North American Monsoon

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, centered roughly on the Rio Grande Valley, and typically occurring between June and mid-September. During the monsoon, 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 monsoon typically loses its energy by mid-September when much drier 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. Mechanism The North American monsoon is a complex weather process that brings moisture from the Gulf of California (and to lesser extent the eastern Pacific and Gulf of Mexico) over northwestern Mexico and southwestern US resulting in summer thunderstorms, especially at higher elevations. 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. In the monsoon area, the late spring period is very hot and dry, because the dry western side subtropical high pressure ridge and dry continental air have not begun to 'shift' northwards yet. During this period, inland areas have extremely low relative humidity, and characteristically very low dew points, which are frequently well below the freezing mark. In some years, this delayed effect is more substantial if the dryline which separates the hot and dry airmass to the Northwest from the humid monsoon airmass to the Southeast, fails to migrate. This can prevent tropical moisture from reaching farther Northwest towards Death Valley until later in the summer. If this pattern prevails, the Nevada deserts may receive almost no monsoon.