Flash flood, dust storm threats to continue as North America monsoon carries on

By Alex Sosnowski, AccuWeather senior meteorologist

July 14, 2018, 11:41:14 AM EDT

An annual event, known as the North America monsoon, will continue to bring the risks of flash flooding, dust storms and lightning strikes over the southwestern United States in the weeks to come.

While not every location over the interior Southwest will be hit with a shower, a gust of wind or a thunderstorm every day, a few communities and stretches of highways can be hit hard on a given day into early next week and beyond.

Storms are not expected to reach the California coast over the next few days, but storms are likely to erupt over part of the Sierra Nevada and perhaps some of the High Deserts in the southeastern part of the state on occasion.

Storms are likely on a nearly daily basis in much of New Mexico, Arizona, Colorado and Utah, as well as southern Nevada and far-western Texas.

What is the monsoon?

A monsoon is merely a change in wind direction that can trigger rainfall or long-duration dry weather. The monsoon is well known in southeast Asia, but it occurs in other parts of the world as well.

In the case of the southwestern United States, this change in wind directions happens when breezes bring up moist air from Mexico that has originated from the Gulf of Mexico and the tropical Pacific Ocean.

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Officially, the North American monsoon is triggered when a high amount of moisture (high humidity) in the air is present for three days in a row.

"However, in 2008, National Weather Service officials in Phoenix decided to make the monsoon easier for people to understand and follow by assigning a permanent start and end to the season, just like hurricane season," according to AccuWeather Senior Meteorologist Kristina Pydynowski.

As a result, the monsoon season now begins on June 15 and ends on Sept. 30.

Expect highly variable rainfall

The North America monsoon brings an average of one-quarter to one-half of the annual rainfall to the Southwest.

The rainfall can breathe life into the landscape, fill some streams and reservoirs and temporarily reduce the threat of wildfires. However, the rainfall can be disruptive as well.

While much of the rainfall occurs in small amounts during several days each week, some of the rain can be excessive and fall in a short period of time, resulting in flash flooding.

For example, in the Phoenix area, between 0.50 and 1.50 inches of rain fell in a few hours this past week.

Vehicles in the Phoenix metro area navigate a flooded street that accompanied severe storms on Monday, July 9, 2018. (AP Photo/Ross D. Franklin)

The recent downpours compare to an average annual rainfall of approximately 8 inches.

Palm Springs, California, was hit by torrential downpours and flash flooding on Wednesday, July 11, when about 1 inch of rain fell in 90 minutes.

A small part of a large city may be slammed by flooding, while other areas barely receive a drop of rain.

Flash flooding can occur in an area that received no rain at all. This is a common occurrence in the Southwest as downpours can occur miles away but cause dry stream beds to rapidly fill with water and inundate canyons and highways. These conditions can be a serious threat to unsuspecting campers, hikers and motorists.

Dust storms, lightning and a fire threat accompany the monsoon

In areas that receive little or no rain from the storms, blowing dust and lightning can be a major problem.

Dust storms, known as haboobs, can overtake highways in a matter of seconds and bring a dangerous, sudden drop in visibility.

While haboobs can occur any time during the monsoon and outside of the season, they can be more potent and disruptive during the early days of the monsoon, prior to the ground being dampened over a broad area.

High winds created by the storms can also down trees, cause property damage and power outages, and the strong winds can create havoc for firefighters battling blazes.

Where lightning strikes occur with little or no rain, small brush fires can rapidly escalate into a major wildfire.

People hiking into high elevations need to constantly monitor the sky for developing storms and be prepared to change plans and head down the mountain to avoid being struck by lightning.

Whether exploring canyons or mountain climbing, be sure to constantly monitor the weather and consider taking along a seasoned veteran who has plenty of experience to spot the developing weather conditions that pose a serious risk to your plans.

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