<strong> </strong>South China is one of China’s rainiest regions, where mesoscale convective systems (MCSs) frequently produce heavy rainfall during the post-flood rainy season (July-September). Using 1-km, 6-min radar mosaics and ERA5 reanalysis, this study identified 123 MCSs from 2019 to 2022. MCS activity is concentrated over southern Guangxi (S-GX) and the Pearl River Delta (PRD), with marked regional contrasts in diurnal evolution and environmental conditions. Variations in the relative positions of the South Asian High (SAH) and the western North Pacific subtropical high (WNPSH) produce distinct circulation regimes over the two regions. S-GX MCSs over the S-GX mainly form from midnight to early morning and are associated with an eastward-extended SAH and a westward-displaced WNPSH, which strengthen the southerly monsoonal flow and produce a deeper moisture transport layer extending to approximately 750 hPa. Together with larger convective instability (mean CAPE ≈ 1843 J kg⁻¹), these conditions allow convection to overcome the relatively larger convective inhibition (CIN) and support nocturnal MCS development. In contrast, PRD MCSs over the PRD mainly form from afternoon to evening under weaker large-scale forcing, shallower moisture transport largely confined below 900 hPa, lower but localized instability (mean CAPE ≈ 1523 J kg⁻¹) and smaller inhibition. These conditions increased the relative importance of daytime boundary-layer heating and sea-breeze convergence, consistent with the afternoon to evening MCS maximum. The results identify a synoptic-dynamical mode over S-GX and a boundary-layer thermodynamic mode over the PRD, providing a physical framework for understanding post-flood-season MCS development and heavy rainfall over South China.