Atmospheric river may lead to landslides, floods and power outages

Alan Torres, Eugene Register-Guard

Mon, December 8, 2025 at 8:03 PM UTC

4 min read

A strong atmospheric river will dump widespread rain and kick up high winds from Dec. 8 through Dec. 10. The system's heavy rain is likely to cause flooding, landslides and power outages, according to the National Weather Service's Portland office, which forecasts conditions in northwest Oregon and southwest Washington.

An atmospheric river occurs when water vapor from the tropics evaporates and travels toward colder regions in a jet stream. The water concentrates into narrow bands of air forming a "river in the sky" that can travel thousands of miles and last for days.

For the people in the path of the atmospheric river, this is experienced as nonstop, downpours and gusty winds. In the worst cases, atmospheric rivers can disrupt travel, induce mudslides, and endanger life and property.

Heavy rain and flooding

Vehicles drive through water over the roadway of Highway 99 at Goshen in March.

NWS said heavy rain will arrive the night of Dec. 8, and last for two more days, resulting in flood and landslide risks through Dec. 11.

The rain in Lane County will be relatively mild compared with northern portions of the region. NWS forecasted a 70%-80% chance the Portland metro will see a quarter inch of rain per hour between 1 a.m. and 9 a.m. Dec. 9. Rain will be even heavier along the coast and in the Cascades.

There's a 44% chance of 2 inches of rain or more falling in Eugene from 4 a.m. Dec. 8 to 4 a.m. Dec. 11.

If rain falls at a quarter-inch per hour for 3-4 hours or more, this typically results in urban floods significant enough to strand vehicles in low-lying areas with poor drainage.

NWS predicted a "major risk" for such urban flooding, as well as floods along creeks and streams in the Portland metro area, the coast and southwest Washington. NWS said creeks in these areas will overflow their banks, depositing debris on roads, and in some cases flooding them, especially where leaves and debris will clog storm drains.

There's a 5% chance of a rain rate over 0.25 inches per hour (enough to cause urban flooding) Dec. 9.

NWS warned commuters to be careful the morning of Dec. 9, predicting there will be car crashes from hydroplaning and motorists driving through flooded roads. The weather service also warned people who live or work in buildings with a history of flooding that they may see such flooding again this week.

Most rivers in the coast range and Willapa Hills have a 40%-70% chance of "moderate" flooding and a 20%-50% chance of "major" flooding. Willamette River tributaries that drain from the coast range or the Cascades have a 40%-60% chance of "minor" flooding and a 20%-40% chance of "moderate" flooding. Chances for flooding of Willamette mainstem rivers remains low, between 5%-15%.

NWS also said the heavy rain may cause landslides impacting people, structures, and roads located below steep slopes, in canyons, and near the mouths of canyons.

The Oregon Department of Geology and Mineral Industries warned people driving along such areas to be cautious, and people living or working in such areas to:

Listen for unusual sounds that might indicate moving debris such as trees cracking, boulders knocking together or the trickle of falling mud.
Watch a nearby stream or creek, because if it suddenly turns muddy or the amount of water in it suddenly increases or decreases, this could be because of an upstream landslide.
Check weather alerts online at weather.gov or over the radio at frequency 162.400, 162.425, 162.450, 162.475, 162.500 or 162.550. Leave if any of those sources indicate a coming landslide.

More: Landslides 'are everywhere' in Oregon and more unpredictable than earthquakes

Rain will lighten on Dec. 11, but flood risk will continue that day, subsiding Dec. 12. It will still be rainy, but less so, through at least Dec. 14.

In addition to river and urban flooding, NWS also predicted tidal flooding on the coast at high tides on Dec. 8 and Dec. 9.

Sneaker waves are possible Dec. 9 through 14. These waves can move logs and heavy debris onto beaches, potentially knocking over or pinning down beachgoers.

High winds and outage potential

A down tree blocks the path near University Hall where a pedestrian was injured during a wind gust on Feb. 24 in Eugene.

NWS also warned wind gusts in northwest Oregon and southwest Washington will reach between 35 and 45 miles per hour. Winds will be highest Dec. 8 in the afternoon and evening, but the wind will last through Dec. 10.

Winds this high, combined with soil saturated by heavy rain, will likely knock over trees and cause power outages.

Alan Torres covers local government for the Register-Guard. He can be reached byemail at atorres@registerguard.com, on X @alanfryetorres or on Reddit at u/AlfrytRG.

This article originally appeared on Register-Guard: Rapidly rising rivers and coastal flooding coming to Oregon

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