Flood

Historic flooding caused by a major atmospheric river earlier this week impacted large parts of western Washington state, prompting evacuations, emergency declarations, and record river levels. As impacts from that event persist, another strong atmospheric river is forecast to reach the Pacific Northwest on Sunday, December 14, 2025, bringing a prolonged period of precipitation to Washington, Oregon, and northern California through mid-week.
A powerful atmospheric river affected western Washington between December 8 and December 11, producing prolonged heavy rainfall and widespread flooding across river basins draining the Olympic Mountains and the western slopes of the Cascades.
Rainfall totals exceeded 250 mm (10 inches) in parts of the Olympic Mountains and Cascade foothills over approximately three days, according to satellite and ground-based observations. The intense precipitation, combined with already wet antecedent conditions, resulted in rapid river rises, inundation of low-lying communities, and extensive overbank flooding.
Several rivers, including the Skagit, Snohomish, Nooksack, and Puyallup, reached major flood stage, with some gauges exceeding historical records.
Floodwaters and debris flows forced closures of major highways, local roads, and rail corridors, isolating some communities and disrupting transportation across western Washington.
Local and state authorities issued evacuation orders and advisories for tens of thousands of residents, particularly in flood-prone areas of Skagit, Snohomish, and Whatcom counties.
The Governor of Washington declared a statewide emergency, enabling deployment of the National Guard and coordination of disaster response resources.
Emergency responders conducted numerous water rescues, including evacuations from flooded homes and vehicles. While river levels began to gradually recede later in the week, soils remain saturated, and rivers are elevated, maintaining heightened hydrologic sensitivity.
According to the Center for Western Weather and Water Extremes (CW3E), a new strong atmospheric river is forecast to develop over the North Pacific Ocean and merge with remnant moisture from the dissipating system that affected the region earlier in the week.
The system is expected to move onshore over British Columbia and the Pacific Northwest on Sunday, December 14. Forecast guidance indicates a long-duration atmospheric river event exceeding 48 hours, with strong southwesterly integrated vapor transport exceeding 750 kg m⁻¹ s⁻¹ over the Pacific Northwest from Monday, December 15, through Tuesday, December 16.
The system is then forecast to shift southward along the U.S. West Coast, bringing atmospheric river conditions to northern California between Tuesday, December 16, and Wednesday, December 17.
Model guidance shows uncertainty in the duration and continuity of atmospheric river conditions. The ECMWF ensemble indicates the potential for a brief break in conditions on December 16, while the GEFS maintains near-continuous atmospheric river conditions, particularly along coastal Oregon.
Ensemble control members from both modeling systems forecast AR3–AR4 conditions along coastal Washington and Oregon, and AR2–AR3 conditions along coastal northern California, although uncertainty remains regarding exact duration and peak intensity. Beyond mid-week, ensemble forecasts from both GEFS and ECMWF show elevated probabilities for additional atmospheric river conditions beginning around December 18, though confidence in timing and intensity remains moderate.
The National Weather Service Weather Prediction Center (WPC) forecasts 72-hour precipitation totals of 75–150 mm (3–6 inches) along the coastal ranges of Washington, Oregon, and northern California, as well as the Cascades and northern Sierra Nevada, for the period ending at 10:00 LT on Wednesday, December 17.
The WPC has issued Excessive Rainfall Outlooks indicating marginal risk for northern Washington from December 14 to 15, marginal to slight risk across parts of Washington, Oregon, and far northern California from December 15 to 16, and continued marginal risk across much of the region from December 16 to 17.
The outlooks indicate an increased probability of flash flooding, particularly in areas recently affected by flooding.
The National Weather Service Northwest River Forecast Center forecasts renewed rises on rivers across western Washington and Oregon beginning December 15, with elevated flows expected to persist for multiple days.
Given the recent flooding, saturated soils, and stressed river systems, additional rainfall could exacerbate river flooding and prolong impacts. Freezing levels are forecast to begin relatively high, then lower as the atmospheric river progresses, increasing the likelihood of significant snowfall at higher elevations, particularly in the Cascades after December 16.
Beyond mid-week, subseasonal multi-model forecasts indicate elevated probabilities for additional atmospheric river activity along the U.S. West Coast beginning around December 18. According to analysis of ensemble and regime guidance, a transition toward a Pacific Ridge circulation pattern during December 15–21 may favor enhanced moisture transport into the Pacific Northwest and northern California. Confidence decreases later in the period, but probabilities remain above climatological norms.

References:
1 Atmospheric River Forecast to Bring Precipitation to Pacific Northwest and Northern California – CW3E – December 12, 2025