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ORZ101>128-WAZ201>211-061400-

Hydrologic Outlook
National Weather Service Portland OR
246 PM PST Fri Dec 5 2025

...AN ATMOSPHERIC RIVER WILL BRING PERIODS OF HEAVY RAIN FROM
DECEMBER 8-10, RESULTING IN AN INCREASED RISK FOR RIVER AND URBAN
FLOODING...

A series of frontal systems will keep conditions wet through the
weekend and much of next week. The most notable period will be late
Monday through late Wednesday, where a prolonged atmospheric river
may bring significant rainfall and rising rivers across southwest
Washington and northwest Oregon. However, river flooding for some
locations could continue into Thursday and beyond as some rivers
take longer to respond to heavier rainfall. Rivers draining the
Coast Range and Willapa Hills have a 30-60% chance of reaching
moderate flood stage and a 10-30% chance of reaching major flood
stage. Additionally, many Willamette River tributaries that drain
from the Coast Range and Cascades have anywhere from a 10-55% chance
of reaching minor flood stage and a 10-25% chance of reaching
moderate flood stage. Flooding of the Willamette mainstem rivers is
unlikely to occur (less than 10%). Probabilities for specific river
points can be found at the National Water Prediction Service
website.

There is also the potential for flooding along small creeks and
streams, as well as urban flooding, including roadway flooding, for
locations in the central Willamette Valley north into the southwest
Washington lowlands, including the Portland metro area. Periods of
heavy rain will also increase the risk for landslides in areas of
steep terrain, and debris flows over recently burned areas.
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