Preparations underway ahead of back-to-back atmospheric rivers rolling into Southern California

Intense rain is expected to batter Southern California in coming days with back-to-back atmospheric rivers on the way, prompting a warning from federal officials who urge residents to be prepared ahead of the powerful storms.

According to the National Weather Service, the strongest rainfall is expected just in time for the Thursday morning rush hour. Anywhere between one to two inches is expected on Thursday in coastal and valley regions, while foothills and mountain areas can expect between three and five inches. 

"While the rain will be quite heavy at times, due to the shorter duration of the event impacts are expected to be relatively minor with mainly typical roadway ponding of water and slick driving conditions and flood advisories should be sufficient to handle this event," according to NWS officials. "However, with a 10-20 percent chance of thunderstorms, rain rates in very localized areas could be high enough to require a flash flood warning."

Weather officials have issued a flood watch for most of Orange County starting early Thursday and lasting through Friday, while a high surf advisory is also in place for the entire Los Angeles and Orange County coastline through Saturday morning. Waves reaching up to 12 feet could hit LA beaches. 

A winter storm warning is also in effect for the eastern San Gabriel Mountains, with as much as 18 inches of snow expected in regions above 7,000 feet. 

The second round of wet weather isn't expected until Sunday. 

The California Governor's Office of Emergency Services activated its operations center and positioned personnel and equipment in areas most at risk from the weather.

Brian Ferguson, Cal OES deputy director of crisis communications, characterized the situation as "a significant threat to the safety of Californians" with concerns for impact over 10 to 14 days from the Oregon line to San Diego and from the coast up into the mountains.

"This really is a broad sweep of California that's going to see threats over the coming week," Ferguson said.

Much of the first storm's heaviest rain and mountain snow was expected to arrive late Wednesday and overnight into Thursday.

"The main impact is going to be runoff from heavy rainfall that is probably going to result in flooding of some waterways," said Robert Hart, a meteorologist with the National Weather Service's western region.

The Federal Emergency Management Agency is also monitoring the incoming storms, warning residents to be on their toes and keep an eye on the evolving forecast. 

"While the current atmospheric river is on track with potential heavy rainfall in southern California, which could result in some flooding and landslides, FEMA is encouraging every community to get prepared this winter season as other storms are expected to follow," said a statement. 

Residents are urged to sign up for emergency alerts in their area, pay attention to the roads for potential flooding while driving and avoid downed power lines. Additionally, they say that people should create emergency kits in the case of severe storms that cut off power and access to public roads. 

With the one-two punch of heavy storms predicted to sweep through, the Los Angeles Homeless Services Authority announced that they will be opening additional shelters throughout the area to provide more space for people to escape the rain. 

The Augmented Winter Shelter Program was activated on Wednesday as the rain is expected to begin overnight, as usual during cold or wet weather conditions. LAHSA's Winter Shelter Program traditionally operates between Nov. 1 and March 31, the wettest part of the year. 

More space will be implemented through the use of motel vouchers for residents who take advantage of the augmented program.

"The Los Angeles region has been cold recently, but the addition of rain this week could make conditions especially dangerous for anyone living on the streets," said LAHSA CEO Va Lecia Adams Kellum. "Thanks to the support of our partners at the city and county, LAHSA is glad to offer this resource to keep people safe and dry during storms like we expect this week."

More information is available by calling 211 or 800-548-6047.

LAHSA says that more than 5,400 took advantage of the program in 2023, a 652% increase over the previous year.