Abstract:Atypical snowfall forecast is a complicated issure in Beijing area. It is especially difficult to determine the role played by the east wind in light snow process. Based on the conventional observation data and various intensive sounding data，two cases including a false snowfall prediction and a first snowfall process occurring in winter 2010 are analyzed. The analysis shows that: Wet or dry conditions for the east wind, depend mainly on the humidity of the upstream eastern region. When the upstream eastern region is dry, the east wind is dry, and thus could not provide moisture for Beijing. The cause for the false snowfall prediction is that the east wind is a dry advection which is unfavorable to humidification at night on December 12, 2010. Low humidity in the boundary layer is the main reason in spite of favorable vertical upward motion. The dry and cold airs in areas north of Beijing moving southwards rapidly and invading are the other major reason for the forecast deviation. Under the favorable condition of abundant water vapor in the boundary layer and weak convergence and ascending motion， a steady rainfall process occurred. The results show that moisture conditions in the boundary layer are very important for winter snowfall prediction. It is difficult to cause effective precipitation even with strong ascending motion in upper air but poor water vapor condition in the boundary layer. In fact， distinct precipitation may occur in case of warm and wet air mass and weak convergence and ascending motion in the boundary layer even without apparent convergence systems.