By Melanie Lenart | The University of Arizona | September 12, 2008
Drizzly winter rains and raucous summer thunderstorms differ in how they affect the Southwest’s climate and landscape. This section describes these different types of storms and their impact on the landscape and water resources.
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Credit: Peter Kresan
Tropical convection powers thunderstorms, including those we experience during the monsoon and hurricanes. Convection occurs when surface heating causes air near the ground to become less dense than the surrounding air, forcing it to rise. Much like how bubbles might rise in a pan of water boiling on a hot stove, clouds and other air parcels rise through the atmosphere above the hot surface (Figure 1).
As clouds cool when they rise in the atmosphere, they sometimes reach temperatures too cold to hold all the moisture they contain. In that case, some of the humidity they contain converts back from its gaseous state into rain or snow.
Mountains can help the process of convection in two main ways. They present a physical barrier that forces air and clouds higher in the atmosphere. Also, mountainsides facing the sun can heat faster than the surrounding landscape, generating energy that also helps lift clouds.
Cold fronts, too, can serve as a means of convectively lifting clouds. Where fronts collide, the hot air rises over the front of cold air, increasing the odds that clouds will reach heights cold enough to promote condensation of the moisture they contain, causing precipitation.
In the right circumstances, a collision of fronts can create the major storms known as mid-latitude cyclones. Winds circle high in the atmosphere, bringing rain or snow to areas under low-pressure zones of these frontal systems. Tornadoes can occur where cold fronts meet warm fronts, but they are uncommon in the Southwest.
In tropical climes, cyclonic winds can circulate at ground level as tropical storms and hurricanes. These tropical cyclones form in the tropics during summer and fall. They can maintain their status as hurricanes as they enter the subtropics and temperate zone, but degrade into tropical storms within about 100 miles of hitting land.
By the time these tropical storms reach the Southwest, the intense rainfall typical of a land-falling hurricane has weakened into softer, gentler rains that can soak into soils—or cause floods. Some of the Southwest’s largest floods have occurred when remnant tropical storms were swept into the jet stream and mingled with frontal storms.
In the Southwest, winter precipitation often involves large-scale frontal systems. It can fall as snow in some of the higher elevations, such as along the Colorado Plateau’s Mogollon Rim in northern Arizona.
Where northern cold fronts and southern warm fronts meet, the jet stream flows as a current of swift-moving air high in the atmosphere. That’s why climatologists focus on the jet stream when considering precipitation patterns. In fact, El Niño exerts its influence on southwestern precipitation largely by affecting the position of the jet stream (Figure 2).
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Credit: Smithsonian Institution
Growth rings have been used to reconstruct Southwest climate back to A.D. 1000. See CLIMAS' Reconstructing Past Climate in the Southwest for details.
Winter precipitation tends to arrive in blankets of clouds lingering over states for days, delivering moisture in drizzles. This gentle rain is more likely than the pounding summer storms to soak into the ground. As a result, winter precipitation tends to influence regional water supplies more than summer rains.
It also plays an important role in forest health. The growth rings of high-elevation southwestern trees typically reflect cool-season precipitation, with more growth occurring in years of abundant snow and rain. Because of this, researchers can use tree rings to reconstruct winter precipitation patterns far beyond the instrumental record.
Winter and spring precipitation also have a large influence on when the wildfire season starts and the susceptibility of grasslands and forests to fire. A lack of cool-season precipitation can make forests more vulnerable to wildfires. Meanwhile, wet winters actually can spur fires in grasslands made lush by cool rains and then cured into kindling during a subsequent dry heat.
In much of Arizona and New Mexico, roughly half of the annual rainfall comes during the summer monsoon.
The monsoon arrives with much flash and fanfare, with its trademark thunderstorms and flooded streets in southwestern cities. Monsoon rainfall events tend to be short and spotty, with intense, local storms peppering some neighborhoods but not others. The water the storms bring quickly flows off the landscape into streets and rivers, with most remnant moisture soon evaporating in the summer sun.
The conditions that initiate the summer monsoon begin in the typically sunny southwestern spring, when lowland temperatures often reach into the 90s, heating up the land. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, the winds shift to fill in the vacuum left behind by the rising air.
Figure 3. The air circulation patterns at 18,000 feet show the signature “anticyclone” that helps define the North American monsoon.
Credit: National Weather Service
The shift opens the region to the influence of tropical convection, and the monsoon begins.
The shift in wind direction is what earns the summer rainfall pattern the label “monsoon,” a word more commonly applied to India’s seasonal rains. The North American monsoon starts in southwestern Mexico, around the Sierra Madres. Over the course of the summer, it moves north into the U.S. Southwest.
In 2008, National Weather Service officials decided to consider June 15–September 30 as the U.S. Southwest monsoon season, although the thunderstorms that bring the rain may form in different times and places across the region (Figure 3). Previously, officials used more complex measures of airborne moisture to determine the monsoon’s arrival and departure.
The monsoon helps bring the southwestern wildfire season to a close—but usually only after a couple of weeks of additional lightning-sparked blazes.
The monsoon also influences the survivability of cattle and wildlife. Southwest summers can be especially harsh in years when the monsoon falters, leaving shriveled plants and baked soil in its wake.
Rainfall during any of these seasons can unleash local floods.
Read more about tropical storms in Arizona and New Mexico in CLIMAS' Tropical Storm Impacts on Arizona and New Mexico.
While the thunderstorms generated by the annual monsoon occur at small spatial scales, the summer and autumn storms from remnant hurricanes tend to affect a larger area. In this way, they are more similar to winter storms. In 1983, a series of four tropical storms passing through Tucson culminated in a flood when the remnants of Hurricane Octave arrived in early October, causing about $500 million in damages.
The remnants of tropical storm Norma were behind Arizona’s deadliest storm in history. The 1970 storm led to 23 deaths, many of them in high-elevation campground areas around the Tonto Creek drainage area about 60 miles northeast of Phoenix. A nearby gauge at an elevation of 7,000 feet recorded 11.4 inches of rainfall within a 24-hour period ending on the night of September 5.