Abstract: In 2024, the “June 20” rainstorm event during the Meiyu season in northwestern Zhejiang was characterized by its long duration, large cumulative precipitation, highly localized impacts, and extreme intensity. To investigate the features and mechanisms of this extreme rainstorm, multiple datasets were employed, including automatic weather station precipitation data from Zhejiang and surrounding areas, ERA5 reanalysis data, dual-polarization radar observations, and mesoscale numerical model data. Combined with synoptic diagnostic methods, the study analyzed the extreme precipitation characteristics, the mechanisms, the degree of climatic anomalies, terrain effects, and radar echo characteristics. The results are as follows. (1) The internal mechanisms of the rainstorm exhibited a clear phased evolution. In phase I, convection was triggered by convergence in the exit region of the boundary layer jet and sustained by energy release from a conditionally unstable layer in the lower troposphere. In phase II, dynamic forcing resulting from the coupling of upper-level northwesterly divergence and lower-level southwesterly jet convergence created a pumping effect that sustained precipitation. (2) The extreme nature of the rainstorm was significantly influenced by notable climatic anomalies in water vapor flux and low-level jet. (3) Topography exerted a dynamic forcing on low-level wind, with a pronounced increase in precipitation in front of mountains, where the average accumulated precipitation amount was 1.78 times that in other areas. (4) Radar echoes of precipitation displayed phased differences: Phase I was dominated by convective clouds with convergence of low-level southwesterly radial velocity, while phase II featured stratocumulus mixed-cloud precipitation, with convective clouds corresponding to the coupling of radial convergence and divergence at high and low levels. The study reveals that the extreme rainstorm during the Meiyu season resulted from sustained precipitation due to the phased evolution of weather systems, and that the combined influence of weather systems and topographic forcing was key to its localized nature.