Strong Pacific Northwest Atmospheric River Produced Record Flooding In Parts Of Washington State (RECAP)

Moderate To Major River Flooding Ongoing In WA

Days of heavy rain from a strong atmospheric river triggered major flooding in western Washington state that topped all-time record crests in some areas.

(MORE: Latest Flood News Updates)

Record Flooding

The heavy rain during the week of December 8-11 pushed rivers to record flood in five locations in western Washington:

- Skagit River near Mt. Vernon crested just under 10 feet above flood stage on Dec. 12, topping the previous record from Nov. 25, 1990. Records date to at least 1906.

- Snohomish River near Snohomish crested just over 9 feet above flood stage on Dec. 11, topping the previous record from Nov. 25, 1990. Records date to at least 1942.

- Cedar River at Renton crested just over 5 feet above flood stage on Dec. 11, topping the previous record from Nov. 24, 1990. Records date to at least 1906.

- Nooksack River at North Cedarville crested about 4 feet above flood stage on Dec. 11, topping the previous record from Nov. 16, 2021. Records date to at least 2003.

- Grays River at Rosburg crested just under 5 feet above flood stage on Dec. 9, topping the previous record from Dec. 3, 2007. Records date to at least 2005.

This prompted evacuations of areas in the 100-year floodplain in Skagit County from Mt. Vernon upriver. Flooding from the river has long plagued Mount Vernon, the largest city in the county with some 35,000 residents. Flooding in 2003 displaced hundreds of people.

The city completed a floodwall in 2018 that helps protect the downtown. It passed a major test in 2021, when the river crested near record levels.

The city of Snohomish issued an emergency proclamation due to flooding, while in Auburn, south of Seattle, workers installed temporary flood control barriers along the White River.

How Much Rain Fell?

What made this event so impactful not only was the amount of rain, but that it soaked high elevations of the Cascades, Olympics and far northern Rockies. This warm air commonly seen in atmospheric rivers melted existing snowpack, which then ran off from the higher terrain adding even more water to swollen creeks, streams and rivers.

Numerous areas of western Washington's higher terrain measured over 10 inches of rain from Dec. 8-11, including one report of almost 17 inches of rain in the Olympics near Quinault.

That included Mt. Rainier's Paradise Ranger Station, known more for heavy snowfall this time of year, where over 3 inches of rain fell in 10 hours Tuesday night into Wednesday morning, according to the National Weather Service.

This volume of water pushed rivers into major flood stage, including stretches of the Carbon, Cedar, Cowlitz, Grays, Naselle, Nisqually, Puyallup, Skagit, Skykomish, Snohomish, Tolt, and White rivers, among others.

Other Flooding, Landslides, Wind Damage

Other rivers that narrowly missed a record crest included the Carbon River near Fairfax, Washington, and the Skykomish River near Gold Bar, Washington. Both reached their highest crests in 19 years.

Flooding along the Snoqualmie River in the Cascades east of Seattle was highest in almost 17 years, producing a stunning spectacle at Snoqualmie Falls.

Water rescues were reported in an RV park near Orting and near Fall City.

Several landslides were reported in Washington, including one on US 2 in Stevens Pass, a rockslide north-northwest of Lucerne coming off the Pioneer wildfire burn area, and a pair of landslides that twice shut down a stretch of Interstate 90 in the Washington Cascades near North Bend.

One landslide near Winton, Washington, sent a large tree through the windshield of a truck, injuring the driver. Another Thursday heavily damaged a home near Enumclaw.

On Wednesday, mountain winds gusted up to 108 mph at Chinook Pass near Crystal Mountain. Downed trees or power lines blocked numerous roads in western Chelan County. One home in Bremerton was heavily damaged after a tree fall, and state route 900 through Issaquah was closed Wednesday due to downed trees.

Idaho, Montana Impacts

As we alluded to earlier, this atmospheric river also surged well inland into the far northern Rockies.

One northern Idaho reporting station measured 6.5 inches of rain in 24 hours, their third heaviest total in 44 years.

The Coeur d'Alene River flooded homes near Kingston, Idaho, about 55 miles east-southeast of Spokane, Washington, prompting a swift-water rescue of three people, according to the National Weather Service. A road washed out by Spring Creek trapped 300 to 400 residents near Clark Fork, Idaho.

In far northwestern Montana, flooding in Lincoln County, washed out at least four bridges and also flooded roads and some homes around the town of Libby, about 65 miles west of Glacier National Park.

Montana Gov. Greg Gianforte declared a state of emergency for the area.

What Is An Atmospheric River?

You may have heard this term used before. Basically, this is a long plume of deep moisture in the atmosphere that often extends for thousands of miles.

In this case, the atmospheric river extended over 3,500 miles westward into the Pacific Ocean, between Alaska's Aleutian Islands and Hawaii, as you can see in the satellite loop embedded below.

(MET 101: What Is An Atmospheric River?)

Atmospheric rivers that are strong and slow-moving can produce heavy rain that can lead to flooding, especially along the West Coast of the U.S. in the cold season.

This atmospheric river was rated a level 4 of 5, according to the Center for Western Weather and Water Extremes (CW3E), meaning it had a lot of moisture and is relatively long-lasting.

Way Too Much Of A Good Thing

Believe, it or not, the Northwest was in drought prior to this storm. In western Washington, this drought had been in place since February. The map below shows all the areas that were in at least some drought. But too much rain too fast doesn’t really help as much as you would expect.

The drought-stricken soil cannot absorb heavy rainfall fast enough, and as a result, runoff commences. Rivers get saturated, and the drought doesn’t decrease as much as you would expect.

The Associated Press has contributed to this report.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.