During the period of June 28 to July 1, 2016, a Tibetan Plateau (TP) vortex was generated, which developed and moved eastward to the subtropical region of China, resulting in precipitation in the lower reaches of the Yangtze River. In this study, we used the potential vorticity (PV) to understand this process based on data from the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) and precipitation data from the Tropical Rainfall Measurement Mission (TRMM). The results indicate that surface heating over the TP causes obvious diurnal variation, changing from a heat source in the daytime to a source of cold at night and directly influencing the vertical gradient of diabatic heating. Negative PV is generated near the surface in the daytime and positive PV is generated at night, which reflects a prominent diurnal cycle. When the nighttime generation of positive PV becomes very strong and cannot be compensated for by the daytime generation of negative PV, a low-vortex forms over the TP. By the time this low-vortex system moves to the eastern TP, diabatic heating associated with strong precipitation reinforces this vortex. As the low-vortex system continues to propagate eastward, the PV advection increases with height and serves as a large circulation background over the middle and lower reaches of the Yangtze River, which favors the development of rising air and results in the occurrence of heavy precipitation.