CW3E Event Summary: 8-12 December 2025

23 December 2025

Click here for a pdf of this information.

Long-Duration Atmospheric River Produces Heavy Precipitation and Flooding in Pacific Northwest

The AR:

• A strong atmospheric river (AR) transporting very moist air from the subtropical western and central North Pacific made landfall over Washington and Oregon early 8 Dec.
• After the initial AR landfall, a second pulse of moisture transport moved onshore late 9 Dec and prolonged AR conditions over Washington through 11 Dec.
• Based on GEFS analyses, this AR ranked as an AR 4 on the Ralph et al. (2019) AR Scale over coastal Washington/Oregon and an AR 3 over the western Cascade foothills and central Washington/Oregon.
• This AR was noteworthy for its long duration (~96 hours in some locations) and its significant penetration into the interior northwestern US.
• CW3E’s AR Reconnaissance field campaign carried out four IOPs that sampled the AR as well as nearby essential atmospheric structures and regions of forecast sensitivity.

Impacts:

• This AR produced more than 10 inches of precipitation over the Olympic Mountains, Washington Cascades, and Cascade foothills.
• Snowfall was limited due to high freezing levels. CW3E’s MicroRain Radar in the Green-Duwamish watershed measured snow levels near 9,000 feet during the second pulse of moisture transport.
• The combination of saturated soils, heavy rainfall, and high snow levels resulted in widespread significant riverine flooding in Washington. Five stream gages experienced record flooding, and 16 others exceeded major flood stage.
• The National Weather Service received numerous reports of flooding and landslides in Washington and northwestern Oregon.
• Flooding and landslides caused significant damage to sections of US 2, US 12, and SR 410. A 49-mile stretch of US 2 is currently closed and not expected to fully reopen for months.

Summary provided by C. Castellano, S. Bartlett, B. Moore, S. Roj, M. Steen, M. Warner, and J. Kalansky; 23 December 2025

To sign up for email alerts when CW3E post new AR updates click here.