A rare convective event known as a thundersnow storm tracked across eastern Iowa, northern Illinois and central Illinois on the evening of February 11th, 2003. The storm produced lightning, heavy snow, and winds that were by definition severe in nature prompting the National Weather Service in Lincoln to issue a severe thunderstorm warning for several counties in central Illinois. An analysis of key synoptic and mesoscale processes in various levels of the atmosphere illustrates that when specific ingredients phase together, rapid intensification into a severe thundersnow storm can occur. Some influential features of this particular storm include the evolution of the cold front into an anafront, organized upper level dynamics associated with a strong jet streak, and locally enhanced moisture and warm air advection at the surface. The uncharacteristic severe nature of this winter storm was not anticipated by forecasters since atmospheric conditions leading to the development of thundersnow storms have not been well documented. The goal of this paper is to distinguish key parameters such as those mentioned above that are involved with the evolution of severe thundersnow storms to aid in the dissemination of warnings by forecasters in winter storm situations.