Two successive atmospheric river pulses are affecting the Pacific Northwest this week, delivering multi-day heavy rainfall and flooding risk across western Washington and northwestern Oregon. The first made landfall early December 9, and a stronger second pulse is forecast to arrive late December 9–early December 10, maintaining high integrated vapor transport and major flood potential through December 11.
Satellite image of atmospheric river impacting the Pacific Northwest acquired at 19:50 UTC on December 8, 2025. Credit: NOAA/GOES-West, RAMMB/CIRA, The Watchers
A prolonged atmospheric river continues to impact the Pacific Northwest, bringing several days of heavy rain, gusty winds, and mountain snow across Washington, Oregon, and northern Idaho, according to the National Weather Service Weather Prediction Center (WPC).
The system began its first landfall early December 9 and will remain active as a second, stronger pulse of Pacific moisture approaches late December 9–early December 10, extending the event through December 11.
The WPC reports steady heavy rainfall observed by radar and surface stations across the region, supported by a potent trans-Pacific jet stream transporting subtropical moisture into the coast. This long-duration pattern is expected to produce several more days of rainfall and mountain snow before weakening late week.
Image credit: CW3E “The arrival of widespread heavy rain today [December 8, LT] signals the beginning of several days of heavy rainfall expected to impact western Washington and northwestern Oregon,” NWS forecasters Asherman and Kong noted. “The potent trans-Pacific jet stream will continue to transport sub-tropical moisture from the Pacific and dump the moisture as heavy rain closer to the coast, and more than a foot of new snow for the northern Rockies in northwestern Wyoming.”
Flood Watches remain in effect across western Washington and northwestern Oregon, where scattered flash flooding and riverine flooding are expected as soils remain saturated. Data from the Northwest River Forecast Center (NWRFC) show rapid rises on the Chehalis, Snoqualmie, Skagit, and Willamette rivers, with several gauges forecast to reach major flood stage by mid-week.
Inland, heavy precipitation will fall mainly as snow over higher terrain. WPC forecasts more than 30 cm (1 foot) of new snow in the northern Rockies of northwestern Wyoming and the higher elevations of Idaho, while lower elevations continue to receive heavy rainfall. Since Monday, totals have exceeded 125 mm (5 inches) in parts of the Olympic Peninsula and northwestern Oregon, with additional accumulation expected through Thursday.
Supporting model guidance from the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography indicates greater than 90% probability of AR 4 or stronger intensity between the Olympic Peninsula and central Oregon coast, and greater than 50% probability of AR 5 strength across coastal Washington during the second pulse late December 9-early December 10.
CW3E’s ensemble forecast projects more than 250 mm (10 inches) of total precipitation in the Washington Cascades and more than 125 mm (5 inches) in lowland western Washington through the five-day period.
Snow levels near 1 500 m (5 000 feet) will confine most accumulation to higher elevations, where totals above 60 cm (2 feet) are likely. Wind gusts of 65–90 km/h (40–55 mph) are expected along the coast and through Cascade passes through Wednesday, with localized power outages possible due to saturated soils and downed trees.
The Atmospheric River Reconnaissance (AR Recon) program, led by CW3E in coordination with the U.S. Air Force 53rd Weather Reconnaissance Squadron, continues targeted aircraft missions to sample moisture and temperature profiles within the storm environment, improving real-time modeling of vapor transport and precipitation potential.
Outside the Northwest, the same Pacific jet stream is supporting a clipper system moving across the Upper Midwest and Great Lakes, producing 10–20 cm (4–8 inches) of snow and strong winds through December 10. At the same time, Arctic air has spread across the Mid-Atlantic and New England, while the central and southern Plains remain dry and milder than normal.
The atmospheric river is forecast to weaken gradually after December 11 as the Pacific jet shifts southward, allowing a drier pattern to re-establish across the region by late week.
References:
1 Major Atmospheric River to Bring Heavy Rain and Flooding to the Pacific Northwest – CW3E – December 8, 2025
2 Short Range Forecast Discussion – NWS/WPC – Issued at 03:00 EST on December 9, 2025
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Teo Blašković
I’m a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.