Atmospheric river brings ‘one-two punch’ of rain, wind, possible floods to Western Washington

A powerful atmospheric river sweeping across the Pacific Northwest this week is expected to dump more than a foot of rain and snow in the North Cascades and Olympics and cause major flooding in rivers that flow down from the mountains.

Kirby Cook, science and operations officer with the National Weather Service in Seattle, said forecasters are expecting two waves of water — one lasting through Monday and into Tuesday morning and a second wave Tuesday evening through Wednesday.

“It’s sort of a one-two punch for precipitation,” Cook said.

The three-day total could add up to 15 inches or more in the mountains, where freezing levels remain around 7,000 feet.

“It’s not inconceivable that [Mount Rainier] will receive, at least at the top of the volcano, somewhere in the order of 20 inches of precipitation,” Cook said. “So, a lot of rainfall is going to fall over the next 72 hours.”

That means a substantial risk for major flooding along rivers that flow down from Mount Rainier, the North Cascades, and the Olympic Range. The flood risk spreads out from the mountains into in Snohomish, King, Pierce, and Lewis counties, as well as areas below the Olympics.

The National Weather Service currently has nine forecast points on area rivers expected to hit major flood levels, including several points along the Snoqualmie and Skykomish rivers in King and Snohomish counties, the Puyallup River near Orting in Pierce County, and further south, the Cowlitz River at Randle and Packwood in Lewis County.

In terms of wind, Cook said the Washington coast could see gusts to 45-50 mph Tuesday afternoon into early evening. He said the Seattle metro area could see wind gusts topping 35 mph Tuesday afternoon.

Cook said the storm is expected to dip south into Oregon Tuesday and re-drench Washington state Tuesday night through Wednesday, which could mean flood waters peaking twice during the week.

“It will be a complicated pattern where we'll get a surge and then flow will come down, and then we'll get a second surge on a lot of area rivers because of resurging precipitation from the south,” he said.

Cook, who has been with the weather service in Seattle for close to 20 years, said atmospheric rivers are part of the climatology of the Pacific Northwest. But he added, this is a significant storm.

“It’s probably going to be one of the stronger events that we’ve had in at least a couple of years,” he said.