The Warm Sector Heavy Rainfall (WSHR) is one of the most typical weather events during the early summer monsoon season in South China with instantaneous torrential rain with high locality and complex atmospheric conditions, which results in difficulties in nowcasting and hazard warning. Four dynamical and thermodynamical indices within the lower atmosphere are employed as precursor signals of WSHR over South China in 2019, including the Low-Level Jet Index (LLJI), the Vertical Wind Shear (VWS), the Atmospheric Lifting Intensity (ALI), and the Boundary Layer Height (BLH), by utilizing wind profiler radar and high-density surface observations. Regional heterogeneity in precursor signals are detected 1–4 hours preceding WSHR onset. Significant ALI and WVS signals in western regions are concentrated at approximately 1.5 km height, which is affected by warm, moist advection and orographic lifting. The central region, dominated by urban agglomerations, exhibited complex precursor signal interactions, where anomalies of LLJI and BLH are significant due to combined effects of urban heat island and the presence of the double LLJ at 1 km and 2.5 km, respectively. In contrast, precursor signals are moderated by the upper-level jet and moisture transport. In addition, monsoon activities and geographical factors play important roles in the spatiotemporal distribution of precursor signals. Urbanization effects on wind field at the boundary layer have significantly changed the features of dynamical precursor signals. The urban heat effect makes the low-level wind field more unstable. This research provides fundamental insights to enhance nowcasting and hazard warning for WSHR in South China.