The heaviest rain is predicted overnight until about 10 a.m. Sunday, putting the Tolt and Snoqualmie rivers at risk for major overflows.
Western Washington is in for yet another drenching as an unusual storm pattern brings heavy rain overnight and into Sunday morning, setting the stage for lowland flooding.
The National Weather Service (NWS) issued a flood watch Saturday afternoon for all counties with rivers flowing out of the Cascade mountains: King, Pierce, Snohomish, Skagit, Whatcom, Thurston and Lewis counties.
The storm could dump up to 6.5 inches of precipitation in a few mountain areas, with an average 2 to 4 inches across most the western Cascades, said NWS meteorologist Johnny Burg.
Falling on already-soaked ground, the downpour will elevate the risk of shallow landslides. Sadly for snow-lovers, most of the mountain precipitation will fall as rain, Burg said.
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“The snow level is going to be high, between 6,500 and 7,000 feet,” he said. “There will be some snow melting, but mostly the flooding will be caused by rain that’s going to run off into the rivers.”
In the Seattle area, up to an inch of rain is possible Saturday night and early Sunday, with another quarter inch likely throughout Super Bowl Sunday.
Unlike most “atmospheric rivers” responsible for the region’s heaviest rainfalls, this system is coming from a different direction: out of the northwest, rather than from the southwest.
Part of the reason it’s going to be so wet is because the system is also very warm, Burg explained. High temperatures will reach the mid-50s over the next couple of days, with overnight lows as high as 49 degrees.
The highest flood risk appears to be on the Tolt River at Carnation and the Snoqualmie River at Carnation. Moderate flooding is possible on the Snoqualmie River at Snoqualmie Falls, the Snohomish River at Snohomish and the Cowlitz River at Randle.
Flooding could start as early as Sunday and extend through Tuesday, depending on local rainfall and river conditions.
After the heavy rain sweeps through on Sunday, showers and clouds will linger through most of the week. The seven-day forecast holds out a tiny glimmer of light on Friday, when the sun might shine again in Seattle.
But don’t get your hopes up too high, Burg cautioned.
Forecasts that far out are very unreliable. “Even I wouldn’t bet any money on it right now,” he said.