Moist air moving across the Great Plains is often too shallow to cross the Laramie Range and the Big Horn Mountains. As a result, the surface air is drier on the west side of these topographic barriers and thunderstorms are correspondingly less common. Moisture from the Pacific Ocean fuels thunderstorms in western Wyoming, but the intervening middle third of the state, and especially the Big Horn Basin area, is relatively protected from thunderstorms by topographic barriers to low-level moisture airflow. Thunderstorms occur most often during the summer months of June, July and August when surface temperatures are highest, reaching a peak in the number of thunderstorm days in July. Almost no thunderstorms occur from November through February.
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