What is an atmospheric river?

OAKLAND, Calif. - So, what are atmospheric rivers, and how do they form?

Atmospheric rivers are long bands of water vapor that move through the sky. They can be 250 miles wide and 1,000 miles in length. They are the largest rivers of fresh water on the planet. And they're responsible for more than half of California's annual precipitation. 

Powerful atmospheric rivers can create extreme rainfall and floods, which can also trigger mudslides and cause "catastrophic damage to life and property," according to the National Oceanic Atmospheric Administration. 

A "Level 5" hazardous atmospheric river slammed the San Francisco Bay Area in October 2021, producing 60-foot waves, 80 mph winds and daily rain totals of 11 inches in Kentfield. 

Atmospheric rivers hit again in 2023, battering the Capitola wharf in Santa Cruz County. 

The federal government estimates that atmospheric rivers have been associated with $1 billion of damage every year, with 80% of the flood damage in California. 

National Weather Service meteorologist Daltton Behringer told KTVU that an atmospheric river is a steady stream of moisture coming from the tropics.

Separately, a bomb cyclone, he said, is how quickly a storm strengthens. 

You can have one without the other, but in this case, Behringer said, both are going to slam into the Bay Area, with the heaviest days of rain expected on Wednesday and Friday.