Abstract: Using a variety of high spatial and temporal resolution observation data, an in-depth observation analysis is carried out on the formation and development mechanism of a snowstorm case happened on 1 Nov 2009 over Beijing and notable by its nature of the most early winter snowstorm in the past 60 years in Beijing. It shows that the snowstorm occurs under the favorable large scale atmospheric conditions. The enhanced 500 hPa trough over East Asian region together with the low level occluded front in Taihang and Yan mountain areas of North China provided strong forcing for the snowstorm. The occluded front is a terrain-driven cold-style occluded front system due to the encounter of the low level (below 1500 m) northwest to southeast cold air with the Taihang and Yan mountains. The low level cold air turns its original moving direction into west, and results in the moisture feeding to the occluded front zone from two different directions (the Bohai Sea and the edge of the water vapor-rich region in southern China).It shows that the snowstorm is the result of the low level occluded front, and precipitation distribution of 1101 process in 2009 is decided by the eastward-tilting structure of the occluded front, and Beijing is just located in the favorable front zone (the east side to the top of surface occluded front). Because vertical structure (below 1500 m) of the occluded front is shallower than the classic occluded front developed from the typical frontal cyclone, the vertical motion of the snowstorm case is not very strong. Therefore, its precipitation distributes evenly with time but lasts about 15 hours (rain first then it turns into snow) with large amount of the accumulated precipitation. The observation analysis based on wind profile suggests that the occluded front weakens from its bottom to top, which is caused by the cold air invading to the mature occluded front zone starting from layers below 500—800 m and then extending upward.Furthermore, the diagnosis shows that the precipitation transits from rain to snow because the surface air temperature decreases to near freezing point rapidly. The mechanisms for the temperature drop are not the same in different stages of precipitation. Evaporative cooling of the rainfall is the main contributor to the temperature drop before snowfall (from 00:00 to 08:00 on 1 November), while the low-level cold air advection plays the key role for maintaining lower air temperature during the whole snowfall period from 08:00 to 14:00 on 1 November in 2009.