Authorities lifted mandatory evacuation orders Tuesday for communities downstream of the imperiled Oroville Dam amid a lessened flood risk, but they stressed that an incoming storm system means the danger is not over and that many residents might have to flee again.
Although the storms are expected to be far weaker than the ones that inundated Northern California last week, any additional rainfall could exacerbate the problems in the region, where more than 100,000 people were evacuated Sunday amid concerns that a damaged spillway at Oroville Dam could fail.
Officials have been lowering the level of Lake Oroville for several days, preparing for the storm. Both the main and emergency spillways are damaged, but officials are hoping to continue using the main spillway throughout the storm to keep the lake below maximum capacity. By doing that, they hope to avoid more water flowing down the emergency spillway, which was so damaged Sunday that officials feared it could collapse and cause major flooding downstream.
"All residents are advised to remain vigilant and prepared as conditions can rapidly change," the Butte County Sheriff's Office said in a statement. Residents, authorities said, should be prepared to evacuate again at a moment's notice should new problems arise.
The storm system is expected to arrive late Wednesday or early Thursday morning and could bring 2 to 4 inches of rainfall over Lake Oroville, said Tom Dang, a meteorologist with the National Weather Service in Sacramento. Another storm Friday could drop an additional 1 to 3 inches on the region and is expected to have a much greater impact on Southern California.
"There's no real sign at this stage of any let-up in the precipitation," Dang said.
The Northern California rain is also expected to be much cooler than last week's. That's good news when it comes to flood concerns. Warmer storms cause mountain snowpack to melt more quickly, sending runoff coursing into rivers, canals and reservoirs.
Last week's storm was "very warm," with snow levels as high as 8,000 feet and higher, Dang said. This week, snow levels are forecast at 5,000 to 6,000 feet, which is much more typical for a storm this time of year, he said.
In the five days from Feb. 6 to Friday, Oroville received more than 6 inches of rain, according to the National Weather Service. The surrounding mountains and foothills received up to 24 inches of rain and snow in the same time period.
The storm's runoff sent water into the Oroville reservoir at an average rate of 115,260 cubic feet per second, data show. The lake's water level climbed 50 feet in five days.
In addition to the crisis at Oroville Dam, several levees throughout the region have seen structural damage, adding to the flood threat, Dang said. Many reservoirs in Northern California are having to release large amounts of water, causing rivers to rise.
Portions of the Sacramento and San Joaquin rivers are at or near flood stage, he said. The Feather River, downstream from Lake Oroville, has seen flooding for several days.
Although this week's storms are expected to be smaller, they're "certainly impactful," Dang said.
"The storms last week have really left Northern California in a vulnerable state right now, and any amount of rainfall isn't helpful at this stage."
Forecasters say there is a potential for another series of strong storms in Northern California early next week that could bring additional flooding, though they are less confident about the specifics because it is still early, Dang said.
Christina Widener, 51, evacuated with her husband and her 94-year-old grandmother, who suffers from dementia. They drove to Red Bluff, about 60 miles away, before finding a motel room.
"I think it made sense at the time," she said of the hasty evacuation order Sunday evening. "But I wish she hadn't left."
The mother of four has lived in her bungalow near the river's edge for 32 years, and in 1997 defied an evacuation order despite the threat of arrest.
"We didn't leave then and nothing happened," she said, grabbing boots and a kitty container out of the back of her Ford SUV.
Widener had nothing but praise for the hospitality of Red Bluff — waitresses offered to pay for their meals, strangers invited them to stay in their homes. Someone even offered to do their laundry.
"They were awesome," she said.
Although some evacuees drove far from Oroville, others made arrangements to stay with family or friends who reside in the foothills behind the lake — safe ground should the dam fail.
"As long as you are up above, you should be good, right?" said Jerry Smith, a 26-year-old solar installer. He finished loading frozen pizzas and bottled waters at a newly reopened grocery store, and was anxious to move on with his wife.
"I'm sorry, but we've got to go. We haven't been home yet."
Was he anxious?
"No, we should be OK."
In Southern California, meanwhile, the storm system expected to arrive Friday could provide a walloping, with possible flash flooding, mudslides and rock slides.
"The Friday storm in particular could in fact become the strongest of the season in the Los Angeles region," said UCLA climate scientist Daniel Swain.
A slow-moving storm is expected to debut into the Southland with some light rain Thursday night or Friday morning, but is expected to dump large amounts of rain in a short time frame during its peak Friday afternoon and evening, said Ryan Kittell, a forecaster with the National Weather Service in Oxnard.
Rainfall totals for the Los Angeles metropolitan area are predicted to be between 2 to 4 inches, with 6 to 8 inches expected in the mountains and foothills, he said.
"The raw numbers don't look that scary, but if we get the bulk of that coming over a small period, that will cause a lot of issues," Kittell said. Much of that rain could fall within a 12-hour period Friday, and it could fall at a rate of more than an inch per hour, he said.
There is a high likelihood, Kittell said, for a "lot of roadway flooding," as well as falling trees because the storm also is expected to usher in powerful winds.