Abstract: An extreme warm-sector rainstorm occurred over coastal Guangxi on 27 April 2026, leading to severe disasters. Using observations from S-band dual-polarization Doppler radar, wind-profiler radar, automatic weather stations, and ERA5 reanalysis data, this paper analyzes the precipitation characteristics, synoptic setting, environmental conditions, radar-echo evolution of the event. Particular attention is given to the synergistic enhancement of moisture transport and coastal low-level convergence by a synoptic-scale low-level jet and a boundary layer jet, as well as the initiation and intensification of the mesoscale convective system (MCS). The result is as follows. The rainfall was highly localized and exceptionally intense, with a maximum 3-h accumulation of 323.4 mm, setting a record for Guangxi. Upper-level divergence, positive vorticity advection ahead of the southern-branch trough, and blocking by a high pressure ridge jointly maintained a favorable synoptic environment. Persistent southerly and southeasterly low-level jets transported warm, moist air from the South China Sea and enhanced convergence along the coast. A synoptic-scale low-level jet (SLLJ) and a boundary layer jet (BLJ) formed a double-low-level-jet configuration that further strengthened moisture transport and coastal low-level convergence. Prior to convection initiation, high humidity, a low lifting condensation level (LCL), a deep warm-cloud layer, and rapidly increasing precipitable water favored high precipitation efficiency. The MCS underwent an organized evolution characterized by convection initiation, cold-pool feedback, and intensification associated with meso-γ-scale vortices. The results indicate that the double low-level jets enhanced moisture transport and low-level convergence and, together with mesoscale convergence, cold pools, and meso-γ-scale vortices, promoted the development and intensification of the extreme rainfall. These findings provide useful insights into the monitoring and forecasting similar extreme precipitation events over coastal South China.