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 seasonal increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico.
Geographically, the weather pattern is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua, and typically occurs between June and mid-September. Thunderstorms are fueled by daytime heating and build up during the late afternoon and early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The cycle typically loses its energy by mid-September when much drier conditions are reestablished over the region.
