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What is the Southwest Summer Monsoon?

UPDATED July 7, 2014

By WeatherBug Sr. Meteorologist, James West


Every July, a subtle but important change occurs across the Southwest, ushering in more stormy, humid weather and usually an end to the prolonged extreme heat of early summer. This weather shift is caused by the Summer or Southwest Monsoon.

Monsoon: Its definition

The word "monsoon" is derived from an Arabic word meaning season. It is defined as a seasonal wind that shifts direction from season to season and often brings abundant rain and thunderstorms during one time of the year and hot, dry weather at other times.

Monsoons happen in several locations around the world, including along the Brazilian coast of South America, in Sub-Sahara Africa, across northern Mexico and the Desert Southwest of the U.S. The most famous monsoon occurs in India and Bangladesh, where devastating drought and flooding is influenced by the shifting and unpredictable nature of the monsoon.

The Southwest U.S. Monsoon

Every year starting in early July and lasting through September, large stretches of the Southwest U.S. from southern Arizona east into western Texas see monsoon-fed scattered and strong thunderstorms, often daily.

These storms can bring up to 70 percent of the year`s rain for many of these arid locations. Downpours brought with the storms often trigger flash flooding in the dry river beds, canyons and arroyos scattered throughout the region.

Although it doesn`t start to feed summertime thunderstorms until the second half of summer, the ingredients needed for the Southwest monsoon begin to take shape during late spring and early summer.

During this time, the sun gets higher in the sky, heating the ground throughout the deserts of northwestern Mexico, Arizona, New Mexico, Utah and Nevada. The ever increasing temperatures and dry desert air cause the air density to decrease, which creates a thermally-induced area of low pressure to form.

Meanwhile, an upper-level high pressure area begins to take shape over the southern Plains. Couple this with the relatively cooler air over the Gulf of California and a southwesterly or even southeasterly monsoonal flow begins to take shape across northern Mexico.

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 gulfs of California and Mexico with it.

This more moisture-laden air, as it crosses northern Mexico and into the U.S., is lifted high in the atmosphere by the rising hot air at the surface and the mountains and higher terrain prevalent throughout southern Arizona, New Mexico and extreme western Texas. This lift causes clouds and thunderstorms to form over the mountains later in the afternoon, bringing downpours, strong wind gusts and frequent lightning.

Under the right conditions, these storms will move out of the mountains and deeper into the U.S., bringing storms to more of Arizona and New Mexico and possibly making it into southern parts of Nevada, Utah and Colorado. Flash flooding is a real danger from the strongest monsoon thunderstorms.

During summers when the monsoon is weaker than normal, fewer thunderstorms make it into the Southwest. This increases the likelihood of drought and causes the fire danger to spike.

By early September, the cooler weather of autumn will begin to flip the winds from southerly to more westerly, cutting off the moisture flow and decreasing the monsoonal instability.

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Story 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)

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