Snow in San Bernardino mountains
Ice covers communication towers as massive amounts of snow trap residents of mountain towns in San Bernadino County. REUTERS/David Swanson

Millions of Californians are under threat from an approaching storm that could trigger intense downpours and widespread flooding in many parts of the state, including in some mountainous areas already buried under a near-record snowfall.

After a weeks of unusually bad weather on the West Coast, another storm known as an atmospheric river could dump as much as 3 inches of rain in the San Francisco Bay area and across the Central Coast through the weekend, the National Weather Service said in its forecast.

Hilly inland areas and some coastal ranges inundated by snow in recent weeks could get 6 inches of rain. Some 8 inches was expected in the Santa Cruz Mountains, south of San Jose, and up to 10 inches in the Santa Lucia Mountains, along the coast near Big Sur, the weather service added.

But the storm “is expected to decrease toward the south across San Diego,” the agency said, with rainfall Friday and Saturday of a half inch near the coast to an inch or more in the mountains.

It will be a warmer storm locally, with highs 59 to 64 along the coast and inland, 50 to 60 in the mountains and 73 to 78 in the deserts.

Atmospheric river storms such the one in the forecast are akin to rivers in the sky that dump massive amounts of rain and can cause flooding, trigger mudslides and result in loss of life and enormous property damage.

The growing frequency and intensity of such storms and bouts of extreme drought, are symptomatic of human-caused climate change, experts say. The storms make it more difficult to manage California’s precious water supplies while minimizing the heightened risks of floods and wildfires.

“Most of the flooding concerns are for the lower-lying areas susceptible to rapid river and stream rises,” said meteorologist William Churchill at the national Weather Prediction Center. “It’s really a combination of all this heavy rainfall coming and also rapidly melting snow.”

The looming deluge follows a barrage of nine such storms that triggered widespread flooding, mudslides, rockfalls and sinkholes across California from late December through mid-January. At least 20 deaths were attributed to the earlier storms.

The approaching weather system is relatively warm. It will bring rain and strong winds not only to low-lying parts of northern and central California but to mid-elevation mountain areas still struggling to dig out from back-to-back blizzards that over the last couple of weeks dumped more than 100 inches (2.5 metres) of snow in some places, forecasters said.

More than 15 million people in the metropolitan areas around San Francisco Bay and Sacramento and in parts of the region were under excessive-rainfall and flood advisories from the weather service on Thursday. Some of watches and warnings in California were to remain in effect until Sunday.

Some waterfront communities along major rivers and their tributaries also braced for the possibility of overflowing streams swollen by heavy showers and runoff of melting snow.

In Tulare County, Sheriff Mike Boudreaux issued an evacuation warning on Wednesday for homes and businesses along a stretch of the Kings River, which drains the Sierra Nevada mountain range, in advance of “this rain-on-snow event.”

Elsewhere, the weather service issued “prepare now” alerts for residents along the Big Sur, Carmel, Salinas and Pajaro rivers.

Crews were scrambling until the last minute to shore up flood-weakened levees that had been breached along the Cosumnes River south of Sacramento earlier this year during a previous storm, the San Francisco Chronicle reported.

Higher-elevation mountain areas — above about 8,000 feet — were expected to get even more snow, or snow mixed with rain, posing the risk of widespread roof damage to older structures under the mounting weight of frozen precipitation, forecaster Churchill said.

Although damaging, this winter’s storms have eased a historic four-year dry spell in California, replenishing some badly depleted reservoirs and the Sierra snowpack, a critical source of fresh water for the state.

Updated at 7:40 a.m., Friday, March 10, 2023

Reuters contributed to this article.