The Southwest US Monsoon Was Strange. Phoenix Was Record Dry, But California and Nevada Were Soaked

 Jonathan Erdman

Published: October 2, 2023

The Southwest monsoon has wrapped up, and it featured a weird split of record dry conditions in parts of Arizona and soaking rain, including from the remnants of a former hurricane in California, Nevada and other states.

W​hat is the Southwest monsoon? As we detailed in a previous piece, a monsoon is simply a seasonal change in winds. In the Southwest U.S., this happens from intense heating of the land in summer, compared to surrounding water. A switch in winds taps moisture from the eastern Pacific Ocean, Gulf of California, even the Gulf of Mexico generally from late June through September.

T​his leads to an increase in thunderstorms with dust-laden winds, heavy rain and prolific lightning. In just three months (July-September), a typical monsoon brings half of the year's annual rain in Tucson, Arizona.

One city was record dry*. On one end of the spectrum, Phoenix had its driest monsoon in 128 years, managing only a paltry 0.15 inch of rain from mid-June through September, according to the National Weather Service. Typically, Phoenix gets just under 2.5 inches of rain in that 3.5 month period.

B​ut there's a caveat. Many other Arizona locations reported much more rainfall than Phoenix, including some metro area suburbs. That's common, since most monsoon thunderstorms develop over the higher terrain east, north and southeast of downtown Phoenix.

But even the wetter locations were all generally drier than usual this season.

W​hy so dry? Bubbles of high pressure aloft known as "heat domes" are important in setting up the wet monsoon pattern, helping to steer moisture into the Southwest.

But this past summer, these heat domes were unusually persistent over areas from southern Arizona and northern Mexico to Texas, much farther south than usual.

T​hat configuration baked parts of Arizona, New Mexico and Texas in searing heat and shunted the usual flow of moisture into these areas.

N​ot surprisingly, it was also the record hottest monsoon season in Phoenix. Following a wet winter and spring, this dry monsoon allowed drought to rapidly develop in parts of Arizona, much of New Mexico and southwest Colorado by the end of September.

This map shows anomalies in the upper-level weather pattern from mid-June through the end of September 2023. That includes a stronger than average heat dome or ridge of high pressure in northern Mexico and parts of the Southwest, responsible for a drier monsoon, there.

Other parts of the desert were soaked. This monsoon couldn't have been more different once you neared and crossed the Colorado River.

M​uch of the Mojave Desert of California, including the L.A. Basin into Nevada, far northwest Arizona and southwest Utah, was wetter than average.

F​or example, Palm Springs, California, had its fourth soggiest monsoon and wettest in 40 years. Las Vegas also had a top 10 wettest monsoon, picking up almost 3 inches of rain. That's impressive for a city that averages just over 4 inches of rain for an entire year.

This map shows the percent of average precipitation over the Desert Southwest over a 90-day period ending Oct. 1, 2023. Blue and purple contours show much wetter than average areas, from California into Nevada, southwest Utah and northwest Arizona.

I​t was largely Hilary. The biggest component of this soggier Southwest was Tropical Storm Hilary.

I​n mid-August, Hilary became the first tropical storm to enter California since 1997. But unlike 1997's Nora, Hilary's center tracked directly over Southern California and pulled a deep plume of moisture as far north as the northern Great Basin.

D​eath Valley smashed their all-time calendar-day record, with a year's worth of precipitation in just one day. U​p to 9 inches of rain in the mountains west of Las Vegas triggered damaging flash flooding to roads and some structures in Kyle Canyon.

E​ven areas that typically don't pick up much rain during a monsoon were soaked by Hilary, including downtown L.A. (2.48 inches) and San Diego (1.82 inches).

O​ne benefit of all this rain was it kept soil moisture a bit higher than otherwise in California, Nevada and parts of the Great Basin, lessening both short-term drought and wildfire concerns, at least into early fall.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He studied physics at the University of Wisconsin-Madison, then completed his Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads and Facebook.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.