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 indicate the following: The heavy rainstorm in northern coastal Weihai was situated within the Western Pacific Subtropical High (WPSH), between a northern short-wave trough and a weakened southern typhoon. Here, warm, moist airflows from the typhoon periphery and the WPSH interior over the Yellow Sea in the middle-lower troposphere converged with weak cold air advected southwards from northeastern China. This convergence enhanced the low-level lifting and sustained the 925-hPa shear line (surface convergence line). Ultimately, it established a coupled dynamic mechanism, characterized by low-level convergence beneath upper-level divergence on the right side of the subtropical westerly jet. The coastal topography of northern Weihai provided additional lifting, further intensifying the precipitation. In contrast, the rainstorm in the southern coastal area of Weihai was primarily associated with an intensification of the low-level southerly flow from the typhoon periphery, without direct influence from cold air. Even in the absence of a low-level jet, continuous moisture transport and instability accumulation were achieved through the combined effects of the southeasterly flow within the WPSH and the typhoon. The triggering of convective instability was forced by the lifting from the intruding 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.