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Arizona’s Monsoon Season

Lightning behind the Santa Catalina Mountains—Tucson, Arizona.

In late June, the mercury rises to uncomfortable levels. Dry grasses bake in relentless sun and parking is determined by shade, not distance. Then the wind picks up. Clouds start rolling in. Cumulus thunderheads darken the sky, and most Arizonans wait eagerly for the first quenching drops of rain. The monsoon season has arrived.

Monsoon clouds

Though the word monsoon is often used to refer to a single thunderstorm, it is actually the name of the large scale weather pattern. The word derives from the word mausin, Arabic for “season” or “wind shift.” It is now used to refer to a seasonal wind shift and the precipitation produced as a result. Parts of Africa, Asia, North America, Australia, and Europe experience a monsoon season.

A monsoon is caused by warm air creating surface low pressure zones that in turn draw moist air from the oceans. Arizona winds usually come from the west, but shift to a southeasterly wind in the summer, bringing moisture, most often from the Gulfs of Mexico and California. The wind shift and increase in moisture combine with the surface low pressure from the desert heat to produce storms in a cycle of “bursts” (heavy rainfall) and “breaks” (reduced rainfall). Before the rain, the wind shift can trigger dust storms known as haboobs, which appear as loose swirling walls of dust several hundred feet high.

Flash flood video

The North American monsoon occurs over northwest Mexico, Arizona, New Mexico, and parts of Colorado and Utah. In the U.S., southeastern Arizona and western New Mexico usually experience more strong winds and receive the most rainfall. The North American monsoon season’s existence was initially debated, but it was legitimized in by organizations like the Southwest Arizona Monsoon Project (SWAMP) and the North American Monsoon Experiment (NAME). Although not as strong as the Asian monsoon, the cycle is typical of monsoon seasons worldwide.

The monsoon season begins on June 15 and ends on September 30, but the storms peak between mid-July and mid-August. On average, about half of Arizona receives about half of its annual rainfall during the monsoon. Arizona receives a statewide average of only 12.5 inches of rain per year. However, rainfall can vary tremendously between distances of just a few miles. Mountainous areas tend to receive the most. In fact, mountain ranges such as the Sky Islands and the Mogollon Rim can act as a focusing mechanism for thunderstorms. Get rainfall information and measure rain data at the rainlog citizen science project.

Downpours are often short in duration, but rainfall is heavy. In mountainous areas, the torrent can cause flash floods capable of downing vegetation and relocating boulders. When water picks up debris from fire damaged areas, the combined force of the roiling water and heavy objects is capable of causing mudslides that can wipe out trees or trails in a matter of minutes.