MOUNT VERNON, Wash. — Tens of thousands of Washington residents were under evacuation orders Thursday after a barrage of torrential rain that has overflowed rivers, sent mud sliding onto highways and trapped people in floodwaters.

“The flooding levels we’re looking at are potentially historic in nature, so we just want to emphasize how serious the situation is,” Gov. Bob Ferguson said at a news briefing Thursday, one day after declaring a statewide emergency. “This situation is extremely unpredictable.”

Skagit County, in a major agricultural region north of Seattle, has ordered everyone within the Skagit River’s floodplain to evacuate. Some 78,000 people live in the floodplain, according to the county’s emergency management chief Julie de Losada.

The high water mark upstream at the to

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