Treacherous flooding is about to get worse in California as another atmospheric river closes in on storm-battered residents

    (CNN) -- Californians pummeled by an onslaught of dangerous storms will get deluged again as another atmospheric river threatens more deadly flooding.

Over 18 million people in California are under flood alerts that cover much of the state, including San Francisco, Oakland, Sacramento and Fresno.

"Major wind and water impacts are expected," the National Weather Service's Bay Area office said Monday. "Now's the time to rush any preparations to completion before conditions go downhill later today."

The brunt of the storm is expected to lash Northern and central California beginning late Monday and Southern California starting Tuesday.

Already, a levee breach in Monterey County blamed on devastating weekend flooding has prompted evacuation alerts for thousands. And Monterey Bay residents could soon find themselves on a virtual island, cut off by more floodwater.

About 20 swift-water rescue teams in more than 10 counties have been positioned ahead of the looming storm, the California Governor's Office of Emergency Services said Monday, following dozens of water rescues in recent days.

This new wave of menacing weather comes on the heels of a deadly atmospheric river -- a long, narrow band of moisture that can carry saturated air thousands of miles like a fire hose. It will be the 11th to pummel the West this winter season.

From severe flooding to lengthy droughts, the intensity of water-related disasters around the world has increased over the past two decades as global temperatures climbed to record levels, new research shows.

California's new storm could exacerbate flooding and damage in some places. Already, residents in central and Northern California are crowding into shelters amid mudslides, rushing rivers, collapsed bridges and impassable roads.

Intense rainfall totals up to 8 inches are possible across parts of Northern and central California.

South of San Francisco, parts of Monterey County -- including Salinas -- could get cut off by flooding on the Salinas River, officials said. Those at risk "can and should seek shelter at one of the County Evacuation Shelters, with family or friends," Monterey County officials said.

The storm also could complicate efforts to repair a levee in Monterey County that was breached around midnight Friday by the swollen Pajaro River. Evacuation alerts for 5,000 residents could expand.

Water rushed through the more than 120-foot break and into nearby Pajaro, forcing thousands to flee as crews rescued close to 200 others, Monterey Sheriff Tina Nieto said.

Many Pajaro residents are farmworkers who may not only lose property but also the ability to earn a living for some time if the continued flooding impacts agriculture, said Luis Alejo, chair of the Monterey County board of supervisors.

"These are the folks who can least afford this type of hardship," he said.

What to expect as the storm moves in

This new wave of storms is bearing down on areas already buried by heavy snowfall from the past two weeks. Melting snowpack will also play a role in prolonging flooding over the upcoming days, forecasters say.

The National Weather Service's Storm Prediction Center issued a Level 3 out of 4 risk for excessive rain across Northern California on Monday and across portions of the central California coast and Sierra Nevada on Tuesday.

The rain is expected to start intensifying late Monday and the heavy rainfall, combined with the snowmelt, is forecast to fuel more flooding from Tuesday into Wednesday, the National Weather Service said.

Forecasters warned of "considerable flooding impacts" below 5,000-foot elevations across large portions of central California into Tuesday.

"In addition, heavy rain and snowmelt may lead to renewed (more widespread) flooding from Monday to Tuesday, particularly in low elevations and shallow and warming snowpack areas," the National Weather Service said.

Creeks and streams already bloated from previous inundations are at risk of overflowing due to more rain and snowmelt.

"Even though precipitation totals during this upcoming storm will be far from historic, the impacts will be probably greater than the precipitation totals would otherwise suggest," UCLA climate scientist Daniel Swain said.

In Southern California, rain could fall as fast as 1 inch per hour over the mountains and foothills.

Residents can expect shallow mud and debris flows in recent burn areas, falling trees, downed power lines and travel delays due to flooded roads and mudslides, the National Weather Service office in Los Angeles said.

Across 12 California counties, about 500 people are at 30 shelters -- most in hard-hit Santa Cruz County, just north of the Monterey area, the California Governor's Office of Emergency Services said Sunday.

President Joe Biden has approved a state of emergency declaration requested by Gov. Gavin Newsom, freeing money for the millions of residents hit with severe weather this year. Newsom expanded the declaration to include six additional counties Sunday, including Calaveras, Del Norte, Glenn, Kings, San Benito and San Joaquin.

'We are now an island'

Rescue crews have plucked dozens of residents from perilous floodwater as torrential rain over the past week pushed rivers and creeks over their banks.

More than 90 people have been rescued in Monterey County alone, Sheriff Tina Nieto said Saturday.

The California Highway Patrol posted dramatic video of a helicopter rescue in King City, where a driver was trapped in the Salinas River.

"The rising river washed a driver and his car away but the driver was able to escape the vehicle and get to an island in the middle of the flooded Salinas River," the highway patrol posted Sunday.

Evacuation orders remain in parts of Kern County as the swollen Kern River swept away a shed, a hot tub and full-size trees in Kernville, a resident said.

"The river is now surrounding some RVs and mobile homes. It's really unbelievable," said Danny Housh, who has worked in Kernville for 17 years and said he's never seen anything like this.

To the north, intense rainfall Friday in Santa Cruz County left about 700 Soquel residents trapped after a pipe failure collapsed the only road linking the community to the region, said Steve Wiesner, the county's assistant public works director.

"We are now an island," resident Molly Watson told CNN.

Another hard-hit area was Tulare County, where video from Springville showed devastating damage after Friday's severe flooding.

"It's quite heartbreaking," Hatti Shepard told CNN. "Many hardworking people displaced with losses of home and possessions."