Main Menu
Responsive menu icon

What is the Southwest Summer Monsoon?

July 29, 2019
By WeatherBug's Alyssa Zimmerman
Just as the Southwest gets sick and tired of the hot temperatures and extremely dry air each summer, a cyclical weather pattern brings much needed heat relief in the form of heavy rain and storms. This change in the weather can produce flooding and even raise the wildfire threat.

People typically relate the word “monsoon” with heavy rain, and although they do produce thunderstorms, the word actually refers to a seasonal wind shift. This wind pattern causes the weather to switch between humid and stormy to hot and dry.   

Although monsoons are most commonly known for forming over India and Bangladesh, they also form along the Brazilian coast, Sub-Saharan Africa as well as northern Mexico and the Desert Southwest. Each year, after a hot and dry early-summer, the Southwest U.S. receives monsoon-fed scattered showers and thunderstorms. Historically, the monsoon begins during the first few weeks of July and runs through September.

The storms brought on by the monsoon typically produce up to 70 percent of the year’s rain for much of the Southwest and Texas. During an average monsoon season, cities across the Southwest typically receive anywhere from 1 to 8 inches of rain. The wettest monsoon in history was in 1986, dropping 20 inches of rain over Flagstaff, Ariz., while many of the driest years brought only trace amounts of rain.  

Although the season doesn’t start to feed thunderstorms until the second half of summer, the ingredients needed for the Southwest monsoon begin to take shape during late spring and early summer. A thermally-induced area of low pressure forms over the Desert Southwest and northwest Mexico due to hot temperatures and dry desert air. This coupled with an upper-level high pressure area over the southern Plains and cooler air over the Gulf of California drives the Southwest monsoon.

By early July, the temperature and pressure differences are usually big enough for the monsoon flow to makes its way across the U.S. and Mexico border, bringing tropical moisture from the Gulf of California and Mexico. The moisture-laden air is lifted high in the atmosphere by the rising hot air over the Desert Southwest. This lift causes clouds and strong thunderstorms with frequent lightning to form over the mountains and desert later in the afternoon each day.

Residents across the desert should stay weather aware as these storms can bring flash flooding, downburst winds, lightning and dust storms. Wildfires are also known to spark up during the monsoon due to the volatile mix of extreme summer temperatures and lightning.

Image: Rainstorm in Eastern New Mexico associated with the North American Monsoon, which draws moisture from the Gulf of Mexico during the late summer months. (Wikimedia Common)