An intense and deadly winter storm is barreling into California, unleashing damaging winds, heavy rain and flooding, and feet of mountain snow.
In Southern California, the same storm is dragging an atmospheric river into the coast, delivering heavy rain, snow and strong winds that will continue into Wednesday.
Statewide, more than 200,000 customers were without power Tuesday evening, down from nearly 250,000 earlier, according to utility tracker PowerOutage.us. Stanford University said it had canceled Tuesday’s exams because of a power outage impacting its Stanford campus; its campuses in Redwood City and Menlo Park were also impacted.
As the storm neared central California on Tuesday afternoon, intense thunderstorms barged into the area between San Francisco and Monterey, prompting severe thunderstorm warnings for destructive winds up to 80 mph. Santa Cruz, San Mateo and Santa Clara counties were particularly hard hit. Wind gusts of up to 59 mph were clocked in Monterey and up to 76 mph in the Santa Cruz mountains, downing numerous trees and power lines. Downpours associated with the storms also spurred several instances of flooding.
Meteorologists were awestruck by two intense zones of low pressure orbiting each other just offshore the Bay Area, which triggered the powerful storms. Early in the evening, the low-pressure zones consolidated and made landfall near San Francisco, presenting a hurricane-like eye that further astonished meteorologists.
The San Francisco Bay Ferry operated sporadically Tuesday evening as strong winds at one point created “unsafe conditions” at its boatyard in Alameda, across the bay from San Francisco, preventing the ferries from being pulled out of their berths.
The atmospheric pressure dropped to a March record low in San Francisco as winds gusted to 62 mph. The lower the pressure, the stronger the storm.
High winds also hit Southern California earlier Tuesday, with gusts over 100 mph in the high terrain and topping 50 mph at the coast.
Mesoscale low pressure off the Santa Cruz and San Mateo coast has developed two centers that are trying to rotate around one another. This is similar behavior to that seen with hurricanes and is called the "Fujiwara Effect". 1:30 pm pdt, Mar 21, 2023. pic.twitter.com/QrSq0SZR0V— Jan Null (@ggweather) March 21, 2023
Unrelenting stormy weather has left the ground waterlogged, with creeks, rivers and reservoirs swelling. Snow continues to accumulate in the Sierra Nevada, reaching historic levels in many areas.
In the foothills and mountainous parts of Southern California where it’s too warm to snow (generally below 5,000 feet), more than 7 inches of rain could fall by Wednesday. “Numerous flash floods are likely,” the Weather Service cautioned.
Through early Tuesday evening, the Weather Service office in Los Angeles reported up to almost 3 inches of rain and 8 inches of mountain snow in the region. Downtown Los Angeles had picked up over an inch. Around San Diego, where an inch of rain had fallen, minor flooding of low-lying and poor drainage areas was reported.
Just six months ago, California was entrenched in an extreme drought that dragged on for three years. Now it seems to be locked in a never-ending cycle of snow and rain, as snowmelt season looms this spring. Flood-control systems are likely to be increasingly stressed in the coming weeks.
A bomb cyclone with dangerous winds
In the 24 hours before the storm made landfall in the Bay Area, its pressure dropped so fast that it met the meteorological criteria of a “bomb cyclone.” The resulting strong winds prompted high wind warnings and wind advisories for much of the southern two-thirds of California.
Gusts up to 70 mph were forecast for parts of the Central Coast, which were observed in several locations Tuesday evening.
“People should avoid being outside in forested areas and around trees and branches,” the Bay Area National Weather Service wrote in a warning message. “If possible, remain in the lower levels of your home during the windstorm, and avoid windows.”
Impacts could mount depending on how long the low-pressure center spins along the coast, delivering prolonged high winds and rain.
Models are starting to converge on a bomb cyclone making landfall near the Bay Area tomorrow as California gets hit by yet another significant storm.— Colin McCarthy (@US_Stormwatch) March 21, 2023
Widespread heavy rain, flooding, damaging winds, and heavy mountain snowfall is likely, especially for Central CA/SoCal. pic.twitter.com/7RfNTnD3u6
Atmospheric river for Southern California
The developing storm will help to pull tropical moisture toward Southern California, where an atmospheric river — a jet of subtropical moisture sourced from near Hawaii — could drop up to 8 inches of rain on mountain slopes and up to 6 feet of snow at the highest elevations.
“Numerous flash floods are likely” as heavy rain along the coast and valleys combines with runoff from steep mountain slopes, according to the National Weather Service’s Weather Prediction Center. A level 3 out of 4 risk of excessive rain is forecast for a populated swath along the Southern California coast.
There is now a 🔴MODERATE risk of Excessive Rainfall for part of Southern California on Tuesday 3/21 where the heaviest rainfall is expected with the atmospheric river event. Use extra caution if traveling in this area tomorrow! https://t.co/ObZSLmDrPZ— NWS Weather Prediction Center (@NWSWPC) March 20, 2023
High rain rates from the “very dynamic pattern” will increase the threat of mudslides and debris flows along the coast, while thunderstorms could intensify that risk, the Weather Service wrote. Last week, a landslide in the Orange County seaside town of San Clemente sent debris tumbling to the beach below, nearly claiming several properties. More rain could further destabilize coastal bluffs, many of which are lined with homes.
In the San Bernardino Mountains east of Los Angeles, communities are just beginning to emerge from recent historic and deadly snowstorms. Alex Tardy, a meteorologist with the National Weather Service in San Diego, said those areas will receive about half of the snow they saw between Feb. 22 and March 1 — up to an additional 5 feet. The Weather Service warned of a significant threat of avalanches.
Flooding escalates in the San Joaquin Valley
Runoff from recent storms continues to plague the agricultural valleys west of the Sierra Nevada.
During an online update Monday, UCLA climate scientist Daniel Swain said the greatest flood concerns remain in the southern San Joaquin Valley, even though that won’t be the zone of heaviest precipitation Tuesday.
“Any precipitation at this point is going to be a problem because we’re getting sunny-day flooding now from all of the water flowing out of the southern Sierra,” he said.
The Tulare County sheriff issued evacuation orders Sunday for the towns of Alpaugh and Allensworth. SJV Water, an independent, nonprofit news site serving the San Joaquin Valley, reported that a levee was intentionally cut, sending floodwaters into the low-income rural area Saturday, as agricultural interests in the basin attempt to avoid what seems like inevitable flooding this spring.
Farther north, the San Joaquin River is hovering at “danger stage,” meaning that “flows are greater than the design capacity of the levees and there’s a threat to life and property,” according to the California Department of Water Resources. While the river is not in danger of overflowing, the surging water could stress the levee system.
“The longer the duration of this very high flow period, the longer these forecast points spend above danger stage, the higher likelihood there is that something goes wrong,” Swain said.
Meanwhile, snow keeps piling up in the Sierra Nevada, up to 5 more feet with this storm, with snowmelt season yet to begin.
“We’re not even really seeing the effects of the melted snowpack yet,” Swain said. “Ninety-eight percent of what’s up there is still up there, stored in the snow water.”
Bryan Pietsch contributed to this report.