What is an ‘atmospheric river,’ anyway? Here are 5 things to know

This is what to expect as atmospheric river moves through Sierra, Sacramento

See the winter storm system approaching Norther California spinning off the coast and check out the snowfall and rainfall predictions.
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See the winter storm system approaching Norther California spinning off the coast and check out the snowfall and rainfall predictions.

An atmospheric river is sending rain Sacramento’s way this week, dropping as much as 4 feet of snow in the Sierra and giving a little bit of relief to Californians concerned about another drought.

A wet March so far has aided the Sacramento Valley in inching closer to the average totals for the water year.

The atmospheric river will no doubt help, but 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. In some Southern California counties, mudslides are a risk. 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. Winds from the approaching system in inland Northern California are expected to see winds top out at 20 mph, but the Bay Area could see gusts reach 35 mph by midweek.

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.

That's the type heading for the Bay Area, as well as parts of Southern California, this week.

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.

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