<P> The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon, is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, typically occurring between July and mid September . During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon - early evening . Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily . The monsoon typically loses its energy by mid-September when drier and cooler conditions are reestablished over the region . Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua . </P> <P> The North American Monsoon is not as strong or persistent as its Indian counterpart, mainly because the Mexican Plateau is not as high or as large as the Tibetan Plateau in Asia . However, the North American Monsoon shares most of the basic characteristics of its Indian counterpart . There is a shift in wind patterns in summer which occurs as Mexico and the southwest U.S. warm under intense solar heating . As this happens, the flow reverses . The prevailing winds start to flow from moist ocean areas into dry land areas . </P>

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