PHOENIX – When people think of the Desert Southwest, their first thoughts may not gravitate to rainfall, but during three months of the year, images of thunderstorms and flooding are actually common.
While the start and end dates of each year’s activity can vary, the monsoon season generally runs from June 15 through September 30.
During this stretch of the summer, a ridge of high pressure becomes established and winds turn more southerly, allowing moisture from the Pacific, Gulf of California and Gulf of America to converge.
While some of the heaviest precipitation occurs over northwestern Mexico, communities from Southern California to Colorado and West Texas benefit from the increased chances of precipitation.
Near where the heaviest precipitation falls, flash flooding, mudslides, dangerous lightning and even dust storms – known as haboobs – often develop.
The severity and the length of the monsoon pattern can vary depending on the status of features such as the El Niño Southern Oscillation or what is often referred to as the ENSO for short.
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According to NOAA’s Climate Assessment for the Southwest team, El Niño years tend to delay the arrival of monsoon rains and can suppress overall storm activity.
In contrast, La Niña events are typically associated with an earlier onset and more consistent rainfall, although the pattern usually produces fewer tropical cyclones in the eastern Pacific.
Overall, cities across the Desert Southwest receive up to half of their annual precipitation during the monsoon months of June, July, August and September, but these totals can vary dramatically from year to year.
The wettest monsoon pattern occurred in 1984 when 9.56″ of rain fell over the Phoenix Sky Harbor International Airport, with two of the more nonexistent monsoons occurring in 2023 and 1924, when 0.15″ and 0.35″ of precipitation fell, respectively.
The summer precipitation is considered to be vital for the replenishment of water resources and the survival of ecosystems.
According to the Arizona Department of Agriculture, cabbage, melons, citrus fruits and tomatoes are popular products and serve as important sources of food during the winter months.
The absence of any type of monsoon coverage can cause lower water levels of rivers and reservoirs, economic losses and increased wildfire dangers.
Water systems that are dependent on monsoon precipitation are generally south of the Colorado River, which relies heavily on snowmelt versus the summertime rains.
The largest reservoir in the U.S. is Lake Mead, which is regulated by the Hoover Dam, but an estimated 96% of its water comes from snowmelt along the Rocky Mountains.
For much of the region, the wet weather pattern fades by mid-September, giving way to a drier fall and winter before mountain snowfall becomes the main focus.
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