Successive Atmospheric Rivers to Slam the Pacific Northwest
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A Week Dominated by Atmospheric Rivers
A long corridor of deep Pacific moisture is lining up over the Pacific Northwest, setting the stage for multiple rounds of heavy rain, strong winds and mountain snow from western British Columbia, Canada to Washington and Oregon in the United States. As the first plume approaches, flood and weather alerts already stretch across the western half of the region.Skies over Seattle, WA and Portland, OR will darken in waves as each surge of moisture arrives. The ground is already wet, and each new pulse of rain sinks deeper into soils that can no longer absorb the water efficiently. The rivers respond first. Then the slopes begin to move.
By Friday, rainfall totals may exceed 200 mm across the western half of Washington and northwest Oregon. Cities including Seattle, WA and Portland, OR could see 100 to 150 mm. On the western slopes of the Cascades, amounts approaching 400 mm are possible, while elevations above 1,800 to 2,100 metres will pick up more than a foot of new snow.
Flood Watches and Rapid River Rises
The National Weather Service has issued flood watches for northwest and central Washington. The concern is clear.Sharp increases in river levels are expected along waterways draining the Cascades. With the landscape already soaked, the risk of mudslides and ground instability grows by the hour.
Residents near the foothills describe the atmosphere as tense. “You can feel when the rain gets heavier," one homeowner said. “The hillside behind us starts to sound different. You hear water moving underground."
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Powerful Atmospheric Rivers to Drench Pacific NorthwestUnderstanding the Atmospheric River
Atmospheric rivers are narrow, focused bands of water vapour that form when warm tropical moisture is drawn northward. As that moisture encounters mountain ranges, it is lifted, cooled and condensed into prolonged rain or snow.These events are a natural part of the western North American water cycle, but they are also responsible for 80 percent of flood-related damage along the west coast, costing roughly 1 billion dollars each year.
This particular atmospheric river stretches more than 3,500 miles from the central Pacific between Alaska’s Aleutian Islands and Hawaii, and it has been rated level 4 of 5 in strength by the Center for Western Weather and Water Extremes.
Round One: Heavy Rain and Strong Winds
The first round begins Monday and continues into early Tuesday as rainfall intensifies. Rates may reach an inch per hour. Totals near 5 inches are possible along the coast and over the Cascades.Winds will also strengthen, bringing widespread gusts up to 30 mph with isolated gusts above 40 mph. Downed powerlines and fallen branches may lead to scattered outages.
Urban areas from Seattle to Portland will experience periods of rapid ponding on roads, reduced visibility and slow commutes. For many, the storm will feel relentless.
Round Two: Increasing Flood Risk as Soils Saturate
A second strong surge of moisture will come ashore Tuesday night into Wednesday. While this plume may not be as intense as the first, its impact will grow because the soils will already be saturated. Flooding is likely to become more widespread, especially along the foothills of the Cascades.By Thursday, rainfall will gradually ease, but the accumulated totals will remain significant. Lower elevations across northwest Oregon and southwest Washington may reach 3 to 6 inches, while higher elevations in the Cascades could collect 6 to 12 inches.
Flooding, Debris Flow and Safety Concerns
The Weather Prediction Center has highlighted several days with a level 2 of 4 risk for excessive rainfall, pointing to a sustained flood threat across the region.Communities along the Seattle-Tacoma and Portland-Salem-Eugene corridors face:
- flash flooding in urban zones
- major river flooding in the Cascades and adjacent foothills
- landslide potential in steep terrain
- debris flows in wildfire burn areas
In the words of one resident north of Seattle, WA: “The river comes up fast here. When the rain sounds heavy enough, you start checking the bank every hour."
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