Based on multi-source observation data and ERA5 （0.25°×0.25°） reanalysis data, the causes of the first rainstorm in the local warm sector of Zhejiang in the Meiyu period over the night of 9 June 2021 are analyzed. The results are as follows. The circulation situation of this process was different from that of the typical Meiyu, and belonged to the rainstorm in the local warm sector under a weak weather background. The maintenance of the southeast air flow between the South China Sea low pressure and the western Pacific subtropical high provided abundant water vapor for the rainstorm area. The strengthening of 925 hPa ultra-low level southerly jet was conducive to the increase in temperature and humidity in the low layer and the aggravation of unstable stratification, and the rainstorm area was located on the left side of the jet axis. The weak lifting condition caused by the high humidity background triggered convection. The strengthening of cyclonic convergence and rotation in the middle and high layers strengthened the rainstorm, and the thicker warm cloud layer was benefitial to improving the precipitation efficiency. The formation of mesoscale convergence zone stimulated the initial convection, and its maintenance and strengthening stimulated the formation of convective cloud clusters, producing train effect and leading to the formation of rainstorm. The small-scale topography of Longmen Mountain was favorable for the forced uplift of southerly air flow on the windward slope, strengthening the convection. The development of vertical velocity had a good corresponding relationship with the height of the topography. The higher the topography height, the stronger the verticle velocity to be stimulated.