MoxVib

It’s Day 3 of an atmospheric river’s influence in California, and moderate to heavy rains continue to plague the state. Rainfall totals are approaching double digits in the mountains north of Santa Barbara, and another day of sporadic downpours and isolated thunderstorms could exacerbate conditions further.

Flood watches affecting more than 35 million people blanket virtually the entire coastline from Eureka to the Mexican border and encompass most of the Central Valley. The threat of additional flooding will last into Wednesday.

On Tuesday afternoon, torrential rains triggered a flash flood warning for San Francisco County until 4 p.m. local time. “Doppler radar indicated heavy rain across the warned area,” the National Weather Service wrote. “Between 0.2 and 1 inch of rain has fallen. Additional rainfall amounts of 0.5 to 1 inch are possible in the warned area. Flash flooding is ongoing or expected to begin shortly.”

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Scenes from social media showed flooded streets in San Francisco.

Heavy rain on Tuesday afternoon also moved over Los Angeles, which was under a flood advisory until 4:15 p.m. The Weather Service cautioned that minor flooding was possible in low-lying and poor drainage areas and that mudslides and landslides could affect canyons and vulnerable hillsides.

High water, debris flows and mudslides also affected parts of Central and Southern California on Monday, closing numerous roads.

The westbound lanes of the Pacific Coast Highway were blocked by a mudslide in Santa Monica, while large boulders blocked both lanes at the intersection of Malibu Canyon Road and Piuma Road in Agoura Hills. Flooding also blocked the HOV lane and a second lane on Interstate 5 in Burbank for a time late Monday morning.

The hardest-hit areas appeared to be between Los Angeles and San Luis Obispo. Montecito reported 8.84 inches of rain, with 9.71 inches a few miles to the northeast in the mountains near Toro Canyon Creek. Most of the Ventura County mountains have received at least 6 inches of rain, with 5 inches just north of Malibu and 4.36 inches at Pepperdine University. Downtown Los Angeles has seen considerably less rainfall, but more is on the way. It has already received 11.64 inches of rain this month, fifth most on record; if two more inches fall, it would be the wettest February on record.

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In Southern California, there is the risk of an isolated tornado or waterspout, following several tornado warnings in the Sacramento Valley on Monday. At least one of the storms that triggered tornado warnings marched into the mountains and produced snow.

Highs winds in the central and northern parts of the state toppled trees and power lines. While the strongest winds have eased, more than 10,000 customers remained without power Tuesday.

The atmospheric river’s influence is expected to finally wane by Wednesday evening, but active weather looks to return in time for the weekend.

Where is the atmospheric river now?

The atmospheric river — a filament of deep, tropical moisture from near Hawaii — is being swirled ashore by low pressure stationed off Oregon. It worked its way down the coast and is now aimed at Southern California. San Diego and Los Angeles are in the crosshairs of the moisture plume, and they will be for much of Tuesday.

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Behind it, a pocket of frigid air aloft is moving overhead in association with the core of the upper-level low pressure. That will lead to scattered downpours and perhaps a few thunderstorms across most of California through Wednesday evening.

A few of those thunderstorms could produce small hail, an isolated funnel cloud or even a rogue tornado.

What’s next

A Level 3 out of 4 risk of flash flooding and excessive rainfall has been drawn to include the Los Angeles metro area. “Individual showers and any embedded storms will be moving very quickly,” wrote the Weather Service, but “training will be constant,” meaning downpours will continuously move over the same areas like train cars on a track.

An additional 2 to 4 inches of rain is probable in the mountains north of Los Angeles, while downtown areas see closer to an inch, or perhaps more if downpours do indeed train.

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The issue with this storm isn’t so much the rainfall totals — which will be markedly less than with the previous storm, which dropped upward of 7 inches of rain in two days. Instead, the concern is for rainfall rates topping half an inch per hour. That will quickly overwhelm soils and the already saturated ground, leading to additional flooding.

Additional downpours are also expected in parts of Central California but should be short-lived.

Heavy snows and high winds

Snow levels in the Sierra Nevada will drop from 7,000 feet to about 6,000 feet, leading to greater accumulations at the higher altitudes. A few more inches are probable above 6,000 feet at the resort levels, but the high Sierra peaks might see another 2 to 4 feet of snow.

Through Monday, many areas in the Sierra had received double-digit totals, including Northstar, Bear Valley, June Lake, Kirkwood and Mammoth Mountain.

Winds along the Sierra Ridge may also gust over 40 mph, but elsewhere the strong winds have largely subsided. At the Central Sierra Snow Laboratory at Donner Pass, maintained by the University of California at Berkeley, high winds cut power Monday. Numerous gusts topped 60 mph in the Sierra, including a 99 mph gust near Mammoth Mountain.

In the mountains of Southern California, the Weather Service predicted 10 to 20 inches of new snowfall, with snow levels dropping from 7,500 to 6,000 feet.

Looking ahead

By Thursday, the parent upper-air disturbance behind this atmospheric river is expected to have exited. Thereafter, tranquil weather is forecast before the next atmospheric river is anticipated to arrive late Sunday.

Jason Samenow contributed to this report.

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