By AccuWeather meteorologist
Updated Jan. 25, 2021 11:15 AM EST
After a weekend storm brought some much-needed rain and snow to California and much of the Desert Southwest, forecasters say the weather is going to kick it up a notch this week. An "atmospheric river" will pound the Golden State with days of relentless rain and mountain snow.
The week is already off to a stormy start as a system is forecast to slide southeastward from California to the Four Corners region into Tuesday. Drenching rain is forecast to sweep southeastward at low elevations, with enough rain along the Interstate-5 corridor to cause travel delays.
Snowfall will cause difficulty for motorists venturing over the passes in California, and the snow will not just be limited to the Sierra Nevada. Snowflakes may be seen at elevations as about 1,000 feet above sea level over Southern California into Tuesday.
Malibu, California, already saw a surprising snowfall on Saturday when a brief burst whitened the ground in parts of the Malibu hills.
Major thoroughfares such as Interstate 5 through Tejon Pass and Interstate 15 through Cajon Pass in Southern California could become treacherous or impassable at times due to snowfall. Heavy snow will also spread across the Sierra and interior Southwest.
Over a foot of snow is likely in some populated and well-traveled mountain locations into Tuesday, with an AccuWeather Local StormMax™ of 48 inches over the high country.
However, forecasters say this first storm will pale in comparison with what's on the way this week.
"A storm with copious amounts of moisture is expected to move into California from late Tuesday to Friday," said AccuWeather Senior Meteorologist Adam Douty. "This storm is expected bring very heavy snow into the mountains and rain into the lower elevations."
The storm will stall just off of the Northwest coast this week. That will aim what is known as an "atmospheric river" directly at California, where it will persist for days.
"The storm system will 'wobble' at times as it spins just off the coast. This will cause a plume of the greatest moisture to shift at times, shifting where the heaviest rain and snow is directed through the week," Douty said.
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This could lead to a widespread flooding threat, as large portions of the state will spend time under the plume of heavy rainfall. The greatest rainfall amounts will be along the west- and southwest-facing coastal ranges of Northern and Central California, as well as in the western foothills of the Sierra, below the snow line.
Widespread rainfall totals of 4-8 inches are anticipated in these areas. Rainfall amounts of 8-12 inches will be possible where the heavy rain is more persistent, with an AccuWeather Local StormMax™ of 15 inches.
"Not only can this lead to a significant threat for flash flooding, but in
the many burn scars across the state, mudslides and debris flows would be a significant threat as well," Douty said.
Forecasters say some of the areas of greatest concern are with the burn scars associated with the SCU Lightning Complex and Creek Fire in Central California, August and North Complexes in Northern California and the Bobcat Fire in Southern California
While lower elevations are drenched, the higher ground will see prolific snowfall. Accumulations of 3-6 feet are expected from Tuesday night through Friday in elevations over 4,000 feet, with an AccuWeather Local StormMax™ of 10 feet. At times, snow levels are expected to be as low as 2,000 feet in Northern California and 2,000 to 4,000 feet across the Sierra Nevada Mountains, which will be low enough to impact nearly all passes and cause significant travel delays.
"There is the potential for all of the major passes in California and southern Oregon to be shut down at about the same time from this storm this week," according to AccuWeather Senior Meteorologist Alex Sosnowski.
"Travel along Interstate 80 through Donner Pass could completely come to a halt this week as snow heaps up and gusty winds whip," said AccuWeather Senior Meteorologist Danny Pydynowski.
"With so much snow on the way in a relatively short period of time, the risk for avalanches will greatly increase in the Sierra as well," Douty explained.
In Southern California, snow levels are expected to be higher with the storm from Wednesday to Friday, which should keep the majority of accumulating snow above pass levels. However, snow can still be heavy across the higher elevations, adding even more to the snowpack at some mountain resorts in the region.
Gusty winds are also forecast to accompany the new storm this week. However, while winds are not expected to raise the fire risk due to rain and snow accompanying the system, there can still be strong enough gusts to lead to sporadic to regional power outages. Gusty crosswinds can add to the difficult driving conditions caused by the rain and snow.
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While this storm will be no joke and will bring some serious threats to California, the precipitation is also much needed.
As of Wednesday, Jan. 23, 2021, the mountain snow drought was serious in parts of the Southwest, according to the National Weather Service. The amount of water locked up in existing snow was less than 50% of the 30-year average from 1980 to 2010.
According the United States Drought Monitor, 78% percent of the state of California is in moderate drought or worse, with nearly 40% in severe or extreme drought. This storm will likely help to put a huge dent in the drought in the area, and could completely erase it for some.
The current U.S. Drought Monitor map for California. (U.S. Drought Monitor)
"As beneficial as the stormy pattern will be for shutting off the wildfire threat and boosting snow depth over the high country, which later feeds runoff into streams and rivers during the spring and summer and replenishes the water supply in the region, rounds of heavy rain over low elevations are likely to be too much for the landscape to handle without problems," AccuWeather Senior Meteorologist Alex Sosnowski warned.
"Motorists should be prepared for substantial delays on the highways with the risk of some roads being closed from flooding, mudslides and snowfall."
While the next storm this week will largely set its sights on California, the entire Southwest will still have chances to get back into the action into early February.
"The storms will keep coming in the Southwest," said AccuWeather Lead Long-Range Meteorologist Paul Pastelok. "More snow and rain will fall across the Southwest, leading to some flooding and slippery travel in the higher elevations into the first few days of February."
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