A "bomb cyclone" has hit the western coast of United States, disrupting transport, knocking out power for half a million people and causing at least two deaths.

The storm brought wind gusts of nearly 90 miles (145 kilometers) per hour to the region, uprooting trees and power lines. It also dumped up to three inches (eight centimeters) of rain on parts of Oregon, Washington and northern California, according to the National Weather Service. 

While the storm first hit on Tuesday, extreme rainfall warnings were in place until Friday, with flood risk warnings extending until Saturday in areas north of San Francisco.

A "bomb cyclone" occurs when a storm rapidly intensifies in the span of 24 hours or less. This generally takes place when a cold air mass from the polar region meets warm tropical air — a process that meteorologists call bombogenesis. 

Scientists blame human-caused climate change for the increasing intensity of storms across the world. Global heating and warmer oceans provide additional energy to storms which can then wreak havoc in the form of faster winds and torrential rainfall.

Storm causes deaths, power outages

A woman was killed in the north of Seattle when a tree fell on a homeless encampment, local officials say. Another woman was killed near Seattle when a tree fell on her home, local officials said.

Two people were also injured when a tree fell on their trailer, southeast of Seattle. 

Meanwhile, the website Poweroutages.us showed that more than 500,000 homes and businesses in Washington, southwest Oregon and northern California were without power as of Wednesday morning. By the afternoon, that number lowered to 460,000. 

Schools canceled or postponed classes in Washington, where the storm was expected to linger until midday Wednesday.

The Snohomish Regional Fire & Rescue service in western Washington urged residents to stay home. "Trees are coming down all over the city & falling onto homes," the fire department of Bellevue posted on social media.

"If you can, go to the lowest floor and stay away from windows. Do not go outside if you can avoid it."

In California, the weather service extended a flood watch for areas north of San Fransisco into Saturday. Up to 16 inches (40 centimeters) of rain is expected in California and southwest Oregon through Friday. Officials have warned of flash floods, rock slides and debris flows.

"In the Cascades and mountains of northern California, a heavy snow threat will unfold as several feet of accumulation are possible. Strong winds will combine with snow to produce potential blizzard conditions," the Weather Prediction Center said. 

mk/nm (AFP, AP, Reuters)