From June 26 to 28, 2022, a regional heavy rainfall event occurred in Shandong Province, characterised by significant precipitation intensity and extensive spatial coverage. This event ranks among the most severe heavy rainfall processes that induce disasters in recent years. The precipitation process is divided into three distinct stages, with warm-sector precipitation driven by a low vortex and low-level jet playing a dominant role in this heavy rainstorm. The instability mechanisms of warm-sector precipitation, as well as the triggering and maintenance of heavy precipitation, present significant challenges for operational forecasting and are the primary focus of this study. Based on conventional surface and upper-air meteorological observation data, ERA5 hourly reanalysis data, and radar data, this study analyses the water vapour conditions, atmospheric instability, and frontogenetic characteristics of this heavy rainfall event. The results indicate that the precipitation process in the warm sector of the low vortex primarily occurred within the low-level jet zone south of the warm shear region of the low vortex. The low-level jet provided ample water vapour conditions for precipitation, while a region of strong water vapour flux convergence north of the jet stream served as an effective indicator for heavy precipitation. At the onset of precipitation in the warm vortex region, strong convective instability was observed in the mid-to-lower troposphere, with upward motion initiated by the release of convective instability, exhibiting vertical convection characteristics. During the peak precipitation period, the ascending motion was influenced by both convective instability and symmetric instability, with symmetric instability being predominant, resulting in a combination of vertical and oblique convective processes. This precipitation process was accompanied by pronounced frontogenesis, featuring strong geostrophic wind deviation convergence in the frontogenesis region, which supplied essential dynamic uplift conditions for precipitation initiation and intensification. Analysis of the deformation term and divergence term in the frontogenesis function reveals that the divergence term was the main factor of frontogenesis in this process. Low-level convergence not only provided dynamic conditions for precipitation initiation but also integrated radar precipitation echoes with mesoscale convergence lines at the surface, influencing the morphology of cumulus precipitation echoes and forming mesoscale convective rainbands aligned along the guiding airflow. The mesoscale convective rainband moved along the guiding airflow, creating a “train effect” that resulted in heavy precipitation. These findings enhance operational forecasting capabilities for warm-sector precipitation associated with low vortices and similar processes, providing valuable insights for forecasters to develop systematic models of warm-sector precipitation weather systems.