Abstract The objective of this study is to perform observational data analyses, including European Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite imagery, radar reflectivity, and rainfall data, and numerical simulations with the Weather Research and Forecast (WRF) model to investigate both the synoptic and mesoscale processes responsible for causing the heavy rainfall event which produced up to 379.5 mm over southwestern Taiwan on 7 June 2003. We found that an 850 hPa subsynoptic cyclone, composed of both a low-pressure center and a vortex, formed over the eastern Tibetan Plateau and moved with a 500 hPa shortwave trough toward a wind shear zone over southeastern China in the early morning of 7 June. The wind shear zone was generated by a southwesterly monsoonal flow and a northeasterly flow associated with a high pressure in eastern China. The intensifying 850 hPa subsynoptic cyclone served as a precursor of the heavy rainfall episode in Taiwan. The 850 hPa subsynoptic cyclone extended downward to form a surface subsynoptic cyclone over the southeastern coast of China by coupling the ascending motions associated with the 500 hPa shortwave trough and the 850 hPa subsynoptic cyclone. The mesoscale convective system associated with this surface subsynoptic cyclone migrated from southeastern China to the southwestern Taiwan Strait and then toward southwestern Taiwan. Meanwhile, the speed of the 850 hPa low-level jet on the southern side of the subsynoptic cyclone exceeded 20 m s − 1 over the southern Taiwan Strait. This jet then served as a conveyor belt of the moist airstream and enhanced the inland-moving convection over the mountain slopes. In addition, the low-level convergence resulted from the confluence of flow due to flow deflection over southern Taiwan topography also facilitated rainfall there.