The Southwest monsoon, explained
A monsoon is a seasonal wind shift, not a single storm. In summer, high pressure moves north over the Four Corners and the winds over Arizona and the Southwest turn to come from the south and southeast, pulling moisture up from Mexico and the Gulf of California. That moisture fuels afternoon thunderstorms over the desert.
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Why the wind turns around
Most of the year the desert Southwest gets dry westerly winds. In early summer the land heats intensely and a large area of high pressure builds over the Four Corners region. Air flows clockwise around a high, so on its southern and western side the wind blows from the southeast and south, straight from the warm waters of the Gulf of California and the Gulf of Mexico. Dew points that were in the 20s and 30s °F jump into the 50s and 60s.
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What it brings
- Afternoon thunderstorms build over mountains as the sun heats the ground and the moist air rises.
- Dust storms, sometimes huge walls called haboobs, when a storm's cold outflow wind races across dry ground.
- Flash floods in washes and canyons, often miles from where the rain fell.
- Muggy nights that cool off far less than normal desert nights.
When it happens
The National Weather Service in Arizona defines monsoon season as June 15 through September 30. The moisture usually arrives in early July and peaks in July and August. Some of it reaches Southern California too, bringing humid days and the occasional mountain thunderstorm.
Questions people ask
- Is the monsoon just the rain?
- No. The monsoon is the seasonal shift in winds that brings the moisture. The storms are what that moisture produces.
- Why do monsoon storms happen in the afternoon?
- Sunshine heats the ground through the day until the moist air near the surface rises fast enough to build towering storm clouds, usually from early afternoon into the evening.