August of 2001, continuous heavy rains occurred in Longnan, triggering flash floods, debris flows, landslides and other geological disasters. This paper discusses the weather condition, atmospheric circulation background and their physical characteristics. The rainfall mechanism is also analyzed in terms of nephograms and precipitation characteristics. The low groove at the edge of the subtropical high pressure developed into low vortexes as it moved eastward, thus storms formed. The vortexes and storms were so strong at certain points as to cause disasters.