Arizona Monsoon Season

If you spend any time in Arizona's 100°F+ summer heat, you'll know why many of us eagerly wait for the Arizona monsoons.

Yes, it brings higher humidity, but it also brings wonderfully dramatic storms, amazing clouds, and cooler temperatures when it rains.

The monsoon season is an important part of Arizona's climate and environment, providing much needed water to the state's plants and animals at the hottest part of the year.

We took this Arizona monsoon photo at Monument Valley Tribal Park on the Navajo Nation

It's so important that the Tohono O'Odham people, who are native to the Sonoran Desert region, have a special and very important ritual, often called the saguaro cactus wine ceremony.

But that simple definition doesn't do justice to the spectacle of Arizona's summer monsoon season.

Every year - sometime between mid-June and mid-July - the sunny, hot weather Arizona is known for begins to change as the regional high pressure zone that sits east of Arizona shifts north.

This shift lets prevailing winds, which come from the west most of the year, change direction and flow from the south and southeast.

Often referred to as the Mexican monsoon, this seasonal shift of winds brings tropical moisture from the Sea of Cortez and the Gulf of Mexico into Arizona.

When this moist Mexican air collides with Arizona's desert heat, monsoon thunderstorms are born.

Of course, other weather patterns can play a role in the occurrence of Arizona's monsoon thunderstorms including the position of the jet stream, a heat dome over the Midwest and East U.S. among other factors.

The Santa Cruz River in Tucson, dry for most of the year, flows with water after a major monsoon thunderstorm on August 23, 2005. The Santa Cruz hadn’t run this high since 1994.

Monsoon thunderstorms are one of the most spectacular and thrilling of nature's displays.

Desert dwellers yearn for the crack of thunder, the brilliant flashes of lightning and the deafening downpour of rain that cools the sweltering desert heat and makes the creosote bushes release their aromatic, herbal fragrance - if only for a few hours.

Most monsoon thunderstorms develop in the late afternoon because that's when heat rises.

When winds push against Arizona's mountain ranges, that air has nowhere to go but up.

For many years, the National Weather Service's Arizona monsoon definition stated that the monsoon starts when the average dew point - the temperature at which air must cool and water vapor condenses into rain - hits 54 degrees Fahrenheit for 3 consecutive days.

Monsoon rains bring much needed water to the Lonesome Valley area along Highway 89A east of Prescott, Arizona

That change generally happens sometime in early to mid-July.

However, as of 2008, the National Weather Service has adopted a new calendar-based method for tracking the Arizona monsoon season.

By using this new system, they hope to raise awareness on storm safety - i.e. dust storms, lightning and flash floods - instead of pinpointing the exact day the winds shift or using the dew point measurement.

In the past, major storms have struck before the dew point formula was reached.

According to the National Weather service's new calendar method, the Arizona monsoon season will start on June 15 and end on September 30.

They chose these dates from the analysis of a decades-worth of data including rainfall and season length.

Of course when we actually see the first Arizona monsoon rains is still up to nature.

When a monsoon thunderstorm moves in, temperatures may drop from 105°F to 60°F in a matter of minutes.

The average amount of monsoon rainfall over the summer is 6 inches for the Sonoran Desert, which includes Phoenix and Tucson.

However, the amount of rain can vary greatly from year to year.

Also, Arizona's monsoon thunderstorms are generally not continuous throughout the season. It may rain for a few days and then dry out for several days or even a few weeks before starting back up again.

The Arizona monsoon season usually ends sometime in September.

An isolated monsoon downpour along the Loop 101 north of Phoenix

Monsoon thunderstorms can be highly localized.

In Tucson, for example, it can be sunny and clear in midtown but pouring out at Sabino Canyon on the eastside.

If you're out hiking or golfing, be aware that these storms can move swiftly and change direction and intensity rapidly.

Our Monsoon Season Experience

One August, we were sitting on our front porch watching a monsoon thunderstorm form over the west end of the Santa Catalina Mountains.

At our house, it was hot and sunny but tall thunderheads rose up over the mountains, darkening and growing ever taller.

Then the wind suddenly shifted and the clouds began to speed towards us.

We knew that was our cue to head indoors.

Within just a few minutes, the storm had crossed 15 miles and was right over us. It was the largest storm we ever experienced in our area.

Ear-splitting thunder and sun-bright bolts of lightning cracked above our house, sending the cats scurrying under the bed.

The wind was so powerful that the saguaros swayed from side to side like stalks of corn. It rained so hard that it sounded like a million nail guns pelting the roof.

Whether you're an Arizona vacationer or a resident, you need to be prepared for the high winds, lightning and rain that are trademarks of Arizona's monsoon season.

• Before going on an outing, listen to the weather reports on radio and TV. The National Weather Service issues Arizona weather watches and warnings via radio and TV.
• A severe thunderstorm warning means that life-threatening storms with winds of at least 60 miles per hour or flash flooding are about to occur or have been reported.
• Have an emergency kit ready in your house and car including first aid items, water, ready-to-eat food, battery-powered radio and flashlights with extra batteries. Do not use candles or oil lamps - they are fire hazards.
• When you're outside, regularly scan the skies 360 degrees around you and overhead. There’s no mistaking the tall, skyscraper-like cumulonimbus clouds or the darkening sky.
• Listen for thunder. Anytime you hear thunder, get indoors immediately. Postpone outdoor activities until at least a half hour after you hear the last sounds of thunder or see the last flashes of lightning.
• Do not wait for official warnings before you seek shelter. If you feel threatened, don't hesitate to take cover. The National Weather Service provides advance warning of most storms but severe storms can arise undetected - especially when the storms are producing mostly high winds and little to no rain.

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