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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.

Monsoon Safety

Summer storms can be dangerous for Arizona residents. Flash floods can be dangerous to drivers, hikers, and people near washes. Winds almost always exceed 40 mph and can create dust storms. Tornados are not common in Arizona, but they are recorded a few times per year. Hail is more of a possibility in mountains. Excessive heat is dangerous during the monsoon season, as it is all summer. High temperatures can be more dangerous when combined with the high humidity.

Drivers should be safety-conscious during dust storms and thunderstorms.

Be safe during the monsoon season! Homeowners who live in flood areas should take necessary precautions to protect themselves and their homes, and drivers should never enter flooded areas. Hikers and campers should avoid canyons with steep sides. If thunder is audible, lightning is close and it is advised to go indoors and remain there until 30 minutes after the last lightning sighting. Be sure to avoid downed power lines. Dust storms are often an underrated danger. Dust storms are especially dangerous to drivers. If caught in a dust storm, immediately pull off the road, turn your car off, and take your foot off the brake pedal.

  Thirty Years of Lightening Photography in Southern Arizona by Ralph Wetmore is available in the Arizona Experience Store.