Based on the ERA5 (0.25° × 0.25°) hourly reanalysis data, this paper analyzes two different high-impact Southwest Vortex (SWV) processes with long-distance movement and stagnation over local area. It is suggested that two physical variables, the low- to mid-level convergence and pseudo-equivalent potential temperature, are the key factors influencing the development and evolution of the vortex. According to these findings, a vortex development index is defined in this study. By comparing the vortex development index with the evolution of the SWV and precipitation, we found that: (1) the vortex development index could indicate the development and evolution trend of the intensity and position of the vortex, whether it is a long-distance moving or a local stationary SWV process. The index could also reveal the occurrence of other weak closed lows or multi-core vortices during intense vortex events, as well as the moving mechanism of the SWV. (2) The vortex development index not only indicates the intensity and coverage area of vortex precipitation but also reflects the coexistence phenomenon between the SWV and precipitation, thereby providing a possible explanation for both the "rain-induced vortex" and the "vortex-induced rain" phenomenon. Results of this study suggest the positive significance of the vortex development index in the forecast of both the precipitation distribution and the position of the precipitation center of the high-impact SWV heavy rainfall events.