546
FXUS66 KPQR 061211
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
411 AM PST Sat Dec 6 2025

.SYNOPSIS...
A prolonged wet pattern continues through early next week as
multiple frontal systems push inland. The most notable period
continues to focus on Monday through midweek, when a prolonged
atmospheric river may bring significant rainfall and rising
rivers across southwest Washington and northwest Oregon. Today
and Sunday bring lighter rainfall, while Monday is expected to
produce the highest totals across southwest Washington and
northwest Oregon.

&&

.DISCUSSION...Now through Friday... Light to moderate showers
persists across the area this morning as a steady southwesterly
to westerly flow feeds moisture inland. Todays precipitation
will be relatively modest with forecast totals from now to 5 AM
Sunday near 0.10-0.25 inch across interior lowlands, 0.2-0.4
inch along the coast, 0.5-1.0 inch in the Coast Range, and
0.5-1.5 inches in the Cascades. Breezy southwest winds are
expected at times with gusts typically 20-30 mph, strongest near
exposed coastal and elevated terrain.

A more organized plume arrives Sunday with IVT values around
300-500 kg/ms. That wave increases rainfall efficiency,
producing about 0.5-0.7 inch for interior lowlands, 0.7-1.5
inches along the coast, 1.5-2.5 inches in the Coast Range,
1.5-3.0 inches in the Cascades, and 0.5-1.5 inches in the Lane
County Cascades. Widespread impacts are not anticipated through
Sunday; however, if rainfall rates sustain around 0.2 to 0.3
inches per hour for several hours, a few faster-reponding
basins, such as the Grays at Rosburg and the Willapa River,
could rise quickly. Breezy southerly to southwesterly winds are
expected at times with gusts typically 30-40 mph, strongest near
exposed coastal and elevated terrain.

The first strong to moderate moisture surge is forecast for
Monday afternoon. Ensemble clusters show concentrated IVT around
650-850 kg/ms for this plume, supporting prolonged, efficient
rainfall. Expected Monday rainfall jumps markedly, with interior
lowlands receiving roughly 1.25-2.50 inches (highest totals in
southwest Washington; 1.8-2.0 inches in the Portland-Vancouver
metro), coastal totals near 2.5-3.5 inches, the Coast Range and
Cascades 3-5 inches, and the Lane County Cascades 0.75-1.75
inches. Given the duration and magnitude of this plume, faster-
responding basins could see rapid rises and localized flooding;
this period warrants close monitoring.

A second strong to moderate surge of atmospheric moisture is
expected Tuesday into Wednesday, with ensemble means near
700-800 kg/ms. High-end ensemble solutions cluster around
850-950 kg/ms while lower-end members fall in the 550-650 kg/ms
range. Even if the second surge trends a bit weaker, the
combination of two consecutive high-IVT plumes and the residual
moisture between them will sustain elevated rainfall totals and
prolonged runoff. By midweek, soils will be saturated across
much of the region and flood risk for rivers and small streams
will increase. After the second surge, the core of the moisture
feed is expected to diminish later in the week.

Winds will be a concern Monday through Wednesday, with gusts up
to at least 30 mph inland and up to at least 40 mph at the
coast. There is a 10-20% chance for max wind gusts over 40 mph
for inland areas, and a 20-40% chance for gusts over 50 mph
along beaches and headlands. If wind gusts of this magnitude do
materialize, expect scattered downed trees and power outages.

Showery conditions will linger into Thursday and Friday as the
flow gradually weakens and dries. Snow levels are expected to
remain high (above 6000 ft) during this warm weather event,
keeping most precipitation as rain in the Cascades. Forecast
confidence in exact timing and magnitudes is still moderate
given ensemble spread. Overall, the Monday through Wednesday
time period remains the primary focus for potential high-impact
hydrologic (see hydrology section below for more details) and
wind concerns.
~12

&&

.HYDROLOGY...
A series of frontal systems will keep conditions very wet
through the weekend and much of next week. The most notable
period will be late Monday through late Wednesday, when a
prolonged atmospheric river will bring significant rainfall and
the potential for widespread river flooding across southwest
Washington and northwest Oregon. Total rainfall amounts from
4am Monday to 4am Thursday (72 hour total) will most likely be
around 4 to 6 inches at the coast and interior lowlands, and 5.5
to 9.0 inches in the Coast Range and Cascades. Lane County
Cascades will be an exception, with 4-6.5 inches expected. There
is a 10% chance for rain amounts up to one foot in the Cascades
and Coast Range, up to 8-9 inches at the coast, and up to 7
inches in the Willamette Valley, Portland metro and Cowlitz
Valley. While rain amounts this high are unlikely to occur, they
represent a reasonable worst case scenario. This outcome would
result in widespread major flooding.

