This study assesses the predictability of an enhanced monsoon trough south of Japan in late August 2016, which is accompanied by Rossby wave propagation over Eurasia and a consequent anticyclonic Rossby wave breaking east of Japan, with a relaxation technique using an atmospheric general circulation model. Three types of the relaxation experiments are conducted, with nudging the model forecast in the upper troposphere toward reanalysis, for regions of the Rossby wave breaking east of Japan, the Rossby wave propagation over Eurasia, and both the regions from Eurasia to the east of Japan. All types of the relaxation experiments show improved reproducibility of the enhanced monsoon trough, which the operational one-month ensemble prediction in Japan Meteorological Agency failed to predict. Compared with a result of a control experiment, the relaxation experiments show the more amplified Rossby wave propagation over Eurasia and Rossby wave breaking east of Japan, as seen in the reanalysis. The upper-level wave amplification contributes to the improved reproducibility of the enhanced monsoon trough, through that of southwestward intrusion of upper-level high potential vorticity airmass toward the southeast of Japan. The results of relaxation experiments indicate primary and secondary contributions from corrected forecast errors of the Rossby wave breaking east of Japan and the Rossby wave propagation over Eurasia to the predictability of the monsoon trough, respectively. Their relative contributions to the enhanced monsoon trough are consistent with a result of ensemble-based simple sensitivity analysis shown in a related previous study.