Much of Southern California remained at risk of flooding on Christmas Day as heavy rains that have triggered flash floods and evacuations since the weekend showed little sign of easing.

The National Weather Service placed portions of southwest California, including Los Angeles, under a flood warning until 6 a.m. local time on Thursday, meaning conditions could be life-threatening. Forecasters said Los Angeles could receive up to half an inch of additional rain on Christmas Day. Parts of Ventura and Santa Barbara Counties could see as much as 2.5 inches of rain.

A flood watch remains in effect through Friday afternoon for Los Angeles, Ventura, Santa Barbara and San Luis Obispo Counties, and Santa Barbara Airport was closed temporarily on Thursday.

“All commercial flights and fixed-wing aircraft are currently grounded,” the airport said on social media.

The rains are being driven by a series of atmospheric rivers, large plumes of moisture flowing over California. Forecasters warned that the rains could trigger mudslides and debris flows, particularly in areas burned by wildfires earlier this year, where scorched soil is less able to absorb rainwater.

On Wednesday, fire crews were evacuating homes in San Bernardino County, and rushing debris flows shut down Highway 2 in the area east of Los Angeles. Heavy rains also flooded portions of Interstate 5 in the San Fernando Valley in Los Angeles, prompting officials to close northbound lanes. There were water rescues in at least two counties, Los Angeles and San Bernardino, and reports of mudslides and rock slides in the Los Angeles area.

The rain has caused particular hardship to the thousands of people in the region who are homeless.

“Pretty horrendous,” said Matthew Alexander Consolato, who left a shelter because he said it was too crowded and was trying to stay dry on Wednesday under a bus stop awning. “It kind of caught me by surprise. I didn’t think it would be this bad.”

In Wrightwood, a ski resort town of about 5,000 in the mountains of San Bernardino County, crews were going door to door to evacuate people from homes and vehicles, as the rain and debris flow overwhelmed roadways. The entire town was under a shelter-in-place warning, and Christopher Prater, a spokesman for the San Bernardino County Fire Department, said the agency had received dozens of rescue calls starting at 7:30 a.m.

The Weather Service forecast as much as 10 inches of rain there overnight.

By Wednesday evening, the rain had subsided in some areas, including parts of Los Angeles. As the sun shined over Santa Monica beach, people began to emerge from their apartments and hotel rooms. One young boy said he had ventured into the water for a swim.

Wednesday’s rain is the first of what forecasters said would be two main waves of downpour. Even though it has died down, Joe Sirard, a meteorologist with the Weather Service, said the precipitation would probably intensify again on Thursday night, bringing a couple more inches.

Many residents spent Christmas Eve in evacuation centers after authorities ordered mandatory evacuations in parts of Orange, Ventura, Los Angeles and San Bernardino Counties. On Thursday, Gov. Gavin Newsom declared a state of emergency in those counties, as well as Riverside and Shasta counties.

The San Bernardino County Fire Department said a child with a minor injury was taken to a hospital, the only injury reported in the county overnight. The Weather Service forecast as much as 10 inches of rain overnight in Wrightwood, a ski resort town in San Bernardino, triggering debris flows and disrupting the town’s peak season.

Matthew Mpoke Bigg contributed reporting