Southwest monsoon season officially begins as dramatic atmospheric flip underway from heat to flash flooding
Now, a large-scale atmospheric shift is officially underway as monsoon season arrives.
FILE: Monsoon season is underway in Arizona, bringing the risk of dust storms and flash flooding
University of Arizona Professor of Practice in Hydrology and Atmospheric Sciences Eyad Atallah joins FOX Weather to discuss this year's monsoon season.
As the Southwest endures another summer of punishing heat, an inevitable threat is beginning to emerge: monsoon season.
For weeks, the Desert Southwest has baked under a relentless summer heat dome, enduring triple-digit heat waves and explosive wildfires. Now, a large-scale atmospheric shift is officially underway.
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Deep tropical moisture will surge northward from the Gulf of California and the eastern Pacific, fueling afternoon showers and storms that could lead to flash flooding through Wednesday.
FILE PHOTO: Dark skies prevail as monsoon storms continue over Glen Canyon National Recreation Area on August 22, 2024 near Page, Arizona.
(David McNew / Getty Images)
A monsoon is a seasonal reversal of prevailing winds driven by the temperature contrast between land and ocean, pumping vast amounts of moisture into a region.
While the term is sometimes used to describe a heavy downpour, it actually refers to an entire season marked by shifting winds, increased humidity and frequent thunderstorms.
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Rain and thunderstorms will persist through the first half of the week, with daily precipitation chances spanning from Arizona as far north as the northern Rockies. While the most notable flash flood threat remains concentrated near the Four Corners, localized flooding will be possible border-to-border.
FILE PHOTO: Monsoon rains wet the desert in in Kayenta, Arizona on Sept. 2, 2025.
(Robert Ray / FOX Weather)
To complicate matters, thunderstorms will erupt over the higher terrain during the peak of afternoon heating before drifting slowly into the desert valleys.
With weak wind shear aloft resulting in slow, erratic storm motion, the heaviest downpours could stall over the same areas or merge together, increasing the flash flood threat.
Rainfall totals will generally average 1 to 2 inches in most locations; however, where these storms park themselves, localized totals of 3 to 5 inches are entirely possible.
FILE: Watch: Intense monsoon downpour pounds Phoenix
A video recorded in Phoenix on Friday, Aug. 22, shows an intense monsoon rainstorm with lightning.
As forecast models continue to sharpen over the coming days, the exact timing and placement of the greatest threats will become clearer. Stay with FOX Weather as we continue to track the evolving monsoon threat across the Southwest.