Mass: Atmospheric river to bring 10-15 inches of rain, major flooding to Western Washington Tuesday-Wednesday

Western Washington residents woke up Monday to their final hours of dry weather before a powerful atmospheric river unleashes 10-15 inches of rain on the region’s mountains and triggers widespread flooding across multiple river valleys.

“The big action’s coming up tomorrow,” University of Washington (UW) atmospheric sciences professor Cliff Mass told KIRO Newsradio’s “Seattle’s Morning News” Monday morning. “This is a very strong atmospheric river. When this moisture hits our mountains, it produces huge amounts of precipitation.”

The storm system (a river of tropical moisture streaming directly into the Pacific Northwest) will turn on Monday but deliver its most punishing blows Tuesday and Wednesday, when several rivers, including the Snoqualmie, are forecast to reach major flood stage.

“The Snoqualmie River is going to get to flood stage, so we can expect some water getting into the lowlands there (the farm areas),” Mass said. “There will be water going over the roads into the fields. I expect that to happen.”

Strongest atmospheric river in years to hit Western WA

Mass characterized the storm as a “two- to five-year event,” making it the strongest atmospheric river to hit the region in several years, though not a record-breaker for Western Washington.

“It’s not the end of the world, but people have to be mindful,” Mass said.

Atmospheric rivers are currents of concentrated moisture that flow from tropical and subtropical regions. When they collide with mountain ranges, they can unleash extraordinary amounts of precipitation in a short time.

“It is the strongest one we’ve had in a few years,” Mass confirmed.

Current forecast models show the heaviest precipitation falling across the southern Cascades, with some locations potentially receiving 10 to 15 inches of rain over 48 to 72 hours. The impact zone extends “from Portland up to Chehalis in that region,” Mass said, where some drainages could see rainfall with return times of five to 10 years.

For Western Washington’s lowlands, forecasters expect two to five inches of rain from Monday through Thursday, with higher amounts south of Puget Sound.

Snoqualmie, Puyallup, Cowlitz rivers forecast for major flood stage

Multiple river systems are forecast to reach at least moderate flood stage, with the Snoqualmie, Puyallup, and Cowlitz Rivers expected to hit major flood stage. The Snohomish and Skagit Rivers are also forecast to reach major flooding levels.

The National Weather Service has issued a flood watch across much of Western Washington, which Mass noted should soon be upgraded to a warning.

For the Snoqualmie River near Carnation, forecasts show a major flood stage lasting three consecutive days (an unusual duration that signals the severity of this event). Mass’s models predict the river will reach all-time high stages on two of those days.

“We do get flooding situations in the Snoqualmie Valley every few years, and this is going to be like that,” Mass said.

The timing of the flood threat is critical for residents in vulnerable areas.

“Monday, everybody will probably be okay, but the action will be on Tuesday and Wednesday,” Mass said. “That’s when the flooding will be most impactful.”

Rivers will begin rising Monday as rain intensifies, but won’t reach their peak levels until Tuesday and Wednesday. Some models suggest rain could continue into late week, potentially prolonging the flood threat.

Monday is last day to prepare for Western Washington flooding

With the worst conditions still 24 to 48 hours away, Mass emphasized that residents in flood-prone areas still have time to prepare, but that window is closing.

“The big flooding is not going to be until Tuesday and Wednesday,” he said. “The rain is going to turn on tomorrow, and it takes a while for the rivers to react.”

Mass’s most urgent warning was simple and direct: “The key thing is you don’t drive through water. Some people make the mistake of doing that (you can get into trouble that way). So don’t go into water.”

He also stressed that residents in low-lying areas, particularly farming communities in river valleys, need to complete preparations immediately.

The heavy rainfall will create multiple hazards beyond river flooding. Urban flooding is expected where storm drains become clogged or overwhelmed. The risk of shallow landslides will increase significantly as soils become saturated, and shallow-rooted trees could topple.

Snow levels will remain at or above 5,000 feet, meaning most precipitation will fall as rain rather than snow. This maximizes runoff into rivers and streams, amplifying the flood threat.

Atmospheric river will refill Western Washington reservoirs

Despite the flooding dangers, Mass noted the storm will provide significant benefits for regional water resources.

“The soils will be nice and moist. The reservoirs (several of them were below normal) will be brought up to normal,” Mass said. “So our water storage will be all normal after this thing’s over.”

He added that filling reservoirs is “good for business” and will ensure adequate water supplies for the next six months.

Looking further ahead, Mass expressed optimism about the winter snowpack. “This is a La Niña year, and La Niña years tend to have good snowpacks,” he said. While this current storm will primarily bring rain to all but the highest elevations, he expects more snow later in the season to build the snowpack needed for next summer’s water supply.

What to expect Tuesday and Wednesday

Western Washington faces its most significant flood threat in years as a powerful atmospheric river takes aim at the region. Tuesday and Wednesday will bring the greatest danger, with multiple rivers reaching major flood stage and rainfall rates that haven’t been seen in this region for several years.

Residents in flood-prone areas have Monday to complete preparations. As Mass emphasized, stay out of floodwater, and be ready for roads to close and low-lying areas to flood.

Charlie Harger is the host of “Seattle’s Morning News” on KIRO Newsradio. You can read more of his stories and commentaries here. Follow Charlie on X and email him here. 

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