The current flooding rivers(1) in the Pacific Northwest now are ascribed to an atmospheric river. Atmospheric rivers are long narrow weather regions that carry enormous amounts of moisture from the tropics in the sky. This sky river carries water vapor and moves with the weather, "carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River(2)." When they make landfall, they can release the water in the form of rain or snow.
They can stall over watersheds and release huge amounts of wind and rain and cause severe flooding. The atmospheric river from Hawaii to the Pacific Northwest is called the "Pineapple Express". "On average, about 30-50% of annual precipitation on the West Coast occurs in just a few AR events and contributes to the water supply and flooding."(2) An atmospheric river is approximately 250-375 miles wide.
"Not all atmospheric rivers cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply(2)." If it were colder we would have an amazing snow pack right now.
The bomb cyclone that came through a few weeks ago caused much less damage. An atmospheric river does not sound as dramatic, but the drone pictures of houses and cars deep in water (1) tell a different story. There is road and bridge damage and already talk of Thanksgiving travel disrupted.
(3) Meanwhile the Olympic Peninsula still has a section cut off: https://www.peninsuladailynews.com/news/flooded-roadways-cut-off-west-end/
(4) Pictures of the Sol Duc River rising: https://drkottaway.com/2021/11/17/atmospheric-river/