Although rain will likely become lighter Thursday into Friday,
the threat of flooding will linger along slow responding rivers.
Rivers draining the Coast Range and Willapa Hills have a 30-60%
chance of reaching moderate flood stage and a 20-40% chance of
reaching major flood stage. Additionally, many Willamette River
tributaries that drain from the Coast Range and Cascades have
anywhere from a 40-70% chance of reaching minor flood stage and
a 20-50% chance of reaching moderate flood stage. Flooding of
the Willamette mainstem rivers is unlikely to occur (less than
5%). Probabilities for specific river points can be found at the
National Water Prediction Service website.

There is also increasing confidence in the potential for
flooding along small creeks and streams as well as urban
flooding, including roadway flooding. Periods of heavy rain
will also increase the risk for landslides in areas of steep
terrain and debris flows over recently burned areas. People,
structures, and roads located below steep slopes, in canyons,
and near the mouths of canyons may be at serious risk from
rapidly moving landslides.

Considering the probabilities of flooding for rivers and urban
areas, a Flood Watch has been issued for all of southwest
Washington and northwest Oregon from 4 AM Monday to 4 AM Friday.

&&

.AVIATION...At 1030z Saturday, visibilities were in the VFR range
with cigs in the high-end MVFR to low-end VFR range, with mainly
high-end MVFR cigs at the coast and low-end VFR cigs inland. Moist
westerly flow aloft is resulting in persistent rain showers in
the Cascades and Coast Range, bringing frequent mountain
obscuration. Expect a mix of MVFR/VFR flight conditions to
continue through Saturday night before MVFR and IFR cigs become
more widespread after 12z Sunday. This is when chances for IFR
cigs increase to 20-30% for inland TAF sites and near 100% at the
coast. Breezy southwest surface winds will continue throughout the
TAF period with occasional wind gusts up to 20 kt. Steady
stratiform rain will develop across the area between 06-12z
Sunday.

KPDX AND APPROACHES...Expect a mix of high-end MVFR to low-end VFR
cigs to continue through 12z Sunday, before trending towards low-
end MVFR or even IFR after 12z as steady stratiform rain
develops. Chances for MVFR cigs at that time peak near 90%, while
chances for IFR cigs peak near 30%. Will note that at around 700
ft MSL, winds are SSW around 30 kt. Not nearly enough to add LLWS
to the TAF, but for east approaches cannot rule out the potential
for isolated periods of gusty crosswinds. -23/27

&&

.MARINE...Seas will remain between 11 and 14 ft around 11 seconds
through today before lowering to 8 to 9 ft tonight. Seas are then
forecast to remain steady until rising to 12 to 15 ft on Monday.
With dominant wave periods around 11 seconds, seas will be steep
and hazardous. In addition, expect west winds to continue today at
15 to 20 kt with gusts up to 25 kt, becoming west-southwest 10 to
15 kt with gusts up to 20 kt Saturday evening. Winds become south
20 to 25 kt with gusts up to 30 kt Sunday morning. Isolated and
brief gale force gusts up to 35 kt are possible Sunday morning,
especially over the inner waters. Have mentioned this possibility
in the ongoing Small Craft Advisory that is currently in effect
for all marine zones through Sunday.

Gale force wind gusts may become more widespread on Monday when
probabilities increase to 50-80% for max wind gusts over 34 kt,
highest over the northern waters. These probabilities have trended
upward since last night`s update. Depending on how strong winds
get on Monday, there is the potential for seas over 15 ft.
Currently, there is a 10% chance for seas up to 18 ft. -23

&&

.BEACH HAZARDS...Coastal Flood Advisories remain in effect from
for minor tidal overflow during high tide from 9 AM to 3 PM Saturday
for the south Washington coast and Clatsop County coast in Oregon.
Minor flooding up to 1 foot above ground level is possible during
high tide in low lying areas near bays, sloughs, and the low
reaches of the coastal rivers. Coastal residents in the warned
area should remain alert for rising water and take appropriate
action to protect life and property. -10

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this
     afternoon for ORZ101.

WA...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this
     afternoon for WAZ201.

PZ...Small Craft Advisory until 4 PM PST Sunday for PZZ210.

     Small Craft Advisory until 7 PM PST Sunday for PZZ251>253-
     271>273.
&&


$$

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