What is an Atmospheric River? The planet's largest freshwater rivers are in the sky

The term describes a narrow corridor of concentrated water vapor -- usually from tropical origins -- that is being carried by the jet stream.

Imagine the mighty Mississippi River that bisects the heart of America and all the water it carries. 

Now imagine *27 times* that amount of water flowing in the skies above, as a storm brings hours of torrential rainfall!

These kinds of storms are a reality along the West Coast multiple times in autumn and winter when the region is struck by what's known as an atmospheric river.

The term describes a narrow corridor of concentrated water vapor -- usually from tropical origins -- that is being carried by the jet stream, according to the Center for Western Weather and Water Extremes (CW3E). Think of it as a river of water vapor in the sky. 

And wherever this "river" comes ashore will be drenched with a period of moderate-to-heavy rainfall -- sometimes for 48 hours or more. 

Heavier rains fall where atmospheric rivers crash into mountain ranges. As air is forced upward on the windward side of mountains, the moisture will be wrung out of the atmosphere like a sponge, dropping several inches of rain into mountain river basins. Intense atmospheric river events frequently lead to extensive river flooding. 

And while atmospheric rivers typically bring their most significant rainfall to the coastal areas, their impacts can travel hundreds of miles inland, bringing heavy rain to interior locations.

When do they occur?

A vast majority of atmospheric rivers happen in the fall and winter -- with the northern Pacific Coast receiving the bulk of theirs in fall and the California coast receiving them more in the winter. While rare, atmospheric rivers are possible in the spring and summer months.

Is it just a West Coast Thing?

In the U.S., nearly all atmospheric rivers occur along the Pacific Coast, but they can happen anywhere in the United States.

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In the spring of 2010, a robust atmospheric river came off the Gulf of Mexico and struck the southeastern U.S., bringing record flooding to the Tennessee Valley, according to the CW3E.

They're challenging to forecast too

Forecasters can use satellite imagery and ground observations to plot atmospheric rivers by measuring the amount of water vapor entrenched in the storm. But predicting where they will go remains quite a challenge.

"They can temporarily stall in place, change their landfall locations, intensify or weaken, interact with other ARs (atmospheric rivers) or remnants of previous ARs, and warm or cool," according to the CW3E. "Any of those outcomes can affect AR impacts and can change with short notice."

But the CW3E now dedicates an entire site predicting the likelihood of an atmospheric river and where it's expected to have its most significant impacts.

Not all atmospheric rivers are bad news

Atmospheric Rivers can cause damaging flooding and mudslides, but they also serve an essential purpose in supplying much of the West's water supply -- especially California. 

Weaker atmospheric rivers that bring copious amounts of rain but not enough to flood are considered beneficial to the area. About half of California’s annual rainfall comes from atmospheric rivers, and without them, the region can find itself in an extended drought.

Forecasters have been working to measure atmospheric rivers better and convey their impacts to the public by creating a new 5-point scale

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The scale aims to delineate between beneficial atmospheric rivers and those that cause more harm than good. 

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