The North American Monsoon is the seasonal shift in the large-scale wind pattern over northwest Mexico and the southwestern United States that turns a hot, dry late spring into a summer of much more frequent thunderstorms. The word refers to the wind reversal itself, with the rain as its result.
What drives the shift
In summer, an upper-level high, often called the monsoon high or subtropical ridge, moves north. The National Weather Service (NWS) office in Albuquerque says that by July and into August its mean position is over New Mexico, and the Tucson office describes the 500 mb ridge center sitting over west Texas and New Mexico by July.
With the high in that position, winds across the Southwest turn more southerly. That flow carries moisture north from the Pacific Ocean, the Gulf of California and the Gulf of Mexico.
When it arrives
Onset moves north through the summer. The Climate Prediction Center (CPC) gives typical onset dates from early June in southwest Mexico to early July in Arizona and New Mexico, and the Grand Junction office says it generally begins around the second week of July for western Colorado and eastern Utah.
For awareness and preparedness, the NWS offices in Phoenix, Tucson and Albuquerque use a designated monsoon season of June 15 through September 30. The Albuquerque office puts average onset closer to early July in its own area.
The hazards it brings
Flash flooding is the leading concern. The Phoenix office calls flash floods the top thunderstorm-related killer in Arizona, and some of the most significant flash floods hit recent wildfire burn areas.
Organized outflow can raise walls of dust miles across, and even small dust events can cut highway visibility to a few hundred feet in seconds. Downbursts bring damaging winds, and any storm, however weak, can produce deadly lightning.
Early in the season, storms often bring lightning with little rain, which can start wildfires, and their gusty, erratic winds can spread fire quickly.
What the season label does not tell you
A monsoon season does not mean storms every day. Activity comes in active bursts, often tied to moisture surges, and drier breaks, and NWS Phoenix's 2025 review describes a quiet first half of the 2025 season when low pressure in the West pushed the subtropical high south and east.
Storms are also very local, so pair the season with daily moisture, instability and steering-flow forecasts rather than treating it as a guarantee.