Nature’s Way: Arizona Monsoons

Gerry Tietje

When we moved to SaddleBrooke Ranch a few years ago in June, we heard people talking about the monsoons arriving soon. I had visions of torrential rains like those that occur in India. Happily, this didn’t prove to be the case. Monsoons in Arizona are relatively tame by comparison, but nevertheless, have a notable impact on our desert ecosystem and on those of us who stay here in summer and witness this unique natural phenomenon.

Monsoons occur when there is a seasonal reversal of wind direction accompanied by increased precipitation. During the North American monsoon season, dry westerly winds are replaced with moisture-laden winds that blow from the southeast and southwest for a few months, bringing tropical moisture first into Mexico, and then into parts of Arizona, New Mexico, and Texas. The monsoons begin when a high-pressure zone develops over the “Four Corners” area and begins rotating in a clockwise direction. The accompanying winds pick up moisture from the Gulf of Mexico. Other moisture arrives from the Gulf of California. When these two fronts meet over central Mexico in early May, thunderstorms develop over the Sierra Madre Occidental Mountains. The monsoons advance in a northwesterly direction and generally reach Tucson by late June or early July. As the moisture-laden air rises to higher elevations, thunderclouds form and precipitation occurs in our area, especially in the Santa Catalina Mountains. The monsoons reach Phoenix a couple of weeks after they reach Tucson, and largely dissipate by the time they reach western Arizona.

The monsoon season in southeastern Arizona can begin by mid-June and last until the end of September. Most of the annual rainfall in the Tucson area occurs from the monsoons. The University of Arizona has an ongoing research program on the Arizona monsoons and has published the following information: in Tucson, the normal rainfall during the monsoon season is 5.65 inches; normal rainfall in Phoenix is 2.43 inches; and normal rainfall for Yuma is 1.13 inches. Tucson received 13.84 inches of rainfall from the monsoons in 1964, a record high, and 1.59 inches in 1924, a record low. In 2023, Tucson received only 4.73 inches of rainfall during the monsoons; the first rain fell on July 17 and the last on Sept. 13.

The monsoons are both dangerous and beneficial. Lightning-laden thunderstorms can ignite wildfires and cause flash flooding, especially in the washes. Fierce winds can blow tiles off roofs and break limbs off trees, especially Palo Verde trees. Dust storms cause vehicle accidents on our roadways and microbursts cause localized damage. But monsoons are also beneficial in providing much needed rainfall to the Sonoran Desert ecosystem, allowing both flora and fauna to survive intense desert heat in summer. Cacti, like the giant saguaros, noticeably increase in circumference as they soak up the monsoon rain. Animals, like Gila monsters, that seldom need to drink water, would die without the monsoons. Monsoonal rains also increase our ground water and can help firefighters quench raging wildfires.