What is an atmospheric river and how does it affect Arizona's weather?

As one storm after another drenches California and buries some of the state in snow, weather forecasters have been talking about “atmospheric rivers” as the driver of the phenomenon. Here’s some of what you need to know about them.

What is an atmospheric river?

Atmospheric rivers are long narrow corridors of air that are heavy with moisture. They're a common phenomenon across the globe but have affected the West significantly this winter. Occasionally called "rivers in the sky," these weather events are the largest natural movers of freshwater on Earth and can carry more water than the Mississippi River.

The flowing column of condensed water vapor is responsible for producing significant amounts of rain and snow, especially in the West this year, and are already helping boost water supplies in western states.

Why are they so damaging?

Storms brought in by an atmospheric river will last around 20 hours in an area over the coast. When these systems move inland and sweep over mountains, the water vapor rises and cools to create heavy precipitation. While many of these systems are weak and provide beneficial rain or snowfall, some of the more powerful rivers can create extreme rainfall and floods capable of disrupting travel, including mudslides, causing catastrophic damage to life and property.

Atmospheric rivers have been wreaking havoc this past winter. There have been 11 such events so far, with California taking the brunt. Severe weather brought on by an atmospheric river caused a levee breach in Monterey County earlier this week, prompting evacuation alerts for thousands and killing at least two people.

Is Arizona weather affected by the phenomenon?

Arizona River Basins are susceptible to atmospheric rivers and the state has felt severe effects from these systems in the past. In January 2010, more than 10 inches of precipitation fell in the higher terrain east of Phoenix. The National Weather Service estimated damages at $11 million across the state.

California and Arizona are both in a period of extended drought, which can be a reason why runoff from recent storms have been so destructive. Conditions from drought can leave soil hardened which causes it to repel excess water it cannot store, instead of soaking it in. Water will then run across the surface, which increases the risk for mudslides and erosion.

“When we're looking at California, they have a different climate system than Arizona, but we're dealing with the same kinds of situations when we're dealing with drought and wildfire and having heavy amounts of precipitation in a short period of time,” said Erinanne Saffel, the state climatologist at Arizona State University, who talked about the rivers in a news release.

In the West, where wildfires are common and have been exacerbated by drier conditions, atmospheric rivers can cause even more destruction and turn dangerous to humans and property. Torrential downpours can unleash wildfire debris during floods and can move large objects like boulders and trees.

“With California, they've had wildfires and burn scars. Those are places that are susceptible to mudflows and dangerous runoffs from these amounts of precipitation,” said Saffel, “That's another consideration when you get all of that precipitation all at once.”

How does this all affect the West’s drought?

Not all atmospheric rivers cause damage. Some are weak systems that serve as a vital recharge of water.  And while the precipitation is not nearly enough to reverse the effects of drought, Saffel says the extra moisture could be good for short-term drought and help replenish water supply.

“At the end of December, California was in extreme drought levels. Thirty-five percent of the state of California had an extreme or exceptional drought; that's the angry red that you see on those drought maps,” Saffel said. “Now, with the moisture that's been coming in, the atmospheric rivers are dropping the precipitation and they've eliminated all of that extreme and exceptional drought.”

Atmospheric rivers can contribute as much as 30% of Arizona’s winter precipitation. Winter precipitation in the state is especially critical when it falls in the form of snow, which will melt in the spring months and help refill reservoirs and recharge aquifers.

Scientists believe climate change will likely alter the frequency of these atmospheric events. Last year, the National Oceanic and Atmospheric Administration used data generated by regional climate models to determine that climate change will likely alter atmospheric rivers in ways that will make managing water more difficult.

Saffel says it's important to know how these systems operate to best prepare for a potential storm.

“Pay attention to forecasts and understand some of the extreme things that could happen so you can stay safe,” she said.

Jake Frederico covers environment issues for The Arizona Republic and azcentral. Send tips or questions to jake.frederico@arizonarepublic.com.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook, Twitter and Instagram.

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This article originally appeared on Arizona Republic: How does an atmospheric river affect Arizona's weather?