Abstract: Based on the NCEP/NCAR reanalyzed data and the conventional observational meteorological observing station data, two strong snowstorms in Hunan province in 2018 were comparatively analyzed. The results showed that both snowstorms were caused in "Ω" circulation situation at 500 hPa, but there was a significant difference in the middle-low atmosphere. The first process （January 24 to 31, 2018） had a shear line and a good vapor condition, and the warm and wet air were forced to climb, because the cold air wedged in from the near ground. In the second process （December 28, 2018 to January 3, 2019）, a strong and stable cold pad had been formed before the snow, and the warm and wet air climbed along the pad at 850 hPa. The snowstorm areas were located in the left side of the large water vapor flux area and near the vapor flux divergence center. The main reason for the difference of water vapor is the different sources of water vapor by the Lagrange air parcel tracking method. The water vapor in the first process mainly came from humidified northerly backflow which through the Yellow Sea and the East China Sea from the bottom of the high pressure and the Arabian Sea, the second process was from Indo-China peninsula and India peninsula. The two processes both had pseudo-equivalent potential temperature intensive zone and the surface front zone near the 22° N. The slope of the second process was higher than that of the first process and the wet air climb obviously, which caused the low-level cyclonic vorticity increases, and the relative humidity, the relative vorticity and vertical movement extend upward significantly at 12 hours before the start of heavy snowstorm, which has precursory significance for the prediction.

Key words: strong snow storm, circulation situation, vapor, pseudo-equivalent potential temperature, northeast of Hunan province