A slow-moving Pacific storm prompted evacuations early Thursday in cities and beach towns along California's southern coast as streets were submerged beneath floodwaters.

The cities of Oxnard and Port Hueneme, both in Ventura County, east of Los Angeles, issued evacuation orders for some coastal areas and at least one shelter was opened for residents fleeing their homes. The Oxnard Fire Department has executed water rescues and, along with the police department, has asked residents to stay off the roads due to widespread flooding. There have not been reports of any injuries or deaths. A brief tornado warning was issued for Ventura County overnight.

In a 7 a.m. advisory, National Weather Service meteorologists said at least 2-6 inches of rain had fallen over the Ventura County area. An additional 1-4 inches was expected to fall by the afternoon. Meteorologists anticipate rock and mudslide activity on and below canyon and mountain roads, leading to “significant travel delays.”

“It was a very, very significant amount of rainfall being produced in a short amount of time,” Ariel Cohen, a meteorologist in charge at the National Weather Service's office in Oxnard, told the Ventura County Star, part of the USA TODAY Network. Rainfall rates – which can cause flooding at 1 inch per hour – reached as high as 3 inches per hour early Thursday, he said.

Developments:

◾ The Oxnard Union High School District, which has schools across Ventura County, remained open despite the storm conditions Thursday morning. Oxnard and Hueneme elementary and middle schools are on winter break, according to the districts' websites.

◾ In Port Hueneme, a beach city in Ventura County, officials have issued evacuation orders for several cities and opened a shelter in the gymnasium of Oxnard College. "The City of Oxnard is requesting all residents remain off city streets for the next several hours until flood water recedes," read a statement from the city. "Many streets and intersections are heavily impacted by standing water, causing vehicles to stall and become trapped in flood water. Please remain indoors until the situation resolves."

◾ About 20 residents of a senior community in Port Hueneme were evacuated early Thursday, county Firefighter Andy VanSciver told the Ventura County Star. About 10 were rescued in a tactical rescue vehicle.

◾ Oxnard Fire Department reported cars stuck in flooded roadways and several water rescues. There have not been any reports of injuries or deaths in Ventura County. "Many of the streets and intersections are heavily impacted by flooding," read a post on X by the fire department. "Please stay off the city streets for the next several hours until the water recedes. Standing water can cause vehicles to stall and may become trapped."

Santa Barbara under flash flood warning

The National Weather Service issued a flash food warning for south Santa Barbara County that'll remain in effect until 4 p.m. local time. Meteorologists urged the estimated 233,800 people in the area to move to higher ground and avoid driving through flood waters.

"This means that flash flooding is occurring or imminent," read a statement from the Santa Barbara Police Department. "A flash flood is an extremely dangerous, sudden, violent, and fast-moving wall of water. ... If you are in one of these areas, go to the innermost room of your home or to higher ground such as a second floor."

Weather in Los Angeles to pick up through the evening

Los Angeles will be hit the hardest by rain and strong winds beginning Thursday afternoon and continuing into the evening, according to AccuWeather. Through the night and into Friday, the area between San Diego and Palm Springs will be drenched, raising concerns over potential flooding.

Coastal areas in Los Angeles and San Diego are expected to receive 2-5 inches of rain, while mountainous areas will get around 5-10 inches, according to the NWS. Wind gusts around 25-40 mph are expected, but some could reach up to 55 mph.

This will be the area's first El-Niño influenced storm, one that's fueled by an atmospheric river of moisture, the National Weather Service said. It's expected to be the biggest rainstorm for the region since Hurricane Hilary dumped up to half a foot of rain in August, AccuWeather forecasters said.

In all, some 22 million people live where a flood watch is in effect, including most of the Los Angeles and San Diego metro areas.

Storm first caused flooding throughout Northern California, tornado in Oroville

The same storm has been drenching areas in Northern and Central California into midweek while remaining largely offshore over the Pacific. Parts of Sacramento flooded, with entire roadways under water, as the storm moved southeast Tuesday and Wednesday.

Oroville, a city about 65 miles north of Sacramento, experienced a tornado on Tuesday with winds of up to 90 mph and which lasted two minutes, according to the NWS. While no one was hurt or injured, the tornado damaged at least one building and downed trees.

Contributing: Doyle Rice, USA TODAY; Associated Press