Focus region: Portland–Vancouver metropolitan area, Willamette and lower Columbia basins

Date: December 9, 2025

1. Executive Summary

Since December 8–9, 2025, the Portland metropolitan area has been experiencing significant flooding and flooding-related impacts driven by a prolonged, warm atmospheric river event aimed at northwest Oregon and southwest Washington. This weather pattern is delivering persistent, moisture-rich onshore flow, leading to several days of heavy rain, elevated river and creek levels, urban flooding, combined sewer overflows, and heightened landslide risk. 

Forecasts from the National Weather Service (NWS) and local media indicate that:

Rain chances remain very high (often 80–100%) through at least Friday, December 12.  Portland could receive on the order of 5–6 inches of rain by the end of the week, with higher totals in the Coast Range and Cascade foothills that drain into local rivers.  Multiple rivers in northern Oregon and southern Washington are at or near flood stage, including smaller basins that respond quickly, while at least one key urban tributary (Johnson Creek at Sycamore) is under an NWS Flood Warning. 

Even if the most intense rainfall wanes, flood conditions are expected to persist for several days because:

The atmospheric river is prolonged and only slowly weakening, maintaining above-normal rain and keeping soils saturated.  Hydrologic response lags meteorological forcing: rivers, creeks, and groundwater continue to rise or stay high for 24–72 hours (or more) after peak rainfall in their watersheds.  Urban infrastructure is stressed by sustained runoff, leading to repeated episodes of ponding, roadway flooding, and combined sewer overflows (CSOs) even between heavy downpours.  Landslide risk remains elevated in steep terrain due to soil saturation, even after rainfall intensity diminishes. 

The combination of continued rainfall, hydrologic lag, and stressed infrastructure explains why “flood conditions” in a broad sense (hazard, risk, and impacts) are expected to remain in the Portland area for several days beyond the peak of the current storm.

2. Current Situation in the Portland Metropolitan Area

2.1 Meteorological Overview

A strong atmospheric river—a narrow corridor of very moist air extending from the subtropical Pacific toward the Pacific Northwest—is currently impacting Oregon and Washington. This event has been widely described as a “Pineapple Express,” indicating moisture transport originating near Hawaii. 

Key characteristics:

Persistent southwesterly flow is funneling warm, moisture-laden air into the region. High freezing levels mean that a large portion of precipitation falls as rain rather than snow over the Cascades, enhancing runoff into rivers and creeks.  Embedded frontal waves (disturbances riding along the moisture plume) periodically enhance rainfall intensity, producing intervals of heavy downpours.

The local forecast for Portland calls for:

Heavy rain and above-normal temperatures on December 9–10, followed by continued cloudy, damp weather with intermittent rain through at least December 15. 

2.2 Hydrologic Conditions

The hydrologic picture is mixed:

Smaller, flashy tributaries and creeks are experiencing or approaching flood stage. NWS has issued and extended a Flood Warning for Johnson Creek at Sycamore, noting ongoing minor flooding and non-trivial odds (10–20%) of reaching moderate or major flood thresholds.  Multiple rivers in northern Oregon and southern Washington—including basins draining the Coast Range—are in or near flood, driven by intense orographic rainfall and saturated soils.  Mainstem Willamette and Columbia Rivers currently have limited flood risk in the Portland reach due in part to flood-control reservoirs and regulated flows, though levels are elevated and sensitive to continued inflows and tidal effects. 

USGS monitoring at the Willamette River at Portland (Morrison Bridge) shows rising gage height and discharge over the past several days, consistent with sustained heavy rain. 

2.3 Urban Flooding and Infrastructure Stress

Portland is experiencing urban flooding and infrastructure impacts in parallel with riverine flooding:

Combined Sewer Overflows (CSOs): The City has issued advisories as the “Big Pipe” CSO control system nears or reaches capacity, leading to sewage overflows into the Willamette River during intense rain.  Roadway flooding and ponding in low-lying areas and at underpasses has been widely reported. Residents throughout the metro area are stocking up on sandbags and taking protective measures in expectation of continued flooding. 

In short, flood conditions in Portland encompass both river/creek flooding and urban/drainage flooding, all of which are being sustained by ongoing rainfall and saturated ground.

3. Meteorological Drivers: The Prolonged Atmospheric River

3.1 What Is an Atmospheric River?

An atmospheric river (AR) is a relatively narrow but elongated band of concentrated water vapor transport in the lower atmosphere, capable of delivering large amounts of rain or snow when it makes landfall and is forced upward by terrain. ARs account for a substantial fraction of cool-season precipitation in the Pacific Northwest. 

