Millions across Southern California are under flood threat that could snarl travel ahead of the holiday weekend

(CNN) — An atmospheric river continues to unload a deluge of rainfall across Southern California Thursday, leaving roadways flooded and prompting evacuations ahead of the holiday weekend.

The atmospheric river poured more than 4 inches of rain over parts of the region from late Tuesday to early Thursday. The heaviest rainfall will continue into Thursday night with rainfall rates over 2 inches per hour, leading to flash flooding, according to the National Weather Service.

Heavy rain triggered flooding in Ventura County early Thursday morning, where parts of the county are under an evacuation warning until noon Friday, the Ventura County Sheriff’s Office said.

More than 2.5 inches of rain fell in just a few hours early Thursday morning in Oxnard, the county’s largest city. This significant deluge marked at least a 1-in-500-year rainfall event for the city.

At least 60 homes were affected by floodwaters in Ventura County Thursday, officials said.

The county’s fire department received 275 calls within five hours overnight – well above the department’s average of 190 received calls in a 24-hour-period, Ventura County Fire Department spokesperson Andy VanSciver said in a video statement on X.

Crews with the department had to prioritize the calls based on severity, and at one point had 90 calls pending, VanSciver said.

Roadways were flooded and left some vehicles trapped, which called for 12 swift water rescues, he added.

To the west, intense rainfall also left some roadways underwater in Santa Barbara. Multiple ramps leading to the area’s Highway 101 were closed late Thursday morning due to flooding, according to the California Department of Transportation.

Across most of Southern California, rainfall totals of 2 to 5 inches are likely, with another 5 to 10 inches of rain possible on and below south-facing foothills and coastal slopes, according to the weather service.

Flood watches are in effect through Thursday evening for parts of the Los Angeles area, San Diego, Santa Barbara and Palm Springs. In addition to the ongoing flooding in Santa Barbara, urban flash flooding is also possible in lower elevation areas closer into the Los Angeles basin.

“Some of this flooding is expected to be significant and potentially life-threatening in nature, with extensive roadway flooding, rock and mudslides, debris flows near recent burn scars, and significant creek flooding all a possibility … into Thursday,” the weather service warned.

A moderate risk for excessive rainfall – a Level 3 of 4 – is also in effect Thursday for parts of Southern California, including Santa Barbara and the Los Angeles metro area.

A few rumbles of thunder are also possible within the swath of rain across Southern California on Thursday, but storms are unlikely to turn severe.

Earlier this week, a small tornado did briefly hit part of Northern California – a relatively infrequent event for the state this time of year.

An EF-1 tornado was reported Tuesday evening in Oroville, about 70 miles north of Sacramento, causing tree and minor structure damage, according to the National Weather Service in Sacramento.

The tornado was on the ground from 5:40 to 5:42 p.m. PT, and no injuries or deaths were reported. California gets about one December tornado on average, according to the Storm Prediction Center.

Extreme weather also has battered the opposite coast this week as torrential rain and devastating flooding tormented parts of the Northeast.

More than 130,000 power customers in Maine have suffered without power since Monday as temperatures plunge near or below freezing in much of the state.

“The concern today is colder temperatures presenting a challenge for those who have been without power for days,” Vanessa Corson, public information officer for the Maine Emergency Management Agency, said in a statement to CNN.

Periods of stronger wind gusts continue to impact Maine on Thursday and “may slow or set back” power restoration efforts, Corson added.

Dozens of roads throughout the state remain closed or partially closed due to downed trees and power lines from the early-week storm.

This story has been updated with additional information.

CNN’s Holly Yan and Maria Sole Campinoti contributed to this report.

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