Another compounding factor, especially along the Central Coast and in the Bay Area, is that thunderstorms and gale-force winds are expected. Wind gusts could reach 55 to 70 miles an hour or more 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.

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 latest atmospheric river — a narrow conveyor belt of water vapor in the sky — arrived first in Northern California late Monday, and then slowly spread into the central part of the state early Tuesday. It is expected to deliver rain in Los Angeles and other regions of Southern California later in the day.

All told, the storm is predicted to bring another two to five inches of rain along the coast and in valleys, and up to eight inches in the mountains. At elevations above 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 cumulative rainfall for this point in the year, according to Mike Wofford, a senior meteorologist for the 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 quickly 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 overflow. The combination means that water-related impact from the storm is a sure bet, forecasters from the Bay Area said on Monday.

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