Rising floodwaters have sent at least 14,000 residents fleeing inundated homes in San Jose, California, and forced the closure of a major road.
The road was closed in both directions south of San Francisco because of water across lanes and officials said there was no estimate when the key commuter link into Silicon Valley would reopen.
Authorities went door-to-door overnight ordering thousands to seek higher ground as creeks and reservoirs over-topped their banks and sent chest-deep water into neighbourhoods.
"The water started to seep in the driveway, and then it started to creep up into the front door. It kept getting worse and worse," said Alex Hilario, who walked in knee-high water to get to his car and leave.
Meanwhile, several other communities downstream from a northern California reservoir gushing water for the first time in 20 years braced for flash floods and evacuations.
The surge released from Don Pedro Dam into the Tuolumne River in the foothills east of Modesto was expected to reach over-topped levees later in the day.
Katie Whitley, who manages the Driftwood Mobile Home Park in Modesto, said residents nearest the river have been moving their trailers out since the start of the weekend.
"We're just holding our own," she said. "That's what we have to do. You just have to hope for the best. But you can expect it when you live on the river."
The water released from Don Pedro is expected to reach its peak along a stretch near Vernalis that is already at a danger stage, said Tim Daly, a spokesman for the San Joaquin County Office of Emergency Services. The water is not expected to spill over the levees but rather increase pressure on them, causing possible breaks in any weak places.
The Anderson Dam in Santa Clara County reached capacity over the weekend, and after heavy rain, it began overflowing into the Coyote Creek.
The rains were the latest produced by a series of storms generated by so-called atmospheric rivers that dump massive quantities of Pacific Ocean water on California after carrying it aloft from as far away as Hawaii.
In the Sierra Nevada range, one of the main routes to Lake Tahoe was in danger of collapsing.
The rains have saturated the once-drought stricken region and wreaked havoc for residents.
At least four people have died in the storms throughout the state in the last week.