What is an ‘atmospheric river,’ anyway? Here are 5 things to know
Editor’s note: This story was originally published in March 2018 and was updated in December 2022.
In the days after Christmas, winds up to 50 mph were forecast for the Sacramento Valley, along with rain and Sierra Nevada snow, amid an atmospheric river storm.
You’re probably wondering: What is an atmospheric river, anyway?
The National Oceanic and Atmospheric Administration defines them as “relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.” For California, this means that streams of moisture are being pulled by wind from the tropics to the West Coast.
Here are five other things to note about atmospheric rivers.
Good news, bad news...
The good news: Atmospheric rivers help provide much-needed rainfall, can give major boosts to snowpack in areas like the Sierra, and National Weather Service forecasts can accurately predict and map them in advance.
The bad news: They also bring heavy rains, sometimes posing significant flood risks to Western states. Always be on the lookout for flood and flash food warnings.Like any moderate to severe storm, atmospheric rivers can lead travel problems and delays, as well as to power outages.
Most are weak systems
Despite sometimes being nicknamed “horizontal hurricanes,” the NOAA notes that many rivers don’t cause significant damage.
How big are they?
Atmospheric rivers span anywhere from 250 to 370 miles, according to the NOAA.
How about some examples?
Pineapple express is one of the better-known types of atmospheric river. It takes moisture streams from Hawaii to California and nearby states, and is one of the stronger AR systems.
Other quick facts
As explained by NOAA’s Earth System Research Laboratory: Atmospheric rivers are an important part of Earth’s water system, and must exist somewhere on earth at any given time. A handful of them account for as much as half of some West Coast states’ annual rainfall totals; therefore, they are critical to California’s water supply.
This story was originally published March 20, 2018 at 2:59 PM with the headline "What is an ‘atmospheric river,’ anyway? Here are 5 things to know."