On the morning of July 21,2017,a localized rainstorm event occurred in Shijiazhuang,with the heavy rain mainly concentrating in its urban area,eastern and northern parts,which was omitted by both numerical and subjective forecasts.In this paper,the triggering mechanisms of the mesoscale convective system(MCS) causing this rainstorm,the propagation direction of the system and the main factors affecting the overall movement were analyzed by using the intensified surface observation data,Doppler radar data,radar wind profile,the four-dimensional variational Doppler radar analysis system(VDRAS) and NCEP reanalysis data.The results show that:1)Before the heavy precipitation occurring,a large amount of unstable energy was accumulated because of the high temperature and high humidity in this area.The surface airflow was turned by Taihang Mountain in the vicinity of Shijiazhuang,forming the shear line of northeast wind and west-northwest wind in front of the mountain.The strong thunderstorm high and cold pool was formed by the heavy precipitation in Qinhuangdao and Tangshan areas.The pressure gradient force generated by the thunderstorm high strengthened the northeast wind,which guiding the cold pool to gradually move southwest in a tongue shape to the north of Shijiazhuang area.In the vicinity of the aforementioned shear line,an unstable stratification of low-level convergence and middle-level divergence was formed,which was combined with the topographic forced uplift of the windward slope triggering the unstable energy release.2)During the development and strengthening of thunderstorm,the cold outflow formed by the precipitation of MCS moved slowly in the northeast direction.In the area where the northeast airflow was converged,new thunderstorms were constantly triggered,which made thunderstorms spread northeast.At this stage,the average wind speed of the storm bearing layer(ie,the translation direction of the MCS) was small,the system was stable and less moving,mainly in the form of “backward propagation”,which was shown as “quasi-static state”.With the increase of the average wind speed in the storm bearing layer,the moving direction of the MCS could be synthesized from the reverse vector of the low-level jet and the average wind vector above 1.5 Km(between 850 and 300 hPa layers).At this stage,the moving direction of the cold pool was closely related to the average wind in the storm bearing layer,causing the thunderstorm cold outflow moving southeast.The northwest cold outflow of the cold pool formed the convergence with the southeast warm environmental flow,leading to the constantly emerging thunderstorm cells in the front of the MCS.The propagation direction was consistent with the advection direction,and the system moved “fast” to the southeast.