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(Adds conditions beginning to ease, severe weather headed to Oklahoma, conditions in Northern California)
SACRAMENTO, Calif., Jan 12 (Reuters) - Heavy rain and snow began to ease on Thursday in the U.S. West, but flooding risks remained high in parts of California after overflowing rivers forced thousands to evacuate in recent days.
Wet weather continued throughout the day even as the power of the storms eased, with as much as a foot of snow (30 cm) anticipated in parts of the region by Friday, the National Weather Service said.
A weather pattern meteorologists call an "atmospheric river" - a dense plume of moisture flowing from the tropical Pacific and into California and the West - has brought heavy snows and rains to the region over the past few weeks.
"The state is pretty much drenched," said Dave Rizzardo, a hydrologist with California's Department of Water Resources.
Severe weather was also making its way to Oklahoma on Thursday, and Governor Mary Fallin declared a state of emergency in anticipation of the storms.
"Because the weather event is expected to include dangerous road conditions and power outages, emergency management authorities recommended issuing this declaration before the full brunt of the storms arrive," her office said in a statement.
In Northern California, the latest round of extreme precipitation forced thousands of residents to seek higher ground in recent days.
While rains slowed on Thursday, rivers remained swollen and hydrologists worked to manage the state's complex flood control system in the face of high afternoon tides expected to raise water levels in the massive Sacramento-San Joaquin River Delta near San Francisco.
Mudslides closed State Route 49 in the Sierra Nevada on Thursday, and flooding blocked roads in the state's San Joaquin Valley agricultural breadbasket, the California Highway Patrol said.
On Wednesday, residents in Wilton, a community of more than 5,000 people near California's state capital, Sacramento, were advised to evacuate their homes because of anticipated flooding along the Cosumnes River.
In wine-growing Sonoma County, north of San Francisco, authorities asked residents of 650 homes in Guerneville to evacuate as the Russian River has flooded parts of the community of less than 5,000 people.
To the north, schools in Portland, Oregon, remained closed on Thursday as the city of more than 600,000 dug out from a blizzard that delivered roughly a foot of snow.
About 6,700 utility customers remained without power early on Thursday in the Portland area after the storm knocked out electricity for more than 63,000 customers, the utility said.
Reporting by Sharon Bernstein in Sacramento, Calif., and Brendan O'Brien in Milwaukee; Editing by Peter Cooney