By using National Centers for Environmental Prediction (NCEP) Final (FNL) operational global analysis data, the formation and developing mechanisms of the two vortices that caused snowstorms, on 16 and 25 February 2015 are investigated in terms of dynamics diagnosis. The results show that the formation and strengthening of vortices are the direct causes for the snowstorms, but the associated mechanisms are quite different in the two snowstorm cases. Positive tendencies of relative vorticity caused by convergence in lower troposphere and vertical transportation are the mainly forcing items for the formation of vortices in the two cases, respectively. On 16 February, there was a vortex moving eastward at 500 hPa. The cold advection near the bottom of the vortex deepened the trough at 500 hPa and the positive vorticity advection strengthened. Therefore, the increase of differential vorticity advection〖JP2〗 resulted in the developing of upward motion, and then the〖JP〗 enhancement of the convergence at 850 hPa. On the other hand, on 25 February, the vertical vorticity transport by strong upward movement was the major developing mechanism of the vortex at 850 hPa.