What are the atmospheric rivers slamming California?

An onslaught of severe storms since just after Christmas has savaged parts of California, with at least 19 storm-related deaths reported along with tens of thousands of customers left without electricity.

The area has been battered by a string of atmospheric rivers, with heavy rain expected to continue through early next week.

In response to the devastation of the severe storms in California, President Biden declared the situation a major disaster and directed federal aid to the state and region to support recovery efforts.

California Gov. Gavin Newsom (D) warned Californians that serious wind and rain were expected to continue throughout the state.

What are these atmospheric rivers that are wreaking havoc on the West Coast?

An atmospheric river is essentially a river in the sky, transporting water vapor from the tropics and dumping it as rain or snow when it makes landfall, according to the National Oceanic and Atmospheric Administration (NOAA). The heaviest rivers can cause extreme rainfall and flooding.

A small number of atmospheric rivers are responsible for about 30 to 50 percent of annual precipitation on the West Coast on an average year. The sheer size of the formations mean they can park over a region for days at a time and dump precipitation.

And a belt of relatively severe forms of these atmospheric rivers have slammed California since just after Christmas. 

Are all atmospheric rivers severe?

Not all forms of these storms cause damage. Many of them are weak systems. The typical formation can be 300 miles wide and more than 1,000 miles long.

NOAA does warn that the strongest of these storms can create extreme rainfall and flooding, saying they can “induce mudslides and cause catastrophic damage to life and property.”

For example, a series of these atmospheric rivers fueled winter storms in 2010 that battered the West Coast, from Washington state to Southern California, with 11 to 25 inches of snow in some areas. 

When these storms follow each other back to back is when an area can face severe flooding and other weather-related damages, like California is confronting now.

Can atmospheric rivers be beneficial?

According to NOAA, atmospheric rivers are actually a key feature in the water supply in the western U.S. They can also result in a beneficial increase in snowpack.

These types of storms account for roughly 50 percent of California’s annual precipitation, according to recent studies. A lag in the number of atmospheric rivers could result in a drought for the region. California has been plagued by a historic drought, and experts say the current storms are at least offering a temporary reprieve.

“It’s definitely too early to say that we’re out of the drought, so to speak,” said Andrew Ayres, research fellow at the Public Policy Institute of California’s Water Policy Center.

Just 0.32 percent of the state was under extreme drought as of Tuesday, down from over 27 percent the week before, according to the U.S. Drought Monitor.