In this case, the AR:

Is oriented southwest-to-northeast, tapping subtropical moisture. Is forecast to be multi-day, with only modest shifts in position and intensity. Interacts with the Coast Range and Cascades, which enhance rainfall through orographic uplift.

3.2 Why This AR Is Particularly Effective at Producing Flooding

Several factors make this event especially conducive to flooding:

High Integrated Water Vapor Transport (IVT): Analyses from NWS and global forecast models describe strong water-vapor flux into the region, increasing the efficiency of rainfall production.  Warm Temperatures and High Freezing Levels: With above-normal temperatures and freezing levels well above typical winter storm elevations, more precipitation falls as rain over mountain basins, rapidly feeding rivers instead of accumulating as snowpack.  Duration and Wave Train: Rather than a single front passing through, waves embedded in the AR keep renewing rainfall over the same region for days. 

The result is not just one burst of heavy rain, but a sequence of episodes with limited recovery time in between.

4. Hydrologic Context: Rivers, Creeks, and Urban Watersheds

4.1 Basin Types and Their Response

The Portland area sits at the confluence of multiple hydrologic systems:

Large regulated rivers (Willamette, Columbia) Influenced by upstream reservoirs and dam operations. Respond over longer timescales (days to weeks). Current forecasts show low probability of major flooding along the Willamette mainstem in Portland, though levels are elevated.  Medium-sized tributary rivers (Clackamas, Tualatin, Sandy) Drain the Coast Range or Cascades. More responsive to intense rain and snowmelt. Some are experiencing high flows and minor to moderate flooding in parts of northern Oregon and southern Washington.  Small, urbanized creeks (e.g., Johnson Creek) Highly sensitive to 6–24-hour rainfall totals. Quickly spill onto floodplains and into adjacent neighborhoods when bankfull thresholds are exceeded. Currently under Flood Warnings with non-negligible risk of reaching moderate or major flood stages.  Urban drainage systems Streets, storm sewers, culverts, and CSO infrastructure. Vulnerable to clogged inlets, limited conveyance, and high inflows. Experience recurrent surface flooding even with “only” moderate rainfall, and are pushed toward or beyond design limits under AR conditions. 

4.2 Antecedent Soil and Reservoir Conditions

NWS hydrologic outlooks and local reporting note that:

Soils had already been wetted by prior fall storms, which reduces infiltration capacity and increases runoff efficiency.  Many Willamette Basin flood-control reservoirs were relatively low before this event—intentionally maintained with storage capacity for winter flood control—which mitigates, but does not eliminate, mainstem flood risk. 

This pre-storm setup means smaller basins and urban areas respond rapidly, while large rivers benefit from managed storage but still feel cumulative inflows over several days.

5. Why Flood Conditions Will Persist for Several Days

Even if rainfall intensity diminishes, several mechanisms ensure that flood conditions, in a broad operational sense, will persist in and around Portland:

5.1 Continued Moisture Supply and Elevated Rain Chances

Forecasts indicate that while the most intense phase of the AR occurs from roughly December 8–10, associated moisture and unsettled conditions linger into the weekend and beyond:

Rain and drizzle are expected daily, with intermittent steadier periods. Temperatures remain mild, keeping precipitation as rain rather than snow. 

This keeps runoff and inflows elevated, preventing a rapid drop in streamflow.

5.2 Hydrologic Lag of Rivers, Creeks, and Groundwater

Hydrologic systems respond more slowly than the atmosphere:

Rivers and creeks: It typically takes hours to days for rainfall in upstream basins to propagate as increased flow downstream. For some basins, forecast flood crests occur after the heaviest rainfall period.  Groundwater and subsurface flow: Continued seepage prolongs elevated baseflow in streams even after surface runoff declines. Tidal effects on the Willamette at Portland: Because this reach is tidally influenced, high river stages can persist or fluctuate with tides even as upstream inflows begin to recede. 

Thus, flood warnings or advisories can remain in place for days after rainfall intensity drops.

5.3 Infrastructure Recovery Time and Secondary Impacts

Urban systems also require time to recover:

Storm drains and culverts may remain clogged with debris until cleared, prolonging localized flooding. CSO systems need time for volumes to pass and storage to reset; multiple back-to-back events increase the frequency of overflows.  Road closures, transit disruptions, and cleanup efforts (e.g., pumping water, clearing mud and rock from slides) mean that community-level “flood conditions” can persist after water recedes.

