California atmospheric river brings record rain, major mudslide risk

An atmospheric river dumping rain across Northern California and several feet of snow in the Sierra was making its way across the state Friday, bringing flooding and threatening mudslides along with it.

The storm, the first major rainmaker of the season, initially strengthened into a bomb cyclone, a description of how it rapidly intensified before making its way onshore.

It’s now moving south, after strong winds and heavy rain hit the northern edge of the state Wednesday and Thursday.

It rained 3.66 inches in Ukiah on Thursday, breaking the record for the city set in 1977 by a half-inch. Santa Rosa Airport saw 4.93 inches of rain on Thursday, shattering the daily record set in 2001 of 0.93 inches.

Across Humboldt and Mendocino counties, the highest rainfall totals were 17.7 inches near Laytonville and around 15 inches near Honeydew in the King Range mountains, according to National Weather Service meteorologist James White in the Eureka office. Over the weekend, the storm could blanket the region with another one to two inches of rain.

Near Sacramento, Whiskeytown in Shasta County got 12.21 inches while Sims in the Klamath Mountains got 13.37 inches, according to NWS forecaster Bill Rasch from the Sacramento office. The region is expected to get another one to two inches of rain, with foothill areas in interior Northern California getting anywhere from two to five inches, he said.

The storm should start bringing snow after Friday, with elevations above 6,000 feet getting between one and three inches by Tuesday, Rasch said.

“Today is the last big day of rain,” he added.

In a Friday morning forecast, the Weather Service warned that “prolonged rainfall will result in an increased risk of flooding, an increased risk of landslides, and downed trees and power lines across the North Bay.”

In the San Francisco Bay area, the rain has been primarily focused on Sonoma, Marin and Napa counties, according to National Weather Service meteorologist Dylan Flynn in the Monterey office. Santa Rosa’s rainfall has been historic, shattering its three-day rainfall record of about 10 inches set in October 2021.

“Over a 72-hour period, we’ve never seen this much rainfall reported anytime of the year,” Flynn added. “This is the most rainfall we’ve seen in the last 123 years.”

The storm is expected to leave the North Bay and impact the rest of the San Francisco Bay area Friday, dumping around one to three inches across the region before petering out on Saturday, Flynn said. The storm could then extend south into the Central Coast and possibly Southern California.

On Saturday, Los Angeles and Ventura counties could see anywhere from a tenth to a third of an inch of rain. San Luis Obispo and Santa Barbara counties could see up to an inch in some areas.

A second round of rain expected to begin Sunday could be “a little stronger than the first but still likely in the ‘beneficial rain’ category,” the National Weather Service said in its latest L.A. forecast.

Chances are low of flooding or any other significant issues in Southern California, forecasters said, though roads could become slick and snarl traffic.

Concern about the upcoming rainy season has been growing among residents living on and near the Portuguese Bend landslide area in Rancho Palos Verdes, because increased rainfall leads to more groundwater — which is the impetus for the ongoing, devastating land movement.

But city officials are hopeful that extensive “winterization efforts,” which include improving drainage, filling in cracks and lining canyon walls, can help minimize the effect of any new rain. Much of those projects has been completed, but some remains underway. The work aims “to best prepare ourselves for the wet weather season ahead,” said David Copp, the city’s deputy public works director.

Parts of the landslide have seen recent slowing, and in some areas, even complete stabilization, the city has reported, but additional rain is always a concern for this unstable region.