Recent satellite observations indicate the existence of plumes above the anvil of some midlatitudes severe thunderstorms but the plume formation mechanism has not been identified. The present study utilizes a three-dimensional, nonhydrostatic cloud model with sufficiently sophisticated microphysics to perform simulations of a supercell storm occurred in US Midwest in August 1981 to illustrate a hypothesis of the plume formation mechanism. The model results show the presence of water vapor plumes above the anvil and their characteristics fit well with the observed plumes. Analysis of the model results shows that the source of water vapor for the plumes is the shell of the overshooting dome. The strong pressure gradients in the shell, caused by the strong updrafts, may overcome the stability above the tropopause and allow water vapor to inject into the stratosphere. Thus the hypothesis suggests that the anvil-top plumes are a mechanism of troposphere-to-stratosphere water vapor transport. This mechanism can also explain the recently observed seasonal variations and north-south hemispheric asymmetry of water vapor in the upper troposphere/lower stratosphere region. It also suggests that other atmospheric trace chemicals can be transported similarly. If the hypothesis is verified, then the plume mechanism represents a source of water vapor and other trace chemicals in the midlatitude lower stratosphere and its impact must be carefully considered.