By AccuWeather senior meteorologist
Published Dec 29, 2022 12:15 PM EST | Updated Dec 31, 2022 5:38 AM EST
An atmospheric river of moisture from the Pacific Ocean is aiming directly at drought-stricken California and other parts of the West, AccuWeather meteorologists say. The sizable storm will unleash heavy rain that will replenish reservoirs and raise the risk of flooding and will produce heavy snow in the mountains through the New Year’s holiday weekend.
Even though a long train of storms from the Pacific will continue to roll ashore through early January, there is the potential for the major storm to unload 8-16 inches of rain in a narrow zone of Northern and Central California during the final days of 2022.
In some locations, an AccuWeather Local StormMax™ rainfall of 20 inches will be possible with the storm, and much of that rain is likely to fall over a 24-hour stretch through Saturday night.
Heavy rain was already reported in Northern California on Friday, including a 4.5 inch heavy rain report in Willow Creek, California, which is located about 30 miles northeast of Eureka.
Mild air will accompany the first part of the storm into Saturday. The warm surge will push freezing levels well up into the mountains. As snow levels shift, the risk of avalanches will increase, but so will the likelihood of flooding, AccuWeather forecasters warn.
"Snow levels rising to between 8,000 and 9,000 feet in the Sierra Nevada will mean that only the highest mountain peaks will be affected by snow through Saturday morning," AccuWeather Meteorologist Ryan Adamson said. "Some of the snow in the Sierra Nevada is likely to be washed away."
Enough rain will fall 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. San Francisco may receive 2-4 inches of rain from the storm while 4-8 inches of rain could fall near Sacramento.
The added factor of melting snow in the mountains will substantially raise the risk of rapid 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," AccuWeather Senior Storm Warning Meteorologist William Clark. "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 week have already dropped a general 0.50 of an inch to 1 inch of rain in the Los Angeles and San Diego metro areas. An additional 1-2 inches of rain will likely fall through this weekend in coastal areas of Southern California from small-scale storms and perhaps a brief visit by the atmospheric river from Saturday night to Sunday morning.
Locally heavier amounts can fall on the west- and south-facing slopes of the Coast Ranges of Southern California. Even a few hundredths of an inch of rain has been known to lead to slick driving conditions. With the level of rain currently forecast by AccuWeather meteorologists, there could be flooding in urban areas.
Mudslides and other debris flows in recent burn scar locations could occur into this weekend throughout California, as well as Oregon and Washington, as the ground becomes progressively wetter and unstable.
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.
As the storm pivots inland from Saturday afternoon to Sunday, snow levels will plummet. Travel conditions along I-80 at Donner Pass, California, and along I-5 at Siskiyou Pass, in southern Oregon, are likely to turn wintry in a hurry on Saturday.
"Invading colder air will allow snow levels to drop to 4,000 to 5,000 feet by late Saturday in the Sierra Nevada," Adamson said. "Travel impacts over the mountains will shift from heavy rain and flooding into Saturday morning to slippery and snow-packed roads by Saturday afternoon and Saturday night."
In Southern California, it is possible that some of the higher peaks become capped with snow during the tail end of the storm by early Sunday.
There is some good news for motorists venturing over the Grapevine, as freezing levels are likely to remain too high to bring accumulating snow over the passes in Southern California through New Year's Day, according to Adamson.
In northern Oregon and much of Washington, much of the rain and snow has already fallen and may not be as heavy as the atmospheric river farther to the south. Still, an inch of rain can fall along the I-5 corridor from Bellingham, Washington, to Seattle and Portland, Oregon.
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 next 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.
Additional storms will likely follow in the coming days for the Pacific Coast states and the interior western U.S.
Are the storms in California a sign of brewing El Niño?
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.
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