Based on regular observational data and NCEP/NCAR reanalysis data,the dynamical mechanism of an extraordinary snowstorm during 3—4 March 2007 over the south of Northeast China was analyzed.The results showed that the region around the shear line at 850 hPa,which played a significant role in convergence and lifting,corresponded to the center of the heavy snow and it was also the main influencing system and the key factor for the successful forecast of the event.Vorticity variability could not only reflect the physical mechanism of the formation and development of the snowstorm shear,but also predict the position and intensity of the heavy snowstorm.Among all the factors affecting vorticity,the strong convergence terms in positive voritcity regions at 700 hPa to 900 hPa was the main forcing source of positive voritcity tendency.When voritcity on lower level increased,it transported upwards and made the system develop,which illustrated the function of the main system dynamically.The absolute vorticity advection was related with the movement of trough and its intensity varieties on different levels,which reflected vertical changes of the influencing system on different levels during the snowstorm.The twisting term played a negative role on low level,which meant that the upwards movement distributed asymmetrically in horizontal direction which formed negative vorticity and it negatively affected the development of the snowstorm system.