General Circulation Models (GCMs) are being increasingly used to estimate the ecological impacts of global climatic change. The authors analyzed seasonal weather patterns over the conterminous U.S.A. and determined that regional patterns of rainfall seasonality appear to control the distributions of the major biomes of the country. These regional patterns were compared to the output from three GCMs for validation. The models appear to simulate the appropriate seasonal climates in the northern tier of states. However, the spatial extent of these regions is distorted. None of the models accurately portrayed the necessary seasonal rainfall patterns in the southern tier of states, where biomes are primarily influenced by the Bermuda High.