The opening salvo of a major atmospheric river hit the Pacific Northwest on Monday, drenching much of Western Washington with heavy rain that is forecast to continue at least until Thursday.

The National Weather Service warned of a high risk of widespread and significant river flooding.

South of Puget Sound, the Puyallup, Nisqually, Cowlitz and Carbon rivers were already seeing waters about to spill over their banks by 7 p.m. Monday, with serious flooding expected within hours. Esri (Mark Nowlin / The Seattle Times)

Rivers including the Skagit, Snoqualmie and Skykomish were set to swell near record highs, with major flooding starting late Monday into Tuesday, according to the National Oceanic and Atmospheric Administration.

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