California Braces for Another ‘Significant’ Atmospheric River

Another powerful storm will hit California starting Monday night, compounding the effects residents have already felt from a similar storm that led to turbulent flooding last week and forced thousands of people to evacuate their homes.

Forecasters with the National Weather Service in Los Angeles are calling the latest storm, which is expected to last through Wednesday, a “significant atmospheric river event” and are fairly confident that it will bring widespread, heavy rainfall.

“In fact, it will be a big surprise if it does not do so,” the Weather Service said.

After enduring a historically long stretch of drought, California is now experiencing one of its most ferocious winters on record. Total snowpack and rainfall in many parts of the state have reached levels not seen in decades. Months of precipitation have soaked hillsides, filled creeks and rivers, and covered the Sierra Nevada and other mountain ranges in the state with roof-level snow.

That means that each new storm, like the one arriving Monday, creates a compounding situation that is likely to make things worse. All of this week’s rain will fall on ground that is still fully saturated from last week’s system and earlier winter storms.

“Given antecedent conditions, expect impacts from this A.R. to eclipse and exceed the previous one, with potentially large-scale and long-lasting flooding impacts,” forecasters with the Bay Area office of the National Weather Service wrote on Monday.

In some areas, residents are still coming to grips with the damage caused by last week’s storms. The small farm community of Pajaro remains flooded after a levee broke early Saturday along the Pajaro River, between Santa Cruz and Monterey counties.

The incoming atmospheric river — a narrow conveyor belt of water vapor in the sky — arrives first in Northern California late Monday and then will slowly spread into Southern California early Tuesday, bringing another two to five inches of rain along the coast and in valleys, and up to 8 inches in the mountains. At elevations above 6,500 to 8,000 feet, the Sierra Nevada could receive another foot of snow or more, increasing the weight bearing down on roofs and the risk of roof collapses and roof avalanches, when large frozen sheets of snow and ice suddenly slide off buildings.

The Los Angeles region has already received at least twice the average amount of rainfall for this point in the year, according to Mike Wofford, a senior meteorologist for the National Weather Service office in Los Angeles.

Anything that falls from the sky now is less likely than usual to soak into the ground, and will run off instead, increasing the risk of flash floods. From there, water and snowmelt will flow into creeks and rivers that are also rising, increasing the risk that they will flood. The combination means that water-related impact from the storm is a sure bet, forecasters from the Bay Area said on Monday morning.

Patrick Ayd, a meteorologist with the National Weather Service in the Bay Area, compared the ground to a kitchen sponge that is completely full of water. It cannot absorb any more moisture, he said, so any more water that arrives will just run off.

Waterways like the Salinas River, in the Central Coast region, have had a little bit of time to recede after the last storm, but are expected to begin rising again Monday as the rain starts falling.

Mr. Ayd said that residents who saw flooding during last week’s atmospheric river, like those in the Santa Cruz Mountains or in Monterey County, should expect to be flooded again.

“They’re the ones, especially, that need to have that go-bag and be able to evacuate quickly,” he said.

In mountainous regions of the state, hundreds of creeks are primed to rise quickly, Mr. Wofford said, and Santa Barbara County looks to be one of the areas that could see significant disruptions. U.S. Highway 101 is the main north-south route through the area, and it will be susceptible to closures this week because of creeks that may spill over their banks and onto the road.

Another compounding factor, especially along the Central Coast and in the Bay Area, is that thunderstorms and heavy winds are expected. Wind gusts could exceed 55 to 70 miles an hour along the immediate coastline and at elevations above 1,000 feet in the hills. The winds will spread into the Sierra Nevada, where there could be gusts over 100 miles an hour at mountaintops.

“Be prepared for the likelihood of downed trees for tonight into Tuesday night, with an increased potential for widespread and prolonged power outages,” the Weather Service said.

Mr. Ayd said that when the ground is saturated, trees are more susceptible to toppling over, especially after having been weakened by a previous storm system.