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Monsoon thunderstorms to increase in Southwest following one of driest, latest starts in history

By Kristina Pydynowski, AccuWeather senior meteorologist
July 24, 2019, 11:36:30 AM EDT


The North American monsoon is ramping up across the southwestern United States after one of the driest and latest starts to the season in history.

Despite the advantages of the welcome rainfall the monsoon brings, residents and visitors across the region should remain aware for lightning dangers, flash flooding, strong winds and haboobs (intense dust storms).

"Clockwise flow around an area of high pressure over the Plains will help to steer moisture from Mexico northwestward into the Southwest throughout this week," according to AccuWeather Senior Meteorologist and western U.S. weather blogger Brian Thompson.

Haboob July 22

A large dust storm, or haboob, sweeps across downtown Phoenix, Saturday afternoon, on July 21, 2012. Dust storms are common across Arizona during the summer, and walls of dust more than a mile high can blanket an area in a matter of seconds, sometimes reducing visibility to zero. (AP Photo/Mark Evans)


Combined with the strong heating of the day, that moisture will fuel thunderstorms daily across the Southwest.

"As is typically the case during the monsoon, thunderstorms will tend to blossom over the highest terrain during the afternoon hours and drift toward the lower elevations into the evening and overnight," according to AccuWeather Meteorologist Reneé Duff.

The mornings will be the better time for hikers to head to the Grand Canyon in northern Arizona, Gila National Forest in southwestern New Mexico or San Juan National Forest in southwestern Colorado before thunderstorms erupt in the afternoon.

"Afternoon thunderstorms will pop up as far west as the mountains of Southern California," Thompson added.

"Be watchful for rapidly growing or darkening clouds, which can be a sign that a thunderstorm is brewing," Duff stated. "If thunder is heard or lightning is seen, head immediately downhill to a valley or depression in the terrain."

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Lightning can strike areas well away from where it is raining, expanding the danger zone for those without proper shelter and potentially sparking wildfires.

Monsoon


It may be wise for hikers to not venture too far away from secure buildings that offer proper protection from both lightning and downpours.

Heavy thunderstorms throughout the Southwest can cause streams or dry creek beds, known as arroyos, to rapidly fill with dangerously fast-flowing water. Remember that quick-moving water at a depth of 6 inches can knock a person off their feet, and 12 inches can sweep most vehicles away.

The strongest thunderstorms may also kick up strong wind gusts, which may lead to localized damage and power outages. Dust storms that can rapidly drop visibility to near zero and raise the danger for multi-vehicle accidents can also be triggered in the deserts.

The weather pattern through the first half of this week could steer the thunderstorms and associated hazards along the I-10 corridor from Tucson to Phoenix.

"Thunderstorms that develop over the mountains of southeastern Arizona will move westward into the deserts," Thompson said.

El Paso, Texas; Las Vegas and Palm Springs, California, are other locations where the thunderstorms may spread over this week. These cities and others will be at risk for sudden storms that may bring blinding dust to some neighborhoods and flash flooding to others.

The thunderstorms will be a double-edged sword for firefighters battling the many blazes across the Desert Southwest. This includes the Museum Fire, which has burned 1,800 acres and prompted an evacuation notice for recreation areas north of Flagstaff, Arizona.

Rainfall will help to douse the blazes, and higher humidity will slow the rate the fires burn. However, winds rushing out away from thunderstorms can spread the fires. These winds can also be erratic, causing the fire to steer in different directions and put firefighters and residents in harm's way.

"With increased thunderstorm activity and increase in moisture, it will feel more humid," Thompson said.

Even though actual temperatures may fall short of 110 F, the humidity can push AccuWeather RealFeel® Temperatures to a dangerous mark in the deserts.

"The core of the monsoon moisture will retreat back into Mexico heading toward Friday and the upcoming weekend, which will lead to somewhat of a downturn in thunderstorm coverage," Thompson said.

As thunderstorm activity decreases, actual temperatures will soar back into the 110s in the deserts.

"By July 30-31, the high may drift back east, opening up more of the region for convective activity," according to AccuWeather Long-Range Meteorologist Max Vido.

"Even when the flow of moisture from the south is turned off, moisture can continue to be recycled in the region in the form of spotty showers and thunderstorms," according to AccuWeather Senior Meteorologist Alex Sosnowski.

Monsoon activity can vary in the Southwest on a yearly basis.

Monsoon on hold July 22


The North American monsoon season begins on June 15 and ends on Sept. 30 each year.

Flagstaff, Arizona, only recorded 0.14 of an inch of rain from June 15 to July 21, when 1.71 inches is more common.

Rainfall in Tucson, Arizona, has been held to 0.29 of an inch during that same time frame, making this the driest start to the monsoon season since 2005.

When the monsoon was declared early this week in Phoenix, it marked the second-latest start to the season on record. The 1987 season commenced on July 25, nearly two weeks after the average beginning date of July 7.

Download the free AccuWeather app to know when thunderstorms will threaten your area. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.

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