Tuesday, April 16, 2024

California faces flood warnings, severe rainstorms and atmospheric river – The Washington Post

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It is the third day of atmospheric river influence in California, and moderate to heavy rain continues to plague the state. Rainfall totals are approaching double digits in the mountains north of Santa Barbara, and another day of sporadic showers and isolated thunderstorms could further worsen conditions.

Flood watches affecting more than 35 million people cover virtually the entire coastline, from Eureka to the Mexican border, and encompass most of the Central Valley. The threat of additional flooding will persist through Wednesday. Debris flows and mudslides have already occurred, notably in Los Angeles. And flooding, mud and debris forced the closure of many roads in central and southern California.

Westbound lanes of Pacific Coast Highway were blocked by a mudslide in Santa Monica, while large rocks 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 some time Monday morning.

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

In Southern California, there is a 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 tracked into the mountains and produced snow.

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

The influence of the atmospheric river should finally subside by Wednesday evening, but active weather appears to return in time for the weekend.

Where is the atmospheric river now?

The atmospheric river – a filament of deep tropical moisture originating from near Hawaii – is driven ashore by a low parked off the coast of Oregon. It has made its way up the coast and is now aiming for Southern California. San Diego and Los Angeles are in the crosshairs of the moisture plume, and this will be the case for much of Tuesday.

Behind it, a pocket of frigid air aloft moves overhead in association with the core of upper-level low pressure. This will bring scattered showers and perhaps a few thunderstorms to most of California through Wednesday evening.

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

A Level 3 out of 4 risk of flash flooding and excessive precipitation has been established to include the Los Angeles metropolitan area. “Individual showers and integrated storms will move very quickly,” the National Weather Service wrote, but “the formation will be constant,” meaning showers will continually move over the same areas like train cars on a railroad track.

An additional 2 to 4 inches of rain is likely in the mountains north of Los Angeles, while downtown areas see closer to an inch, or even more if showers actually occur.

The problem with this storm isn’t so much the precipitation totals. — which will be significantly less than the previous storm, which dropped more than 7 inches of rain in two days. The concern is more about precipitation rates exceeding half an inch per hour. This will quickly overwhelm already saturated soils and soils, leading to additional flooding.

Meanwhile, in central and northern California, the rain is largely over, with only an isolated downpour over the next 36 hours.

Heavy snowfall and strong winds

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

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

Winds along the Sierra Ridge can also gust above 40 mph, but elsewhere strong winds have largely subsided. At the Central Sierra Snow Laboratory in Donner Pass, maintained by the University of California at Berkeley, strong winds knocked out power on Monday. Many gusts exceeded 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 to 7,500 to 6,000 feet.

By Thursday, the upper altitude disturbance causing this atmospheric river should have disappeared. Thereafter, calm weather is expected before the arrival of the next atmospheric river on Sunday evening.

Jason Samenow contributed to this report.

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