394
FXUS66 KPQR 052311
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
311 PM PST Fri Dec 5 2025

.SYNOPSIS...A steady succession of frontal systems will keep
conditions wet through the weekend and much of next week. 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.

&&

.DISCUSSION...Friday afternoon through Thursday...Radar imagery
at 2 PM PST Friday indicates rain has tapered off across most
of the area, though showers linger over the coast, Coast Range,
and Cascades into tomorrow. The front that moved through the
area this morning has stalled over the Cascades, and is expected
to move very slowly into eastern Oregon overnight as it
decreases in strength. This has caused tighter pressure
gradients over the region, producing breezy south to westerly
winds this afternoon into tonight. Expect gusts up to 25-30 mph
across the Willamette Valley and up to 35-40 mph in the
Cascades and Coast Range.

Showers linger into Saturday as a westerly flow pattern persists.
Another frontal wave will arrive Sunday and bring yet another round
of steady rainfall to the region. Ensemble guidance suggests IVT
values with Sunday`s system in the 300-500 kg/ms range,
producing a widespread soaking. Rainfall totals from Saturday
through Sunday are currently projected to fall in the 0.5 to
1.0 inch range across the interior lowlands, with 1 to 2.5
inches for the coast and Coast Range. The Cascades are expected
to receive roughly 1.25 to 3.0 inches, except for the Lane
County Cascades where amounts are closer to 0.5 to 1.5 inches.
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.

Attention then turns to the Monday-Wednesday period, where
ensemble confidence continues to increase in a multi-day
atmospheric river event. Over the past several cycles,
ensembles have become increasingly consistent in highlighting
two distinct surges of moisture: one centered roughly on Monday,
and a second sometime Tuesday or Wednesday. Between these
peaks, the moisture feed is unlikely to shut down, leading to a
long-duration event rather than two isolated episodes.

The IVT spread in ensemble guidance has shrunk significantly on
Monday with the first surge of moisture, with GEFS and Euro
ensemble means and their high and low end solutions peaking
within the 600-850 kg/ms range. The ensemble spread is still
wider for the second moisture surge, partially because ensembles
are still struggling to determine the location of where it will
focus along the WA into OR coast along with the timing.
Ensemble means are centered around the 700-800 kg/ms range. with
the high ends peaking around 900-100 kg/ms and the low end
around 500-650 kg/ms. In between the peaks, the ensemble means
indicate the IVT values could remain between 500-700 kg/ms with
the low end forecasts between 300-500 kg/ms, which is still
quite a bit of moisture. The precise timing and latitude of
these plumes remain uncertain, but confidence continues to
increase that the region will experience a prolonged period of
moderate to heavy rainfall. Winds early next week may also trend
breezy, but ensemble spread remains wide. Even moderate gusts
in a 30-40 mph range could be sufficient to bring down isolated
trees due to increasingly saturated soils from over 5 days of
moderate rain. This remains a detail to refine as the event
draws closer. One thing to note is these are warm weather
systems, meaning snow levels are very likely to remain above
6000 feet with little to no snow expected over the Cascades.

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.
Confidence in a multi-day atmospheric river is rising, but exact
rainfall totals and timing details still require several more
forecast cycles to resolve. While rain is looking to continue
through at least Thursday, rain amounts look much less Thursday
and onward.      -12/03

&&

.AVIATION...Radar imagery as of early Friday afternoon depicts
decreasing rainfall as the current frontal system exits the region.
Terminal observations show a mix of LIFR/IFR CIGs and VIS along the
coast, and IFR/MVFR CIGs and VIS across the Willamette Valley.
Expect some gradual improvement to high-end MVFR and low-end VFR
CIGs this afternoon and evening. High confidence (60-80% chance)
that CIGs will be anywhere between 2-4 kft tonight as the low-level
atmosphere remains very moist. Guidance suggests 20-30% chance that
CIGs fall below 2 kft at any given terminal through early Saturday
morning. Southerly to southwesterly winds of 10-15 kt with gusts up
to 20-25 kt along the coast and Valley continue through the evening,
weakening overnight after 06-09z Sat. Scattered light showers and a
mix of high-end MVFR and low-end VFR CIGs continue Saturday as
active weather continues.

KPDX AND APPROACHES...CIGs trend toward high-end MVFR and low-end
VFR (2-4 kft) through the TAF period. Breezy southwesterly winds 10-
15 kt with gusts up to 20 kt, weakening after 08-09z Sat.
Strong crosswinds expected as southwest winds remain around 40 kt
through this evening.     -10

&&

.MARINE...Breezy west-southwesterly winds with gusts up to 25 kt
continue across the waters as a frontal system progresses through
the area. Small Craft Advisories remain in effect across all waters
including the Columbia River Bar through late Saturday evening. Seas
of 7-9 ft at 10-11 seconds build to 11-14 ft at 13 sec Saturday
morning as a northwesterly swell moves in. Will note that a very
strong ebb may lead to seas of 12-13 ft in the Main Channel of the
Columbia River Bar around 530 PM Saturday. There is also a 60-80%
chance that seas build above 13 ft, with the highest chances north
of Cape Lookout and beyond 10 NM. There is also 30-50% chance seas
peak as high as 15 ft.

The parade of fronts continue through the weekend and into early
next week. Chances for widespread and frequent Gale force wind gusts
of 34 kt or greater remain under 15% through this weekend; however,
chances increase to 30-40% on Monday for the inner water zones out
10 NM and Columbia River Bar due to a potential coastal jet with the
next system. Seas are forecast to hover close to 15 ft from late
Monday through late Wednesday, with a 10% chance seas peak around 17-
18 ft.        -10/23

&&

.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

&&

.HYDROLOGY...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 increasing confidence in 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.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for
     ORZ101.

WA...Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for
     WAZ201.

PZ...Small Craft Advisory until 10 PM PST Saturday for PZZ210-
     251>253-271>273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
x.com/NWSPortland