On 5 December 2015, with strong southwest warm and humid air and weak cold air, a heavy snowstorm occurred in the area from Hangzhou of Zhejiang Province to Huangshan of Anhui Province. The forecast for this process was quite inaccurate. The analyses of water vapor, power and temperature conditions in this paper show that the strong southwest warm and humid airflow, the convergence of cold and warm air provided abundant water vapor and dynamic conditions for the generation of a large amount of surface precipitation. The deep wet layer, suitable intermediate temperature and humidity conditions were favorable for the generation of a large number of snow and ice crystals for landing. The abnormal temperature drop at the lower level of Hangzhou Station before the snowfall was the key cause for snowfall. The analysis of the cooling mechanism shows that the cause of the low-level temperature drop at Hangzhou Station was mainly non-adiabatic heating related to water vapor phase transition, and the effect of cold advection was weak. The low-level cooling before 08:00 BT on 5 December was mainly caused by the evaporation of precipitation particles; the low-level cooling during the day was mainly caused by a large amount of ice (snow) crystal melting to absorb the latent heat of the environment, and the cold advection and vertical transport items also contributed. The main reason for the failure of the forecast was the dependence on temperature index and the insufficient analysis of the cooling mechanism. In this case, the cooling mechanism such as advection was weak, and the melting and heat absorption of ice crystals caused the middle and lower layers to form a uniform temperature layer of 0℃, so that the ice crystals reached the ground. The middle and lower temperature was quite different from the temperature index commonly used in discerning precipitation type, which indicates that the temperature index can not be applied mechanically, and the physical mechanism of snowfall formation should be comprehensively analyzed. When precipitation amount is large, melting heat absorption can become the main mechanism of cooling, which should be fully considered in the forecasting operation.