5.4 Extended Landslide Risk

Oregon’s geology and slopes mean that landslides are both common and dangerous, with thousands of slides occurring annually statewide. Saturated soils, steep terrain, and prolonged rainfall set up conditions where landslides and debris flows can occur even after rain has eased, particularly above roads and along developed hillsides. 

This continuing geologic hazard is a key reason emergency managers often treat the overall flood/landslide episode as a multi-day event, not a single-day storm.

6. Risk Outlook for the Next Several Days

6.1 Most Likely Scenario (Through ~December 12–13)

Based on current forecasts:

Rain continues, but with gradually decreasing intensity after Wednesday. Small streams and creeks: Johnson Creek and similar basins remain at high flows; minor flooding persists or repeats with additional bursts of rain.  Urban flooding: Localized ponding and street flooding occur during heavier showers, with continued CSO advisories possible. Landslides: Elevated risk remains in the West Hills, Columbia Gorge slopes, and other steep areas.

6.2 Higher-End Scenario

If additional waves along the AR are stronger or slower to depart than currently expected:

Some smaller rivers and creeks could reach moderate flood stage, with more widespread inundation of low-lying neighborhoods. Infrastructure such as the Big Pipe may experience more frequent or prolonged CSOs, temporarily degrading water quality in the Willamette. More significant landslides could impact major transportation corridors.

The NWS hydrologic outlook notes probabilities on the order of 10–25% for some rivers to reach moderate or major flood stage in the region, particularly in basins draining the Coast Range and Willapa Hills (though mainstem Willamette risk remains lower). 

6.3 Lower-End Scenario

If the AR weakens more rapidly:

Flooding remains mostly minor and localized, focused on small creeks and known trouble spots in the urban drainage system. Rivers crest below serious flood thresholds but still remain elevated for several days.

Even in this case, the combined effect of saturated soils, elevated streams, and lingering showers means that conditions are not “back to normal” immediately after the peak rain.

7. Implications for Local Planning and Resilience

This multi-day flood episode highlights several strategic issues for the Portland area:

Dependence on centralized infrastructure CSO controls, major pump stations, levees, and key transportation links (such as I-5, I-84, and major bridges) represent potential single points of failure if overwhelmed by extended heavy rains. Importance of small-basin and urban hydrology While mainstem rivers like the Willamette often get attention, the current event underscores that small creeks (e.g., Johnson Creek) and urban drainage can produce significant, recurring flood damage even when large rivers stay below major flood thresholds. Need for sustained public communication Because risk persists beyond peak rainfall, messaging must emphasize lag effects and ongoing hazards (high water, unstable slopes, contaminated floodwater) for days afterward. Neighborhood-scale adaptation The sandbagging and localized preparation reported around the metro area demonstrate both community resilience and the need for targeted investments in drainage, green infrastructure, and flood-proofing in known hotspots. 

8. Practical Takeaways for Stakeholders

Residents should: Continue monitoring NWS warnings, river gauges, and local advisories for several days, not just during the heaviest rain. Avoid driving through standing water and stay clear of flooded or closed roadways. Be cautious around slopes, ravines, and creek banks, especially where soils are saturated. Local Governments and Utilities should: Maintain heightened staffing for stormwater, transportation, and emergency response through the full duration of the event and its aftermath. Track small-basin gauges and real-time reports; these systems can change rapidly. Use this event to refine maps of chronic flooding locations and landslide-prone slopes for future mitigation planning. Churches, community groups, and NGOs can: Serve as hubs for information, sandbagging help, and post-flood cleanup coordination. Check on vulnerable community members who may be impacted by power outages, flooded basements, or limited mobility.

9. Conclusion

Flood conditions in the Portland area in early December 2025 are the product of a multi-day atmospheric river event interacting with complex local hydrology and urban infrastructure. Heavy, warm rainfall on already-wet soils is driving high flows in small creeks and rivers, stressing drainage systems, and triggering combined sewer overflows, while elevated landslide risk adds an additional layer of hazard.

Even as the most intense rainfall diminishes, flood risk persists for several days due to:

Continued, though lighter, precipitation from the decaying atmospheric river; The lagged response of rivers, creeks, and groundwater; The time required for urban infrastructure and communities to recover; and Ongoing landslide susceptibility in saturated terrain.

Understanding these interacting factors is essential for effective risk communication, emergency management, and long-term resilience planning in the Portland region.