(CNN) — The next wave of torrential downpour is expected to hit Southern California by daybreak Friday as northern parts of the state brace for whether the Oroville Dam can withstand more rain this weekend.
Flash flood watches affecting more than 21 million people are in effect for Los Angeles, Ventura and Southern Santa Barbara counties, the National Weather Service said.
“The storm looks to be the strongest storm to hit southwest California this season. It is likely the strongest within the last six years and possibly even as far back as December 2004 or January 1995,” according to the National Weather Service.
Parts of the south-facing foothills and coastal mountain slopes could see up to 10 inches of rain through the weekend, meteorologists said.
“The incessant heavy rains, expected to reach up to 1 inch per hour, will dramatically increase the threat of urban flooding, as well as mud and debris flows from recent burn areas near mountainous terrain,” CNN meteorologist Derek VanDam said. “The highest elevations of Los Angeles and Ventura counties will likely experience 1 to 2 feet of snow with near hurricane force winds near the tops of mountain overpasses.”
Oroville Dam ‘is holding up’
While Southern Californians will be seeing the bulk of their rain before the end of the weekend, those in Northern California can expect heavier rain late Sunday and into Monday.
The weather brings more worries for communities south of the heavily damaged Oroville Dam. Rainfall continuing over the next seven days could total more than 12 inches.
On Wednesday, although the threat level has been reduced for residents living near the dam, Butte County officials advised those returning to their homes to “remain vigilant and prepared.”
Helicopters dropped rocks and concrete into the erosion area every minute and a half late Wednesday night in “a manner that kind of glues the areas of concern,” Acting Director of the California Department of Water Resources, Bill Croyle said.
Erosion was a concern in three sites, officials said Thursday. The first site has been completely filled, while the other two sites are 25% and 69% filled.
“The dam is holding up, it’s structurally sound,” California Fire Department spokesman, Jay Smith told CNN.
The Oroville Lake level has dipped to 869 feet, 32 feet below the top and that “everything is going well,” Smith said but added “we’re cautiously optimistic.”
“The next week of storms could potentially bring hundreds of billions of gallons of water into Lake Oroville, adding pressure back onto the already compromised structure of the concrete spillway and emergency spillway next to Oroville Dam,” said Dave Hennen, CNN’s senior meteorologist.
Authorities ordered mandatory evacuations over concerns that an emergency spillway at the dam could fail and threaten nearby communities.
On Tuesday, officials downgraded the evacuation order to a warning, allowing 188,000 evacuees from Butte, Sutter and Yuba counties to return home.
But evacuation warnings remain in place for Oroville and Thermalito, as well as, “all low-lying areas around the Feather River, which includes Gridley, Biggs, Yuba City, Loma Rica, and anywhere south of Butte County along the River,” according to the Butte County Sheriff’s Officer website.
All schools in Butte County will be closed through Friday, except those in Chico and Paradise, officials said.