An extreme event, an unusual snowstorm happening in South China in January 2008, is studied with observational data analysis and numerical modeling experiments. It is found that diabatic heating was centered in the tropical oceans, Central Asia, the Tibetan Plateau, and the North Atlantic Ocean. The results from the numerical experiments demonstrate that global diabatic heating anomalies are responsible for this extreme event. A series of numerical experiments are further designed to identify the key areas where the diabatic heating anomalies occur. The results indicate that the diabatic heating anomalies in Central Asia and the Tibetan Plateau could account for the extreme heavy snow event. Such anomalies generate anomalous air mass rising as well as southerly winds over South China. These features facilitate the water vapor transportation and result in the heavy rain and snow. Meanwhile, an anomalous heating in the North Atlantic enhances the positive phase of the North Atlantic Oscillation, which could contribute to the occurrence of this unusual snow event. On the contrary, a La Nina event happening over the same period, strengthening the heat source in the western Pacific and the heat sink in the eastern and central Pacific, is more likely to counteract the ascent and suppress the rain and snow over South China.