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Dangerous rising waters from record-high rivers in the Pacific north-west presented a significant danger on Friday, with 100,000 people under orders to leave and additional heavy rainfall expected on Sunday.

Torrential Rain Causes Extensive Inundation

Torrential rain on the previous day triggered flooding across much of the region from the state of Oregon north through Washington state and into British Columbia, closing dozens of roads and prompting the evacuations of many thousands of people.

The heavy rainfall began earlier in the week, carried into the area by a storm system meteorologists call an "Pineapple Express", a huge plume of water vapor funneled inland from the Pacific.

Washington's governor declared a state of emergency on mid-week in response to the severe conditions that has caused mudslides and washed out roads and flooded cars.

The Western Part of the State Bears the Brunt

The western portion of Washington bore the brunt of the storm, with flood warnings posted across the mountain ranges and Puget Sound, as well as for a northern section of Oregon. This area is populated by some 5.8 million people.

The same storm system delivered intense rains and flooding to the western part of Montana and an edge of northern Idaho.

Orders to Leave and Life-Saving Operations

Roughly 100,000 residents in west Washington on Friday were under level 3 evacuation orders, urging them to relocate at once to safer elevation. The majority were in rural Skagit county north of the city.

An estimated 3,800 evacuees were believed to be in need of emergency housing.

Emergency crews have rescued several people, including by helicopter in several counties in recent days.

Damage to Roads and Rail

The most severe inundation was observed along the Skagit, Snohomish and Puyallup rivers. Over three dozen major roads and dozens of smaller roads were closed due to flooding across the region.

Several lengthy segments of a major freight line serving the Pacific north-west were washed out or closed due to inundation, with rainfall totals of 10 to 17 inches or more in many areas.

Several waterways have been reaching peak levels several feet above historical records and on Friday morning had not yet gone down.

Forecast and Regional Effects

The prediction is for less intense precipitation on the day and a largely clear Saturday, but forecasters expect another round of intense rainfall on the weekend for the area.

To the north, in Canada's British Columbia, the majority of Canadian highways leading to the coastal city of Vancouver were shut down due to floods, rockfalls and the risk of avalanches.

"This situation is evolving and very dynamic," said the transportation authorities.

Reaching Vancouver depends heavily on a limited highway and rail network that traverses the the Rockies.

Climate Context

While such storms are relatively common on the US Pacific coast, experts say they are expected to grow more frequent and more extreme in the coming decades if climate change from human-caused factors persists at current rates.