A major storm has swept across the northwestern United States, battering the region with strong winds and rain, causing widespread power outages, closing schools and downing trees that killed at least two people.

The Weather Prediction Center issued excessive rainfall risks through Friday and hurricane-force wind warnings were in effect as the strongest atmospheric river – a large plume of moisture – that California and the Pacific Northwest has seen this season overwhelmed the region. The storm system that hit is considered a "bomb cyclone", which occurs when a cyclone intensifies rapidly.

In California, the weather service extended a flood watch into Saturday for areas north of San Francisco. Up to 16 inches of rain (40cm) of rain was forecast in southwestern Oregon through Friday, and more than 10 inches (25cm) in northern California and down into the central coast. Dangerous flash flooding, rock slides and debris flows were possible, officials warned.

A winter storm watch was in place for the northern Sierra Nevada above 3,500 feet (1,066 meters), where 15 inches (28cm) of snow was possible over two days. Wind gusts could top 75mph (120kph) in mountain areas, forecasters said.

Heavy, wet snow was expected to continue along the Cascades and in parts of far northern California. Forecasters warned of blizzard and whiteout conditions and near impossible travel at pass level due to accumulation rates of 2 to 3 inches (5cm to 7.6cm) per hour and wind gusts of up to 65mph (105kph).

Falling trees struck homes and littered roads across northwest Washington. In Lynnwood, Washington, a woman died when a large tree fell on a homeless encampment, South County Fire said in a statement. In Bellevue, east of Seattle, a tree fell onto a home, killing a woman, fire officials said.

Residents of Seattle suburb clean up, survey damage from wind storm. – AP

Tracy Meloy of Issaquah, Washington, felt well-prepared for the storm, with dinner prepped and lanterns ready. But then she spent the night listening to wind-whipped debris hit the outside of her home, including a particularly loud "thump" around 9pm. On Wednesday morning, she ventured outside to survey the damage to her neighborhood about 17 miles (27km) east of Seattle.

"Now that I'm standing here in front of the house, I can tell it's the tree that was across the street," Meloy said. The tree pulled down the power lines in front of her home. Limbs, leaves and other plants were strewn all over the road.

"It looks like a forest floor instead of a street," she said.

The number of power outage reports in Washington fluctuated wildly, but steadily declined to about 460,000, according to poweroutage.us. More than a dozen schools were closed in Seattle alone.

About 2,800 customers were reported to be without power in Oregon, 38,000 in California and 10,000 around Carson City and Reno, Nevada. Three Reno schools were closed and semi-trucks were prohibited on the main highway between the two cities due to high winds. 

A car crushed by a fallen tree during the "bomb cyclone" in Seattle, Washington. – AP