Western Washington residents woke up Monday to their final hours of dry weather before a powerful atmospheric river unleashes 10-15 inches of rain on the region’s mountains and triggers widespread flooding across multiple river valleys.

“The big action’s coming up tomorrow,” University of Washington (UW) atmospheric sciences professor Cliff Mass told KIRO Newsradio’s “ Seattle’s Morning News ” Monday morning. “This is a very strong atmospheric river. When this moisture hits our mountains, it produces huge amounts of precipitation.”

The storm system (a river of tropical moisture streaming directly into the Pacific Northwest) will turn on Monday but deliver its most punishing blows Tuesday and Wednesday, when several rivers, including the Snoqualmie, are forecast to reach major flood s

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