During the period 11–15 July 1981, heavy rainfall occurred over the Sichuan basin in China, resulting in severe floods that took a large toll in human life and property damage. Synoptic analyses indicate that early in this period the southerly monsoon flow was particularly strong near the basin because of a favorable positioning of the Pacific subtropical high and the Indian monsoon depression. The passage of a deep midlatitude trough from the Lake Baikal region brought colder, drier air from Siberia into southwest China. The Siberian air stream met the monsoon current over the eastern plateau and the Sichuan basin, creating a region of large-scale, low-level convergence. Mesoscale analyses show that the flood was directly related to the extreme development of a long-lived mesoscale vortex [called south (SW) vortex by Chinese meteorologists] over the basin as it merged with another mesoscale vortex generated over the Tibetan Plateau. Mesoscale heat and moisture budgets suggest that the latent heat release associated with cumulus convection contributed substantially to the development of the SW vortex. It is found that heating due to convective eddy flux convergence of sensible and Went heat is about half the amount of latent beat release due to condensation. Moisture flux analyses and isentropic trajectories indicate that the major moisture source for the good was from the Bay of Bengal. This moisture was transported over (and around) the southeastern corner of the Tibetan Plateau to the basin. The SW vortex finally dissipated, after the passage of a surface cold front associated with the Lake Baikal trough. We hypothesize that the formation of the SW vortex was a consequence of the blocking of the southwesterly monsoon flow by the mesoscale mountain range (located at the southeastern corner of the Tibetan Plateau). This is suggested by the fact that the SW vortex was initiated completely within this southwesterly monsoon current, that it was strongest at the lower levels during its formation stage, and that the low-level cyclonic vorticity was present throughout the period that the southwesterly monsoon flowed around the plateau.