USviewer: From 3 to 8 inches (7.6 to 20 cm) of rain is forecast for central and northern California and the Sierra Nevada mountains through early on Tuesday, and several feet (1-2 metres) of snow is likely for higher elevations, said meteorologist Andrew Orrison at the National Weather Service's Weather Prediction Center in Maryland.
"We're going to see heavy rain going into the evening and early morning," he said.
Locally heavy snow is expected in Nevada, and the northern Rocky Mountains could get several feet of snow as the system moves east over the next day or two, Orrison said.
The National Weather Service reported that almost 40 rivers or creeks in Northern California or western Nevada were flooded or threatened to top their banks.
The upper Napa River north of San Francisco was expected to cause "extreme damage to all towns along the reach," the California emergency agency said in a statement.
An agency spokesman there had been no reports of fatalities or serious damage from the bad weather.
Several California highways were closed from landslides or high water. In Washington state, high winds, ice and heavy snow shut roads and created hazardous driving conditions.
Iridium Communications said Elon Musk's SpaceX rocket company had delayed Monday's launch of a Falcon 9 rocket carrying 10 of its satellites from Vandenberg Air Force Base, north of Los Angeles. The new launch date was set for Saturday.
The storm is drawing its strength from the interaction between an "atmospheric river," a plume of water vapor flowing from the tropics toward the West Coast, and a low-pressure area near Oregon, the National Weather Service said.
After years of drought, the storm is the latest incident in a strong wet season for California that began in the autumn. Another front is expected on Tuesday.
In an encouraging sign, the U.S. Forest Service said the rain had restored moisture levels in Southern California vegetation to a seasonal normal for the first time in five years.
The eastern United States experienced cold temperatures on Sunday, the day after a massive storm dumped snow from Georgia to Massachusetts.