Arizona’s monsoon is the northern extent of the North American Monsoon, which begins in early June in central and southern Mexico.The monsoon is a season, during which the dry westerly winds that persist through fall, winter and spring shift to moist southerly winds, bringing thunderstorm activity into Arizona.
The monsoon activity accounts for roughly half the annual precipitation in central Arizona, and two-thirds to three-fourths of the annual precipitation in southern Arizona. The short-lived, intense thunderstorms often result in flash flooding in steep terrain, as well as urban flooding through low-lying roads and normally dry washes.
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Long Term Changes of Transient Inverted Troughs Lahmers, et al