Monsoon Weather. Photo by John Sirlin
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Explaining Monsoons in Arizona

by Jeff Payne, Director of Horticulture at Boyce Thompson Arboretum

Monsoon season is here! What does that mean? We usually think about flooding rains, but that’s not really accurate. Let’s take a look at how the National Weather Service defines what a monsoon actually is and what causes the monsoon to occur.

By definition, a monsoon is a seasonal change or shift in wind direction. A monsoon is not a thunderstorm, nor is it a series of thunderstorms or rainy days. In fact, the definition doesn’t mention rain or moisture at all!

Rain is only a by-product of this shift in wind direction – if the moisture is available. Monsoons occur seasonally across our fragile planet in a few familiar locations: Asia (India, Bangladesh, and Thailand), in the southern and central parts of Africa, in the northern part of Australia, and here in the United States, where we have the North American monsoon.

Monsoons occur because a few factors fall into place seasonally every summer. First, in late June or early July, temperatures usually will soar far above seasonal summer norms.

The average high temperature in Phoenix on July 1 is 107°F. But you might have noticed that on July 1, Arizona can have a temperature of 112°F, 115°F, and even 118°F.

In fact, if you go back to June 26, 1990, you’ll find the highest temperature ever recorded in Phoenix: 122°F! It seems that in early summer, these extended periods of above-normal temperatures prime the atmosphere for the onset of the seasonal shift in wind direction.

During the summer, Arizona’s upper and mid-level winds shift from a westerly flow to a southerly or southeasterly flow. This is usually caused by a high-pressure center building and settling over northern Mexico or the Four Corners area. These high-pressure areas will rotate clockwise, drawing in moisture from the Gulf of Mexico. 

As this moisture becomes caught in the rotation, it migrates over mountainous terrain of Mexico. Here, the moisture condenses and, we hope, begins to cross over into Arizona.

At the same time, a low-pressure center also forms over the Baja Peninsula, rotating counter-clockwise. This counter-clockwise rotation draws up moisture from the Gulf of California, enhancing the moisture over Arizona. This seasonal weather pattern lasts anywhere from eight to twelve weeks, on average.

When the condensing moisture from these wind movements reaches the rising heat of the desert floor, convection causes the moisture to rise into the atmosphere, and Arizonans will start to see clouds and thunderheads form.

In the past, the National Weather Service tracked the North American monsoon in a very technical way, by measuring dew points – a measure of humidity, or moisture in the air. For the Phoenix area, the requirement was three consecutive days of dew points of 55°F or higher. When that happened, the Weather Service declared the official start of the monsoon.

In 2004, after an international research project that increased our understanding of the monsoon in North America, the National Weather Service decided to scrap all these details and change up the definition of the monsoon season.

They abandoned the technical aspects that used to define when the monsoon begins and ends, and even came up with a new term for the monsoon season.

Now, statewide, June 15 through September 30 is officially Arizona Monsoon Season. This is the time period when the seasonal shift of winds could occur.

This is a lot easier to understand than all of that technical scientific data about dew points. 

Arizona’s monsoons have been changing over the years. In 2019, many people decided to call the monsoon season a “nonsoon,” because a weakened monsoon pattern failed to bring much moisture.

In fact, the monsoon of 2019 nearly broke records for being hot and dry. It was the ninth driest and third hottest since records started being kept on this in 1895.

Was there a seasonal shift in wind direction last year? Yes, there was. Arizona did, in fact, have a monsoon last year. But now you know that the definition of a monsoon doesn’t mention rain or storms.

In 2019, the winds shifted, but they didn’t pick up enough moisture to bring much rain. Weak flows prevented the winds from bringing the water vapor that, in a normal year, would have caused storms or at least rain over Arizona.

As you see, storms during monsoon can be hit or miss. Sometimes the monsoon season brings strong winds that cause lots of damage to landscapes, buildings, and vehicles.

Dust storms can roll in, reducing visibility and wreaking havoc on the highways and interstates. Lightning and excessive winds can cause power outages. All these things can happen, and yet sometimes it still doesn’t rain.


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