A potent winter storm is pummeling California with flooding rains, winds gusting above 100 mph and enormous amounts of mountain snow, sparking concerns about dangerous debris flows and grinding travel to a halt. Parts of the Sierra Nevada could see up to 10 feet of pasty wet snow, while areas closer to the coast are anticipating up to eight inches of rain.

The blast of chilly air associated with the storm has even brought a rare snowstorm to the northern Sacramento Valley, with snow levels descending below 1,000 feet.

Interstate 5 was shut down at the Grapevine for a time Tuesday, with slow-going, treacherous travel reported on I-80 through Donner Pass.

The National Weather Service warned that “if you risk travel over the Sierra passes, you could be stuck in your car for several hours, if not a day,” forecasting “whiteout conditions, blowing snow … and near-zero visibility.”

Hazardous weather will continue on and off through at least Thursday and into Friday in some spots, coming after days of already wild weather in the Golden State. Strong winds accompanying the system have downed trees and power lines in the lowlands, knocking out electricity to more than 400,000 customers, according to PowerOutage.US.

The setup

Setting off the extreme weather is a strong low-pressure system diving southeast along the Pacific Coast. Southwesterly winds ahead of it have dragged ashore a fire hose of moisture, with an atmospheric river aimed Wednesday morning at areas south of the Bay Area — a narrow funnel of extreme moisture content that, in this case, is contributing to high precipitation rates and snowfall totals across the West.

A strong atmospheric river such as this one can move an amount of water vapor roughly equivalent to 7.5 to 15 times the average flow of water at the mouth of the Mississippi River, according to a National Oceanic and Atmospheric Administration fact sheet.

The corridor of the greatest moisture is focused inland, toward Lake Tahoe, as well.

In San Francisco proper, the weather early Wednesday featured intermittent showers and gray skies, with a total of 0.83 inches of rain. Three Peaks in Big Sur, along the coast between San Francisco and Los Angeles, picked up 4.42 inches in 24 hours.

Nearby Hearst Castle recorded 5.2 inches. Santa Rosa, north of the Bay Area up Highway 101, received 3.19 inches. Scotts Valley and Santa Cruz, both just outside the Bay Area south of San Jose, wound up with between 3 and 4.5 inches.

That ribbon of precipitation has been the source of most downpours and the heaviest snow and will continue to pivot south over through early Friday. Behind it, conditions should gradually improve.

Cold air aloft will contribute to copious snow in the high elevations where moisture from the atmospheric river is forced upward, enhancing precipitation. There’s an increasing risk that the atmospheric river could slow in its southward progress, stalling over San Luis Obispo, Santa Barbara and Ventura counties.

The best chances of showers in Los Angeles were slated for Thursday and Thursday night.

Heavy rain threatens flooding, debris flows on burn scars

Widespread heavy rainfall accompanying the storm poses the risk of flooding, especially on the windward, or western, side of any higher-elevation areas. That includes the Diablo Range and the Santa Cruz Mountains, where a widespread 2 to 4 inches with localized six-inch-plus amounts were expected.

Though the worst rainfall has ended near and north of the Bay Area, the stalling atmospheric river farther south was unleashing torrential downpours over areas burned by wildfires last year. Among the areas at greatest risk are the CZU and SCU lightning-complex burn areas, as well as the LNU Lightning Complex scars. California had its worst wildfire season on record in 2020, putting the state at particular risk for debris flows during the rainy season.

“Significant mud and debris flows are possible and could affect vulnerable roads and structures, including highway 101,” wrote the National Weather Service in Los Angeles, which expressed particular concern about the Avila burn scar south of San Luis Obispo.

Concern was growing in Monterrey County, where local media reported people trapped in their homes and animals missing after a mudslide struck near the burn scar of last year’s River Fire. Ranchers offered heavy equipment to Monterey County Regional Fire officials to make progress into affected areas. The National Weather Service Weather Prediction Center issued a high risk — level 4 out of 4 — of excessive rainfall for a section of Monterrey and San Luis Obispo counties, where the stalling atmospheric river could drop 6 to 12 inches of rain by early Thursday.

The Dolan, River and Carmel burn scars could all be affected with very heavy rain and an increased mud and debris flow potential.

Rainfall rates of 2 to 3 inches per hour are possible, as well, leading to serious runoff concerns and the potential for urban flash flooding. Flash-flood watches blanketed much of central and coastal Southern California, with the faucet unlikely to shut off until Thursday or Friday.

Strong winds topple trees, cause power outages

Fierce winds within and behind the atmospheric river near the core of low pressure have been causing damage, disrupting power and downing trees. Winds have proved particularly strong in the Sacramento Valley, with gusts reaching 63 mph at Sacramento International Airport. Gusts topping 60 mph were likely through early Wednesday morning there and in the northern San Joaquin Valley, where high-wind warnings were in effect. Gusts reached 67 mph at McClellan Air Force Base.

Farther east, winds between 70 and 80 mph were howling through the peaks of the Sierra Nevada, coinciding with heavy snowfall to make for extreme blizzard conditions and total whiteouts.

At the Kirkwood resort about 15 miles south of Lake Tahoe, a gust of 103 mph was clocked, with a 125-mph gust at 8,700 feet near Lake Tahoe.

Extreme mountain snowfall

Crushing snowfall, with localized amounts nearing 10 feet, was falling in the Sierra Nevada. Since atmospheric rivers carry the most moisture about a mile above the ground, tall mountaintops can channel that humidity into producing prolific snow totals.

Many spots within the Sierra Nevada will top six feet by Friday once the atmospheric river and snow on its periphery finally clear, following a days-long onslaught of heavy snow, hurricane-force winds and blizzard conditions.

There was “extreme avalanche danger” in the in the central Sierra Nevada as well.

“Total snow accumulations of 2 to 4 feet, except 3 to 6 feet above 7,000 feet,” were expected, wrote the Weather Service, with “periods of thundersnow” overnight Wednesday.

In addition, bitter cold wind chills around minus-20 or minus-30 were expected, a “life threatening situation” for anyone who becomes stranded.

“Stay indoors until the snow and wind subside,” urged the Weather Service. “Even a short walk could be deadly if you become disoriented.”

Rare snow in the Sacramento Valley

Even the lower elevations in Northern California were seeing a dose of winter’s wrath, with winter storm warnings issued for parts of the northern Sacramento Valley. Redding, Calif., was included in the warning, with Weather Service meteorologists forecasting several inches of snow before an eventual changeover back to rain during midmorning Wednesday.

Snow and reduced visibility were reported at the airport there between 9:30 p.m. Tuesday and 4 a.m. Wednesday. Redding can go years without snow but gets periodic plowable snow every so often.

Northwest of Redding, at an elevation of 2,650 feet, an observer reported just over 30 inches of snow in a little over 10 hours from 1 to 11:30 p.m. Tuesday. Similar totals were tallied to the north near Mount Shasta.