This study investigates a sudden heavy rainstorm that occurred in coastal Shandong on 17 September 2024. By integrating conventional observations, ERA5 reanalysis and Doppler weather radar data with diagnostic analysis and radar wind field retrieval techniques, we elucidate the underlying mechanisms and multi-system interactions. The results are as follows. The heavy rainstorm area in northern coastal Weihai is situated within the western Pacific subtropical high (WPSH) between a northern short-wave trough and a weakened southern typhoon low pressure. Here, the warm, moist airflows from the typhoon periphery and the WPSH interior over the Yellow Sea in the middle-lower troposphere converge with the weak cold air advected southwards from northeastern China, which enhances the low-level convergence and sustains the 925-hPa shear line (surface convergence line). Ultimately, a coupled dynamic mechanism is established, characterized by low-level convergence beneath upper-level divergence on the right side of the subtropical westerly jet. The coastal topography of northern Weihai provides additional lifting, further intensifying the precipitation. In contrast, the rainstorm in southern coastal Weihai is primarily associated with an intensification of the low-level southerly flow from the typhoon periphery, without direct influence of cold air. Even in the absence of a low-level jet, continuous transfer of water vapor and instability accumulation are achieved through the combined effects of the southeasterly flow within the WPSH and the typhoon. The triggering of convective instability is forced by the lifting from the low-level weak cold air, with mesoscale convergence lines or centers near the surface modulated by local topography serving as a crucial mechanism for the heavy rainstorm.