A major storm swept across the northwestern U.S., 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 starting Tuesday 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 of rain was forecast in southwestern Oregon through Friday, and more than 10 inches 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, where 15 inches of snow was possible over two days. Wind gusts could top 75 mph 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 per hour and wind gusts of up to 65 mph.

Falling trees struck homes and littered roads across northwest Washington. In Lynnwood, Washington, a woman died Tuesday night 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 Tuesday night, fire officials said.