SNOHOMISH, Wash. — Heavy rains drenching the Pacific Northwest triggered flooding on Thursday across much of the region from Oregon north through Washington state and into British Columbia, closing dozens of roads and prompting the evacuations of tens of thousands of people.

The intense downpours began earlier in the week, swept into the region by a storm system meteorologists call an atmospheric river, a vast airborne current of dense moisture funneled inland from the Pacific Ocean.

Western Washington state bore the brunt of the storm, with flood watches posted across the Cascade and Olympic mountains and Puget Sound, as well as for a northern slice of Oregon, a region home to some 5.8 million people, according to the U.S. National Weather Service.

The same storm system brought heavy s

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