Abstract: Warm-sector heavy rainfall occurs frequently over South China, characterized by high precipitation intensity and complex physical mechanisms, and often coexists with strongly forced frontal rainfall, posing great challenges to operational forecasting. This study focuses on a warm-sector heavy rainfall event in the southeastern Guangdong, characterized by the rapid organization, development, and evolution of meso- and micro-scale convective systems. Using high-resolution numerical simulations from the Weather Research and Forecasting (WRF) model, the study analyzes the development, evolution, and organization of mesoscale precipitation systems, as well as near-surface thermodynamic and dynamic forcing and moisture conditions. Furthermore, terrain sensitivity experiments are conducted to explore the influence of topography on precipitation initiation and development. The results show that the event exhibits a typical spatiotemporal evolution pattern, with convection developing inland during the early stage and persisting along the coast later on. The low-level jet and the circulation along the periphery of the subtropical high jointly established a persistent moisture transport channel, leading to pronounced low-level convergence, enhanced local ascent, and moisture accumulation in the rainfall center. Terrain reduction weakens the forced lifting and causes the precipitation band to break, whereas terrain enhancement strengthens moisture convergence and instability release, reshaping the precipitation structure. These findings demonstrate that topography exerts a direct influence on the occurrence and evolution of heavy rainfall in this region by modulating low-level circulation, vertical motion, and moisture transport.