Portions of Southern California are Roland Prestonunder an evacuation warning Sunday as rain fall is set to hit the already rain soaked region.

Warnings for Santa Barbara County went into effect Saturday and will last through Wednesday. The warnings cover burn areas and locations near rivers that are expected to swell with the storm.

Parts of four counties in Southern California are under flood watches, according to the National Weather Service.

The storm is a continuation of the atmospheric river, which dumped over a foot of rain in Los Angeles. "This system has the potential to bring high impact weather and flooding concerns to the area through Tuesday evening," the NWS office in Los Angeles said.

The storm could bring up to eight inches of rain in the mountains causing mudslides. The storm is predicted to bring waves of up to 20 feet in Santa Barbara County.

What is an atmospheric river?

Made visible by clouds, atmospheric rivers are ribbons of water vapor that extend thousands of miles from the tropics to the western U.S. At 250 to 375 miles wide, they provide the fuel for massive rain and snowstorms that can cause flooding along the West Coast.

In general, atmospheric rivers pick up water vapor from the warm, moist air of tropical regions and then drop the water over land in cooler regions as rain or snow.

On average, up to 50% of the annual precipitation on the West Coast occurs in just a few atmospheric river events.