Abstract:Using the NCEP/NCAR reanalysis data and conventional observation data, this paper analyzes the circulation situation, dynamic and thermodynamic conditions of the three snowstorms in southwest of the Tibetan Plateau in 2013. The results show that, over the middle and high latitudes, there existed radial circulation situation. According to the ridge and trough positions, they can be divided into the long-wave trough and the horizontal trough. The Western Pacific subtropical high is by west and stronger, and Iran high developing to the northeast slowly plays a key role for the southern branch trough to deepen and move eastward. When the 560 dagpm line of southern branch trough area is south to 30°N, and moves eastward to 70°E near or to the east at the southern part of the Tibet Plateau, snowstorm begins to appear. The vorticity change in the vicinity of snowstorm area reflects the strength characteristics of the south branch trough. The vertical evolution of divergence field in mid-high levels plays an important role in the south branch trough development. Water vapor mainly comes from the Arabia Sea, and the water vapor in the Bay of Bengal plays a complementary role in the eastern snowfall. The southwest upper-level jet plays a key role in water vapor transport. At the same time, the Himalaya Mountain terrain uplift is conducive to the ascending motion and water vapor condensation. Water vapor flux, vapor flux divergence value changes and the center moving direction have better forecasting significances to the forecasting of snowfall intensity, falling area and time.