Abstract:An extreme torrential rain event caused by highly organized severe rainfall mesoscale convection system (MCS) occurred in Yichang on 22 April 2018. Based on conventional observation data, observation data from regional automatic weather stations, radar data and ERA5 reanalysis data, we analyze the organizational characteristics and formation mechanism of extreme severe rainfall MCS. The results are as follows. This extreme torrential rain event occurred under the background of weak forcing at high level and weak forcing turning into strong forcing at low level, accompanied by strong frontogenesis. High temperature, high humidity and extremely unstable atmospheric environment were conducive to the occurrence of the extreme severe rainfall. The warm and wet easterly air at low level was forced to lift by the C-pattern terrain in the east of Yichang, triggering the severe rainfall echoes in the transition area from mountain to plain. The warm and wet southeasterly or easterly wind at top of the boundary-layer reverted warm trough caused the dispersed flocculent convection in the plain area from Yidu to Gongan. The organization of extreme severe rainfall MCS experienced merging stage and vortex stage. Under the influence of weak synoptic-scale forcing, MCS in the mountain-to-plain transition area spread to southeast along the low-lying terrain, and merged along three paths with the warm area flocculent echoes from the plain to north. The east-west MCS formed by the low-level east-west frontal zone and the surface convergence line moved toward west under the guidance of the middle and low level easterly jet, and continuously merged with the south-north MCS in the mountain-to-plain transition area peristently, resulting in the heaviest precipitation stage.The mesoscale cyclonic circulation, composed of MCS cold outflow and ambient airflow, and the latent heat of severe rainfall that heated the middle atmosphere favored the organization, development and strengthening of vortex MCS. The water vapor energy transport of the low-level warm and wet easterly jet was beneficial to the long-time maintenance of the vortex MCS. The extreme severe rainfall mainly occurred in the merging stage and the long-time vortex stage of MCS. Synoptic-scale system forcing, low-level strong frontogenesis, mesoscale topography and positive feedback of mesoscale weather system were the important causes of the formation for this extreme severe rainfall.