather-research-and-forecasting-model, specifically for the TY prediction) of the Central Weather Bureau, Taiwan in the rainfall prediction of Koppu. Moreover, it aims to improve the understanding of the mountainous terrain effect on the rainfall and structural changes of TY Koppu. The TWRF-predicted rainfall initiated at 1200 UTC 17 October is compared with the surface observed rainfall and the passive microwave rainfall product, and the TWRF-simulated TY Koppu structure is verified against satellite infrared cloud imagery. Verifying available best tracks, the largest predicted track error within 72 hours of prediction is 145 km. Rainfall similar to that at Baguio can also be obtained by the TWRF, except that the location of the maximum rainfall is to the south of the observation. Modeling results show that heavy rainfall was caused by the outer rain band of TY Koppu, which gradually reorganized over the western coastal area when Koppu weakened after landing. Results also show that the high terrain located south of the TY center enhanced the rain band. However, the dry air in the lee side and downstream of the high mountains located north of the TY center weakened and pushed the band southward. TY Koppu and the rain band then moved northward. The formation and intensification, the weakening and southward shifting, and then the northward approaching and passing of the rain band resulted in the extreme rainfall at Baguio.