By Nouran Salahieh, Holly Yan and Haley Brink, CNN
(CNN) -- Much of California can't soak up another drop of rain. Yet the state is getting pummeled again with torrential downpours and ferocious winds, causing power outages and treacherous travel conditions.
More than 34 million Californians were under a flood watch Monday -- about 90% of the state's population and 10% of the US population.
Parts of the central California coast got walloped with 1 to 1.25 inches of rainfall per hour, the Weather Prediction Center said. Extensive rainfall there Monday triggered significant flooding, mudslides, debris flows and closed roadways.
Widespread rainfall totals of 3 to 6 inches have been observed from just south of San Francisco to just north of Los Angeles. Isolated amounts of 6 to more than 10 inches have been observed in the higher terrain near the coast.
As the rain shifted slowly to the south Monday toward Los Angeles, the National Weather Service there warned of the risk of flooding, debris flow in land scarred by recent wildfires and an increased risk of rock and mudslides in mountains and on canyon roads.
A flash flood warning is in effect Monday evening for downtown Los Angeles until midnight local time, an area including more than 7 million people.
And hurricane-force wind gusts topping 74 mph thrashed states across the western US. More than 37 million people were under wind alerts Monday in California, Nevada, Oregon, Washington, Utah, Arizona and Wyoming.
A 132-mph wind gust lashed Oroville, California. Residents in Washoe City, Nevada, were hit with a 98-mph gust, the Weather Prediction Center said.
California is now extremely vulnerable to flooding because much of the state has been scarred by historic drought or devastating wildfires -- meaning the land can't soak up much rainfall.
And after an onslaught of storms since late December led to deadly flooding, Gov. Gavin Newsom warned Sunday: "We expect to see the worst of it still in front of us."
'A high-impact event'
Two bouts of major rainfall are expected to hammer the West Coast over the next few days -- without much of a break between events for the water to recede.
The system is part of an atmospheric river -- a long, narrow region in the atmosphere that can transport moisture thousands of miles, like a fire hose in the sky.
The atmospheric river slamming California on Monday could result in a 1-in-50 year or 1-in-100 year rainfall event near Fresno, the Weather Prediction Center said.
A moderate risk -- level 3 of 4 -- of excessive rainfall covers over 26 million people in California, including in San Francisco, Sacramento, Los Angeles and Fresno, where rain could fall at 1 inch per hour.
The San Lorenzo River in Santa Cruz County has risen 14 feet in just over four hours and is in major flood stage. Parts of the county will experience "widespread flooding at shallow depths," and the city of Santa Cruz will have serious flooding, according to data from the National Oceanic and Atmospheric Administration and US Geological Survey.
The threat will shift further south Tuesday, with a level 3 of 4 risk centered over Los Angeles.
"While some of the forecast rain totals are impressive alone, it is important to note that what really sets this event apart are the antecedent conditions," the National Weather Service office in San Francisco said.
"Multiple systems over the past week have saturated soil, increased flow in rivers and streams, and truly set the stage for this to become a high impact event."
'Imminent' flooding spurs evacuation orders
In Sacramento County, officials warned "flooding is imminent" and issued evacuation orders for the Wilton community near the Cosumnes River before roads become impassable.
Wilton residents also had to evacuate during last week's storm, when exit routes flooded quickly, officials said.
Residents in all areas of Montecito, the city of Santa Barbara and parts of Carpinteria and Summerland are being ordered to evacuate immediately due to the threat of the ongoing storm, the Santa Barbara County Incident Management Team said on Monday evening.
"LEAVE NOW! This is a rapidly evolving situation," the team said in a release.
Montecito is a haven for the rich and famous, including Prince Harry and his wife, Meghan, Duchess of Sussex; Oprah Winfrey and Ellen Degeneres. Monday marks exactly five years since heavy rains in the area caused deadly mud- and landslides.
