What is a monsoon?
The amount of rain in Maricopa County is highly variable from year to year, but one thing that County residents can rely on is the monsoon. A monsoon refers to a change in the seasonal wind direction. The term is correctly used to describe a season, much in the same manner as using the term "summer." The terms "monsoon" and "thunderstorm" refer to different weather events, but it is correct to say "monsoon thunderstorm."
When is the monsoon?
Maricopa County has two rainy seasons: summer and winter. Winter usually brings longer-lasting but less-intense storms. Summer brings shorter, more intense thunderstorms. These summer thunderstorms are usually the result of the North American Monsoon (also called the Arizona Monsoon or the Mexican Monsoon). The North American Monsoon impacts the southwestern United States and northwestern Mexico every summer (usually July, August and September).
How does the monsoon work?
During the winter, winds in Arizona normally come from the west or northwest. As summer approaches, winds shift to a southerly or southeasterly direction, allowing moisture to stream into Arizona from the Pacific Ocean, the Gulf of California and the Gulf of Mexico. Summer heating warms the desert and city surfaces, allowing large cumulonimbus clouds to form in the humid air. These thunderstorms bring rain and sometimes severe weather. To learn more about the meteorology involved, visit the ASU School of Geographical Sciences or the National Weather Service.
What happens during the monsoon?
Monsoon thunderstorms not only bring almost one-third of the annual rainfall in Maricopa County, they can also cause flash floods, lightning, strong winds, dust storms and hail. Monsoon thunderstorms have caused significant property damage and, unfortunately, several fatalities.
Dust storms and flash floods contribute to numerous auto accidents. Every year, motorists ignore warning signs and barricades, and become trapped in low-lying flooded washes, forcing expensive and dangerous rescue operations by city and county emergency personnel. In addition, strong thunderstorm winds destroy fences and roofs, knocking down utility poles and creating power outages. Lighting kills two people every year in Arizona on average, with many more sustaining permanent injuries.
Be aware and be prepared!