As of late Sunday morning (local time), the storm was dumping between 1.3 and 1.9cm of rain per hour in parts of California and was set to continue for several hours, according to meteorologist Kenny James of the National Weather Service's Weather Prediction Center in Maryland.
In Oregon and parts of Washington State and Idaho, the storm was expected to bring ice through the evening, creating hazardous conditions.
James said the highest elevations of the Sierra Nevada mountain range in California could get up to 2m of snow by Monday.
In northern California, officials reported power outages and roads closed due to overflowing rivers. An "extreme" avalanche warning was issued for the Sierra Nevada range, which runs through the central and eastern part of the state.
The National Park Service said all roads to the valley in Yosemite National Park were closed because of expected flooding.
In Oregon, a men's basketball game between Gonzaga University and the University of Portland scheduled for Saturday night was postponed indefinitely, while the National Basketball Association's Portland Trailblazers postponed a home game against the Detroit Pistons from Saturday night to Sunday evening.
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, located north of Los Angeles.
The flight, which will be SpaceX's first since a launch pad explosion in Florida four months ago, is now targeted for 9:54am next Saturday.
The storm is drawing its strength from the interaction between an "atmospheric river," a long plume of water vapor flowing from the tropics toward the West Coast, and a low-pressure area near Oregon, according to James.
After years of drought, the storm is the latest incident in a strong wet season for California that began in the fall. Another front is expected to move into the area on Tuesday.
Meanwhile, the eastern United States experienced cold temperatures on Sunday, the day after a massive storm dumped snow from Georgia to Massachusetts, causing hundreds of car accidents and knocking out power to thousands of people.
