Atmospheric river to trigger flooding, ease drought in California

An atmospheric river of moisture from the Pacific Ocean targeted drought-stricken California with torrential rain and feet of snow over the high country of the Sierra Nevada on Saturday. The worst of the storm was over for the Golden State, but rain and snow were pushing inland over the western United States as more storms were gathering over the Pacific Ocean, AccuWeather meteorologists say.

Record daily rainfall occurred in the Bay Area of California on Saturday. Over 3 inches of rain was reported across much of the San Francisco area, with over 5 inches in localized spots. Along the west-facing slopes of the Sierras, which are often among the wettest spots in the state, 8-12 inches of rain have fallen in some areas.

Enough rain fell in the zone from San Francisco to Sacramento and many other locations in Northern and Central California to trigger urban, small stream and river flooding. On Saturday afternoon, major flooding on U.S. Highway 101 in south San Fransisco forced a closure of the highway's southbound and northbound lanes, with local officials telling travelers to avoid the area.

A mudslide was also reported in the San Fransisco area on Saturday, resulting in damaged homes and no injuries.

Flooding was reported at the Golden Gate Bridge exit with additional flooding closing roads near Osage Park in Danville, Calif.

In Sacramento County, a state of emergency was issued Saturday night due to flooding, with county officials saying the storm resulted in "significant transportation impacts" as well as floods in Wilton, approximately 20 miles southeast of Sacramento.

Wind gusts that accompanied the rain pushed up to nearly 60 mph, and over 100,000 power outages were reported in the state capitol's metro area Saturday evening. As of Sunday morning, over 60,000 customers were still without power in Sacramento County, according to PowerOutage.US.

Adding in the factor of melting snow in the mountains will substantially raise the risk of flooding in the short-run rivers that flow out of the Sierra Nevada and into the Sacramento and San Joaquin valleys.

"It is possible that this single storm brings some reservoirs close to full capacity [once the runoff completes]," AccuWeather Senior Storm Warning Meteorologist William Clark said. "Where there are no dams or flood control measures, unprotected areas along some of the rivers can quickly take on water in this situation."

Oroville, Folsom and Shasta reservoirs in Northern California hovered just under one-third of their total capacity as of Wednesday, according to the California Department of Water Resources. Water levels were already rising ahead of the big storm, due to storms this week and earlier on in December.

Farther south in California, the storms that moved through earlier this past week dropped a general 0.50 of an inch to 1 inch of rain in the Los Angeles and San Diego metro areas. Both metro areas picked up an additional inch of rain as the atmospheric river swung through Saturday night.

Locally heavier amounts fell on the west- and south-facing slopes of the Coast Ranges of Southern California.

Mudslides and other debris flows in recent burn scar locations could occur throughout California as the ground becomes progressively wetter and unstable as additional storms were on the way, AccuWeather meteorologists warned

The storms will lead to travel disruptions along the Pacific coast, especially around San Francisco. Motorists can expect slow and slick conditions much of the time along the Interstate 5 corridor. Roadways can also be blocked by debris.

There have been dozens of reports of flooding, mudslides and fallen boulders across Northern California as a result of the heavy rain from the first part of the New Year's weekend.

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As the storm pivoted inland, snow levels plummeted. Travel conditions along I-80 through much of the Sierra Nevada came to a standstill as feet of snow piled up in a matter of hours and clogged the major artery. Long stretched of I-80 remained closed as of early Sunday morning.

There is some good news for motorists venturing over the Grapevine, as freezing levels remained too high to bring accumulating snow over the passes in Southern California into New Year's Day.

Farther inland, a significant amount of moisture from the storm will push across parts of Nevada, Utah, northern Arizona, southern Idaho and Colorado into early week. There is the potential for some of the mountain ranges to pick up several feet of snow at progressively lower elevations from the Sierra Nevada to the Colorado Rockies, where warm air from the Pacific is less likely to reach. A change to heavy wet snow in Reno, Nevada, during Saturday night made roads slippery.

Additional storms will likely follow in the coming days for the Pacific Coast states and the interior western U.S.

The busy nature of the storms is an early signal that La Niña is weakening.

"This weakening of La Niña causes the jet stream and associated storm track to shift around quite a bit, as we expected for this winter," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.

La Niña is a climatological phenomenon that causes sea surface temperatures to become cooler than average in the tropical Pacific Ocean for several months to a few years. The pattern tends to reduce the number of storms that roll into California and the southwestern U.S.

"California is likely to continue to get hit by frequent storms into the first part of January," Pastelok explained. "But, the storms may back off later in January then return in February as other signals [including Pacific Ocean temperature patterns] become stronger as La Niña continues to weaken."

AccuWeather's team of long-range meteorologists expects an El Niño, which is when sea surface temperatures in the equatorial Pacific are above normal for an extended period, to develop by late summer or early autumn in 2023. Under certain conditions, a strong El Niño can lead to frequent and powerful storms that hit California and advance inland over the western U.S.