National Weather Service Weather Forecast Office

Thunderstorms and Associated Weather Phenomena

Wind, flash floods, hail, lightning, and tornadoes.  All are spawned by the cumulonimbus, or thunderstorm clouds, sometimes all at the same time.  This cloud is an amazingly efficient weather factory. 

Nearly 2,000 thunderstorm cells are estimated to be present over the planet at any given time.  It is estimated that globally there are 16 million thunderstorms each year.  In the United States, central Florida has almost 100 thunderstorm days annually.  By comparison the number of thunderstorm days in Wyoming varies across the state from about 20 to 60 per year.  

The map below shows the average annual number of thunderstorm days in Wyoming based on a national map by the National Climatic Data Center (NCDC, 1981).  Thunderstorms occur with the greatest frequency in the eastern counties.  This is partly caused by the warmer
                 
surface temperatures allowed by the relatively low elevations in this region, but it is primarily the result of the more humid surface air there.  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.

Did you know that the Weather Forecast Office in Riverton offers weather spotter and safety training?  Each spring and summer staff from the Riverton office travel across western and central Wyoming providing the two hour multimedia presentations free of charge.  Weather spotters provide important reports to the NWS in Riverton during the thunderstorm season.  Click here for the current training schedule and for more information.

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