An extreme rainfall event with 48-h accumulated precipitation amounts exceeding 500 mm on the north (Japan Sea) side of western Japan occurred when Typhoon Lan (2023) approached and passed over Japan in a weak baroclinic environment. The rainfall event included two local heavy precipitation peaks. In the present study, we perform numerical simulations with a cloud-system-resolving model to investigate the potential roles of two factors in the first event peak: (1) an abnormally high sea surface temperature (AHSST) anomaly (∼ +4°C) and (2) a mesoscale low formed over the Sea of Japan. The results of sensitivity experiments showed that the AHSST increased the total rainfall amount by about 100 mm. The mesoscale low, which was generated by southeasterly flows over the mountain ranges of central Japan, determined the location of the heavy rainfall by controlling the direction and intensity of low-level flows. The role of this terrain-induced mesoscale low provided new insight into the mechanisms producing heavy rainfall in association with typhoons approaching Japan in a weak baroclinic environment.