Atmospheric river, AR, California, storms, Snowmageddon | The Old Farmer's Almanac

What is an Atmospheric River? A River in the Sky?


Atmospheric river on March 13, 2023. Credit: CIRA/ CSU & NOAA

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River in the Sky, Rain and Snow on the Ground!

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Watch overhead! A river may be coming at you. An “atmospheric river” in the sky is dumping rain and snow on the ground! What is an atmospheric river? Learn about this weather phenomenon happening right now.

Updated March 14, 2023: After an “atmospheric river” slammed the West coast this past weekend (starting Friday, March 10), a second atmospheric river is making landfall again (March 13 to 14), dumping even more soaking rainfall on an already sodden terrain along the central and southern California coastline and the Sierra foothills. Already soaked from weeks of rainfall and snowfall, California’s rivers and creeks have risen above flood levels. Record snowpack in the Sierras may lead to even more flooding as the weather warms and the snow melts. Wet weather will also move eastward across the U.S. and could also become concerning for areas of Arizona.  

What is an Atmospheric River?

An atmospheric river (AR) is a band of tropical water vapor caught on the edge of a cold front and carried toward the poles. These 250 to 350 miles-wide bands carry as much as 300,000 tons of water, 7 to 15 times as much as the mouth of the Mississippi River!

Most atmospheric rivers do not damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply—from 30 to 50% of  precipitation along the West Coast, according to NOAA!

If you live on the West coast, many folks (except for some of the damp visitors to the Oscars) are pleased that these giant atmospheric rivers bring rain to a parched state, especially farmers. So much of the food for Americans (and the world) is grown in California. In the East, you might not be so grateful. One of those rivers created Snowmageddon in 2010 and, really, who needs more snow?

Of course, no one embraces landslides, power outages, accidents, and mudslides. The really bad atmospheric-river-driven storms will cause extensive flooding as the water runs out of places to go.

Atmospheric River, March 10, 2023. When cold fronts carry rivers of tropical water vapor north, it brings rain, often too much rain.
Source: NOAA

When Did the Term “Atmospheric River” Start?

We are only beginning to understand these rivers. Discovered by Reginald Newell and Yong Zhu in the 1990s, they were originally connected with West Coast flooding, particularly the infamous “Pineapple Express” that flows through Hawaii. We now know they have a central role in the global water cycle. On any given day, these relatively narrow bands of water vapor account for over 90% of the movement of tropical moisture north and south. Most of the time, they bring warm, welcome rainfall in moderate amounts.

The rains and floods of California in March, 2014, were due to atmospheric rivers. Source: FEMA

The good news is that scientists are increasingly able to warn people where and when the worst atmospheric river floods or snows will hit. They have also learned that these AR events can happen throughout the world. The floods that have hit England are often river storms. Some of the worst Nor’easters, like 2010’s Snowmageddon, were caused by an atmospheric river flooding the chilly East Coast.

Snowmageddon 2010! When an atmospheric river hits the East Coast in winter, it can create a Nor’easter. Source: NOAA

So look up in the skies! It’s a bird, a plane—no—an atmospheric river coming at you. You may not enjoy it, but the flowers this spring will be grateful.