California Atmospheric River Forecast as State Braces for Major Floods

California faces yet more flooding as a fresh atmospheric river is expected to bring heavy rain. The state is still recovering from winter storms that have left snowpacks in mountain regions that are far greater than seasonal averages.

The heavy rain is expected to make landfall on Thursday. It could loosen some of the snow at higher elevations, increasing the amount of water traveling into the valleys.

California flooding January atmospheric river
Flooding pictured near CA 99 in Merced County, California on January 10, 2023. A series of atmospheric river storms brought extensive flooding across the state at the start of the year. Andrew Innerarity/California Department of Water Resources

Atmospheric rivers are narrow channels in the atmosphere that are able to carry a large amount of moisture with them. According to the National Oceanic and Atmospheric Administration, a strong river is able to carry as much as 15 times the amount of water flowing through the mouth of the Mississippi River.

The atmospheric river storm is not the first this year to hit California: in January, the state was battered by a series of deadly storms that brought severe flooding. Thousands of homes were left without power, with thousands of families evacuated.

In late February, a coast-to-coast storm brought freezing-cold wind gusts and blizzard conditions to much of the northern continental U.S. Those states in the Great Lakes and Northern Plains received the brunt of the snowfall.

The winter storm led to the first blizzard warning in 34 years in Los Angeles and Ventura counties. The Hollywood sign was covered in snow, a rare sight for Tinseltown.

The snow caused some communities in the mountains to be cut off for days, and at least 12 people have died because of the disruption. On March 1, Gavin Newsom, California's governor, declared states of emergency in 13 of the 58 counties.

The California Department of Water Resources said on March 3 that surveyors had recorded a snowpack of 116.5 inches at a station near Lake Tahoe—177 percent of the average for the time of year. Across the state, the snowpack is on average 190 percent of the usual size, putting it "just behind" the record snow year of 1982-83.

However, the water contained in this snow could be shifted by the oncoming storm, increasing the potential for flooding.

"It could get really ugly," David Rowe, a meteorologist with the National Weather Service (NWS), told the San Francisco Chronicle. "Probably most of the melt will be in the foothills. The snowpack is so deep in the higher elevations, even though we're expecting a lot of rain, it will probably soak right in at the higher elevations."

In a forecast discussion on Thursday morning, the NWS said rainfall at higher elevations "may contribute to rapid snowmelt, which may further exacerbate the potential for scattered to numerous flooding instances downstream."

The NWS added that there was a moderate risk of excessive rainfall on Thursday and Friday over central coastal California and the central valleys.

California snowpack snow storms
Members of the California Army National Guard Joint Task Force Rattlesnake shovel snow from a rooftop on March 8, 2023 in Crestline, California, after a series of winter storms dropped more than 100 inches of snow in the San Bernardino Mountains. Melting snow from imminent heavy rain will cause further flooding in the state. Mario Tama/Getty Images

California naturally has an atmospheric river that travels from the subtropics, known colloquially as the Pineapple Express. While it usually brings warm, moist air to the West Coast, when mixed with the cold Arctic air of a Pacific storm, it has the capacity to dump a high volume of rain.

The NWS said that warmer temperatures would likely push the threat of flooding into the Great Basin area. It predicts heavy rainfall to spread across into parts of the northern and central Rockies by Friday.

The NWS added that other regions of the northern West Coast would see localized heavy rainfall. However, there won't be the flooding concerns of further south since these areas will not have the tropical moisture of the atmospheric river.