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Climate and Environment

What To Know About Atmospheric Rivers As The Latest One Heads Toward LA

The L.A. River's water flows under an overcast sky, a mix of white and golden light illuminating the scene.
The L.A. River flowing below the 4th street bridge after the storm on Thursday, Jan. 5, 2023.
(Samanta Helou Hernandez
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Southern California will catch the bottom edge of an atmospheric river — and will explain that more in a moment — that's expected to stick around from late Thursday until early Saturday.

While Northern California will be inundated, the only big flooding concern here is in Santa Barbara County.

So, what’s an atmospheric river?

Think of atmospheric rivers as... rivers in the sky.

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These huge bands of moisture — more than 1,000 miles long and hundreds of miles wide — can carry an amount of water that'd put some of the world's largest rivers to shame.

“If you were to slice across one (atmospheric river) and calculate the water vapor transport in it," explained Marty Ralph, with the Scripps Institution of Oceanography, an average atmospheric river "would be something like 25 Mississippi Rivers of water."

[For context: the Mississippi River is more than 2,300 miles long.]

An image that displays the science of an atmospheric river. "A flowing column of condensed water vapor in the atmosphere responsible for producing significant levels of rain and snow."
Atmospheric rivers are responsible for bringing substantial precipitation to California.
(National Oceanic and Atmospheric Administration

Atmospheric rivers show up across much of the planet, but the ones that slam into California form over the Pacific.

They occur when moisture rises from the ocean, concentrates within a narrow region of the atmosphere, and is pushed along by strong horizontal, sometimes hurricane-force, winds.

Why they matter

We make a big deal about them for a few reasons.

One is that, on average, they're responsible for roughly half of our precipitation each year. And just a handful of atmospheric rivers can be the difference between a wet year and another bleak, drought-ridden one.

The second reason is that because they can drop so much water, they're also some of our most destructive storms, causing billions of dollars of flood damage to states across the Western U.S.

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Don't expect the disasters to abate anytime soon. Climate models point towards atmospheric rivers increasing in intensity as the climate continues to warm. In part, because a warmer atmosphere holds more water vapor — the very thing that fuels atmospheric rivers.

What's next

There’s a slight concern that this storm — much warmer than the last — will melt snowpack at lower elevations and increase flood risk.

And even if we make it through unscathed this time around, the risk will rise yet again when another warm atmospheric river shows up next week.

What do you want to know about fires, earthquakes, climate change or any science-related topics?
Jacob Margolis helps Southern Californians understand the science shaping our imperfect paradise and gets us prepared for what’s next.

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