Santa Barbara County authorities are advising residents to "be prepared to sustain yourself and your household for multiple days if you choose not to evacuate, as you may not be able to leave the area and emergency responders may not be able to access your property in the event of road damage, flooding, or a debris flow."
Newsom on Sunday asked the White House for an emergency declaration to support response and recovery efforts.
"We are in the middle of a deadly barrage of winter storms -- and California is using every resource at its disposal to protect lives and limit damage," Newsom said in a statement. "We are taking the threat from these storms seriously, and want to make sure that Californians stay vigilant as more storms head our way."
San Francisco Mayor London Breed on Monday issued a Local Proclamation of Emergency due to the ongoing series of winter storms that began New Year's Eve, according to a news release from his office.
Floods kill more than any other natural disaster
This storm system arrives on the heels of a powerful cyclone that flooded roads, toppled trees and knocked out power last week to much of California. Earlier, a New Year's weekend storm system produced deadly flooding.
At least 12 Californians have died from "storm-related impacts" such as flooding since late December, the governor's office said.
In San Luis Obispo County, dive teams from the sheriff's office and Cal Fire rescuers were searching Monday for a 5-year-old child reported to have been swept away in flood waters near the Salinas River in San Miguel.
"Floods kill more individuals than any other natural disaster," California Emergency Services Director Nancy Ward said Sunday. "We've already had more deaths in this flood storm since December 31 than we had in the last two fire seasons of the highest fire acreage burned in California."
Flood-related deaths can happen when drivers attempt to cross standing water.
"Just a foot of water and your car's floating. Half a foot of water, you're off your feet. Half foot of water, you're losing control of your vehicle," Newsom said.
"We're seeing people go around these detours because they don't see any obstacles -- they think everything is fine, and putting their lives at risk or putting first responders lives at risk."
For anyone who doesn't need to travel during the peak of this storm, "please don't," California Secretary of Natural Resources Wade Crowfoot said. "Be prepared for power outages and other interruptions. Have those flashlights, the candles, batteries, charge cell phones at the ready."
Already, flooded roads, toppled trees and downed power lines are making travel difficult, California Highway Patrol said. Some fallen trees crushed cars and homes over the weekend. On Monday, portions of the Pacific Coast Highway -- US 101, a major north-south highway, were closed.
California is experiencing "weather whiplash," going from intense drought conditions to now contending with its fifth atmospheric river, Newsom said.
Much of the state has already seen 5 to 8 inches of rain over the last week. Two to 4 more inches of rain are expected across the coasts and valleys -- and even more in mountains and foothills through Tuesday.
Rising from swelling rivers could spill over and inundate communities.
The rainfall over the weekend brought renewed flood concerns for streams, creeks and rivers. The Colgan Creek, Berryessa Creek, Mark West Creek, Green Valley Creek and the Cosumnes River all have gauges that are either above flood stage or expected to be in the next few days.
"The cumulative effect of successive heavy rainfall events will lead to additional instances of flooding. This includes rapid water rises, mudslides, and the potential for major river flooding," the National Weather Service said Monday.
The moisture is expected to sink southward Monday night, making flooding "increasing likely" over the Southern California coastal ranges Tuesday, the weather service said. Fierce winds are expected to accompany the storm as it pushes inland.
"Valley areas will likely see gusts as high as 45-50 mph, with gusts greater than 60 mph possible in wind prone areas," the National Weather Service in Reno said. The Sierra Ridge could receive peak gusts between 130 to 150 mph Monday.
8.3 feet of snow ... and counting
For those at higher elevations, intense snow and ferocious winds will be the biggest concerns.
Parts of the higher elevations in the Sierra Nevada have gotten more than 100" -- or 8.3 feet -- of snow in just the past few weeks, the Weather Prediction Center said.
Now, another 6 feet of snow is expected in some parts of the Sierra.
As the storm pushes inland, more than 5 feet of snow could fall along the Sierra Crest west of Lake Tahoe, the weather service said.
The heavy snow and strong winds could lead to near whiteout conditions on roads.