How much water flooded parts of Western Washington? At least 3 Lake Washingtons' worth

Even with words like “historic” and “record-breaking,” it can be hard to get a sense of the scale of the atmospheric river that hit Washington state this month, taking chunks out of highways, sweeping away homes, submerging cars, and taking at least one life.

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But numbers can start to paint a picture. More than nine million acre-feet: That's the amount of water that dumped on the flooded areas of Western Washington over 10 days. It adds up to more than three times the volume of water in Lake Washington.

Still, that number doesn't capture all of the rain that struck the region in such a short period of time.

That’s according to observed precipitation and calculations from the Center for Western Weather and Water Extremes, a U.C. San Diego-based research institute that studies atmospheric rivers. They rated December 2025's atmospheric river as a 4 out of 5, meteorologist Sam Bartlett said. But what made scientists there take this atmospheric river “a little bit more seriously over multiple days” was its persistence.

“This storm was extremely long,” Bartlett said. “It was not among the most intense in the historical record over the region, but it was one of the longest. So that was really what was notable about this event, was the duration for which you had high, elevated atmospheric moisture being transported up over the Pacific Northwest … which did a lot to overwhelm streams and rivers in the region.”

Ryan Torn, a University at Albany professor who got his Ph.D in atmospheric science at the University of Washington, double-checked the math.

“Nine million acre feet is a big amount of water for sure, during an event like that,” Torn said, adding that rivers like the Nooksack and Skykomish were simply inundated over days. “There's a lot of places in the Pacific Northwest that can absorb that kind of precipitation rate for a short period of time. But when you do it for days, very quickly, that saturates the soil and then just the water just runs off very quickly, downhill.”

The nine million acre-feet number captures rainfall from Dec. 9-19, yet doesn’t capture the total amount of rainfall over all of Western Washington. Rather, it’s just how much fell in 17 watersheds KUOW identified east of Puget Sound, from the Nooksack River down to the Lewis River, which experienced heavy flooding. It also doesn’t include everywhere that saw flooding in Washington state, such as Grays Harbor County.

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The 17 watersheds KUOW asked the Center for Western Weather and Water Extremes to calculate rainfall for are the Nooksack, Upper and Lower Skagit, Lake Chelan, Stillaguamish, Sauk, Snohomish, Skykomish, Snoqualmie, Lake Washington, Wenatchee, Duwamish, Puyallup, Nisqually, Upper and Lower Cowlitz, and Lewis watersheds.