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Fox News Weather Center

Monsoon storms to unleash flooding but replenish reservoirs in western US

The return of the monsoon can bring both needed rain and dangerous conditions to parts of the interior West during the third and fourth weeks of July.

As intense heat is forecast to build over the middle of the nation, a flow of moisture will re-introduce thunderstorms to part of the western United States.

"A clockwise flow of air around a massive area of high pressure forecast over the Great Plains will pump moisture northward from Mexico starting next week," according to AccuWeather Lead Long Range Meteorologist Paul Pastelok.

"A west-to-east flow of air from the Pacific Ocean had recently turned off the monsoon, which began in June," Pastelok said.

The influx of moisture will lead to high humidity levels. As the high humidity combines with daytime heating, thunderstorms will erupt.

"The storms typically form over the mountains and drift toward the lower elevations and deserts toward evening," according to AccuWeather Meteorologist Jim Andrews.

Most of the storms will take aim at portions of New Mexico, Arizona, Colorado, Utah, Texas and Wyoming during the second half of July.

Most of the storms will occur east of California and Nevada.

Cities that could be hit by multiple the storms in the coming weeks include Provo, Utah; Albuquerque, New Mexico; Rawlins, Wyoming; El Paso, Texas; Denver and Phoenix.

Approximately half of the annual rainfall in the desert areas occurs from late June to early September. Much of this rain falls during a handful of days when thunderstorms produced by the monsoon arrive.

Just as the storms wet the landscape and add water to reservoirs and rivers, the sudden rain can quickly flood arroyos and fill canyons with little notice. Storms from a dozen miles away can result in a flash flood. Runoff from the storms can wash a significant amount of mud and rocks on roadways.

Campers and hikers will need to keep an eye out for towering clouds. In addition to the risk of flash flooding in the canyons, lightning can strike without warning over the ridges.

Not all of the storms will bring heavy rainfall.

While the vast majority of wildfires are caused by humans, slightly less than 10 percent are caused by lightning strikes, according to the U.S. National Park Service. However, the vast majority of lightning-induced fires occurs in the western U.S. with close to 4 million acres burned annually, according to the National Interagency Fire Center.

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Sparks from a lightning strike can quickly turn into a raging inferno in the high temperatures and dry vegetation.

In certain situations, gusty winds produced by the storms with no rain can kick up dust. The dust can sweep across populated areas and major highways as a threat to pedestrians and motorists.

Most likely, not enough rain will fall from the monsoon to alleviate dry or drought conditions.

"We have concerns that high pressure may settle over the southwestern U.S. later this summer, fall and winter, which would greatly limit the supply of rain [and snow] in the region," Pastelok said.