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What is the North American monsoon?

By Alex Sosnowski, AccuWeather senior meteorologist

An annual event, known as the North American monsoon, brings beneficial rain as well as disruptions, dangers and damage to the southwestern United States.

While many may believe a monsoon is an episode of heavy rainfall, a monsoon is the change in wind direction that can trigger persistent rainfall or long-duration dry weather.

Monsoons are well known in eastern Asia, especially in India, but they occur in other parts of the world as well.

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

Highly localized downpours, flash flooding, dust storms and frequent lightning strikes are common occurrences during a period of about three months, focused mainly during the summer.

AP Lightning cactus

A saguaro cactus is silhouetted by multiple bolts of lightning that strike the ground in the desert north of Scottsdale, Arizona, during a late evening storm Thursday, Aug. 1, 1996. (AP Photo/Scott Troyanos)

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.

Even when the monsoon is in full swing, not every location can expect to get hit on a daily basis, a few communities and stretches of highways can be hit hard on a given day.

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Highly variable rainfall typically occurs during the monsoon

Much of the rainfall is highly sporadic and may only occur at a given location a couple of times a week or less.

However, the cumulative nature of the monsoon over a few months 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.

The rainfall can be intense as well. When combined with the hard, rocky soil and rugged terrain, flash flooding often occurs.

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

Static Phoenix Flash Flood AP Monday Jul 9

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

These conditions can be a serious threat to campers, hikers and motorists if they are not weather aware.

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, called arroyos, to fill with water rapidly and send a wall of water through canyons and across highways.

Static AP Grand Canyon Flooding

Waterfalls normally blue-green turned muddy brown in Supai, Arizona, after flooding the area on Wednesday, July 11, 2018. Heavy flooding forced evacuations and shut down camping and exploration in the area for at least a week.

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.

Large 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.

Static Haboob AP Phoenix 2013

A haboob moves cross the Phoenix metro area on Monday, Aug. 26, 2013. (Ross D. Franklin/AP Photo)

High winds created by the storms can also down trees and 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, especially in windy conditions.

People hiking into high elevations need to monitor the sky constantly 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 monitor the weather constantly 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 lives.

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