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Flooding rain and high winds wallop California while power outages mount

The Washington Post 3/14/2023 Matthew Cappucci, Jason Samenow

Water rescue officers Gino Degraff and Jeoffrey Orman walk through flooded Pajaro neighborhood streets Monday checking on residents who have not evacuated since a levy breach that flooded the community.

© Melina Mara/The Washington Post Water rescue officers Gino Degraff and Jeoffrey Orman walk through flooded Pajaro neighborhood streets Monday checking on residents who have not evacuated since a levy breach that flooded the community.

Waterlogged California faces one of its most serious flood threats Tuesday in what’s already been a historically stormy season. The National Weather Service has declared a “high risk” of excessive rainfall in both coastal and mountainous zones as yet another strong atmospheric river bombards the state.

The storm system is not only producing heavy precipitation, but also damaging winds. Through early afternoon, it had unleashed gusts to over 70 mph near the coast and above 80 mph in the mountains.

Ferocious winds rocked the San Francisco and Monterey Bay areas, where the local Weather Service tweeted that it had received reports of many trees and power lines down. Utility tracker PowerOutage.US reported nearly 300,000 customers without power across the state.

San Francisco International Airport clocked a gust to 74 mph around noon local time, leading to flight delays and a ground stop.

But water — not wind — is the Weather Service’s greatest concern. The agency is warning of “locally catastrophic flooding impacts expected for portions of the coast, Central Valley, and the Sierra Nevada foothills.”

The storm has already produced up to several inches of rain and more than a foot of snow in the highest terrain.

Calif. levee breach forces thousands to evacuate

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The Weather Service office serving the area around Sacramento reported flooded roads in interior Northern California while the Weather Service’s Bay Area office posted a report of roadway flooding near the Salinas River and warned of flooding for interior portions of Sonoma County.

In photos: Another powerful storm intensifying California flood threat

One of the high-risk areas covers the central Sierra Nevada, stretching from Yosemite National Park to east of Fresno. That’s where heavy rainfall totaling 3 to 6 inches will combine with rapid snowmelt to overwhelm area creeks and rivers. Some areas could see in excess of 8 inches.

A second high-risk zone has been introduced for the central California coastline and the coastal range from near Monterey to just west of Santa Barbara.

The Weather Prediction Center's outlook for flash flooding and excessive rainfall. (WeatherBell)

Flood watches expand into Southern California, including the Los Angeles area, and extend across much of central and Northern California.

Burn scar areas in parts of Southern California are at risk of dangerous debris flows. Ahead of the storm Monday, evacuation orders were given to residents in Santa Barbara County residing near the Alisal or Cave burn scars.

Santa Barbara is expecting 3 to 5 inches of rain through late Tuesday night.

Winter storm warnings are in effect for the Sierra Nevada, since primarily snow is expected to fall above 6,500 or 7,000 feet. Totals may reach several feet in the highest mountain peaks.

High-wind warnings also cover much of the Central Valley, with wind advisories occupying a patchwork across the remainder of California. They also extend into Nevada.

The atmospheric river is hitting California just four days after President Biden declared a state of emergency “resulting from severe winter storms, flooding, landslides, and mudslides beginning on March 9, 2023, and continuing.”

The storm now

The American GFS model simulates moisture from the atmospheric river working ashore. (WeatherBell)

A developing low-pressure system was centered just to the north of the Bay Area early Tuesday afternoon. That counterclockwise-spinning low was drawing in a plume of robust subtropical moisture — the atmospheric river — from the west central Pacific as much as 6,000 miles away and slingshotting it east. The core of the atmospheric river was buffeting the coastline, and will continue to soak the already-waterlogged state before pinching off and sliding south into Wednesday.

Atmospheric rivers carry the bulk of their moisture between one and two miles above the ground. That’s why the greater precipitation totals are found in the foothills and mountains — the high terrain literally pokes into the atmospheric river and helps focus moisture.

The heaviest slug of moisture will exit central California to the east Tuesday afternoon, but bands of precipitation will linger in Southern California through much of Wednesday. That’s why Los Angeles will see the majority of its rainfall come down Tuesday night.

Impacts so far

Considerable precipitation totals have already been realized, and some may be doubled by the end of the event. As of early afternoon Tuesday, several locations in California had already reported over a foot of snow from the system. The Central Sierra Snow Lab posted 13.6 inches from the storm, boosting its seasonal total to more than 55 feet, just 7 inches from the second-most on record.

Rainfall reports of between 0.5 and 1.5 inches were common through the Central Valley and around the Bay Area, but several locations along Northern and Central California’s coastal ranges and the foothills of the northern Sierra had seen 2 to 6 inches.

The worst of this atmospheric river was moving into an area with increased vulnerability stemming from heavy rainfall from prior storms.

On Friday, a potent atmospheric river slammed much of California, bringing up to a foot of rain in the mountains of Tulare County in the town of Peppermint. Nearby Springville saw more than 8 inches of rain, then promptly experienced disastrous flooding that prompted evacuations and the issuance of a dire “flash flood emergency.”

In Monterey County, a levee on the Pajaro River failed, inundating the entire town of Pajaro and forcing a mass evacuation. Monterey County officials said it may be months before some homes are habitable again.

What lies ahead

The National Weather Service's forecast for total precipitation. (WeatherBell)

In the Central Valley, precipitation totals of 1 to 2.5 inches are expected, with at least 3 to 6 inches of rain in the Sierra Foothills below the rain-snow line. The rain should change to snow around 6,500 or 7,000 feet, above which 2 to 5 feet of snow is likely, except up to 4 to 7 feet above 8,000 feet.

Avalanche warnings are in effect for parts of the east slopes of the Sierra.

“Heavy snowfall, rain on snow, and strong winds will overload the snowpack, which contains buried persistent weak layers,” wrote the Eastern Sierra Avalanche Center. “Travel will be difficult and very dangerous. Expect widespread areas of unstable snow.”

The National Weather Service's forecast for snowfall. (WeatherBell)

Closer to the ocean, 3 to 6 inches of rain could fall in the coastal ranges south of the Bay Area and north of Los Angeles.

The San Francisco Bay Area is expecting up to about 1.5 inches of rain, while a general 2 to 3 inches of rain is forecast in Los Angeles. The heaviest rain in Southern California may coincide with the evening commute Tuesday and last overnight.

Conditions will improve north to south late Tuesday into Wednesday.

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