Abstract: Based on the conventional observation data,satellite cloud images and radar echo data,the causes of a weather process of drouth turning to rain in Tongliao City of Inner Mongolia during April 18th～21st of 2009 were analyzed.The results showed that the advantageous large-scale circulation,ample vapor conditions,stronger ascending motion and cold air intrusion in low layers caused the occurrence of this rainfall.The eastward shift of upper trough reinforced to form the low vortex and inverted trough appeared on the ground,which led to the precipitation.The data analysis of new-generation weather radar showed that the echoes had obvious and typical characteristics of general rainfall.The forecast effects of using numerical forecast products on this process were better.Further application and research of numerical forecast products was an effective way to enhance the forecast quality of great weather processes.