MM5 model is used to simulate the process of snowstorm in Henan Province from Ja nuary 18th to 19th in 2006. High spacetime resolution data from mo del output are imposed to analyze vapor，temperature，instability，wind and othe r conditions of this process．Results show that the convergence of middlelow l ayer vapor is absolutely necessary condition．It is most favorable for snowstorm that upper boundary of inversion layer over Henan Province corresponds to sout hwest jet flow and lower boundary corresponds to northeast jet flow．The stead y maintaining of highlow jet flow generated two separate secondary circumfluence and the n orth of indirect circumfluence formd convergence ascending air at the exit of up perlevel jet flow．This situation quite favor the generation and development of precipitation．Total helicity changes have th e significant indication to changes of snow intensity．Moist potential vorticity was analyzed to find that MPV2 is more important than MPV1 in this event, where as in the case of rainstorm, MPV1 plays a dominant role．