Area Forecast Discussion
National Weather Service Portland OR
357 AM 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...Now through Thursday...A more robust frontal
system is moving inland early this morning, bringing widespread
rain to the region. This feature carries a stronger plume of
moisture than yesterdays system, with integrated vapor
transport values generally running in the 500-700 kg/ms range.
This is enough to support persistent rainfall through the day,
including areas farther south that were largely missed during
the previous event. Rainfall from now through 5 AM Saturday is
expected to total about 0.25 to 0.50 inches across interior
lowlands. Totals will be notably higher along the coast,
generally in the 1.0 to 1.5 inch range, with 2 to 3 inches
common across the Coast Range and Cascades. Winds today will
turn breezy from the southwest, with gusts most frequently
reaching 25 to 35 mph through most of the day. Expect the
frontal passage to arrive around 7-10 AM today.
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 Sundays 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
0.75 to 1.0 inches along the coast and 1 to 2 inches in the
Coast Range. The Cascades are expected to receive roughly 1.5 to
3.0 inches, except for the Lane County Cascades where amounts
are closer to 0.5 to 1.5 inches. Forecast confidence in the
exact high and low ends of these ranges remains moderate.
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
models continue to suggest a multi-day atmospheric river. 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 completely, leading to a long-duration event rather
than two isolated episodes.
Ensemble IVT guidance continues to show a broad range but
retains a strong signal for an impactful event. For the first
plume on Monday, GEFS and ECMWF are similar, with members
clustering around 600-650 kg/ms and higher-end solutions near
750-800 kg/ms. The second surge maintains mean values in the
500-600 kg/ms range with high-end members closer to 750-850
kg/ms. 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. This remains a detail to refine as
the event draws closer.
Overall, the Monday-Wednesday 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
&&
.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.
Rivers draining the Coast Range and Willapa Hills have a 10-25%
chance of reaching moderate flood stage and a 8-15% chance of
reaching major flood stage. There is also the potential for
flooding along small creeks and streams, as well as urban
flooding. 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. Periods of heavy rain will also increase the risk for
landslides in areas of steep terrain, and debris flows over
recently burned areas.
&&
.AVIATION...At 11z Friday, a mixed bag of flight conditions were
being observed across southwest WA and northwest OR with a warm
front draped over far northwest Oregon. South of this front,
breezy southerly winds, mild temperatures, spotty light showers
and MVFR to VFR ceilings and visibilities were being observed
from KSLE to KEUG. Along and north of the warm front, conditions
were a bit cooler with lighter winds, persistent light rain, and
LIFR to IFR ceilings and visibilities, including KHIO, KPDX, and
KTTD. LIFR ceilings persist at the coast.
A band of heavier stratiform rain is expected to move north to
south across the area between 15-21z Friday, resulting in
visibility reductions down to 2 to 4 SM much of the time, mainly
during the morning hours. LIFR to IFR cigs this morning will trend
towards low-end MVFR towards 00z Saturday, which is also when
precipitation will become more showery. The coast is expected to
remain in LIFR to IFR thresholds through the TAF period. South to
southwest winds are expected to increase Friday afternoon, with
gusts up to 20-25 kt inland and up to 30 kt at the coast.
KPDX AND APPROACHES...LIFR cigs are expected to persist this
morning, potentially rising to low-end IFR towards 16z Friday.
Low-end MVFR cigs become likely by 00z Saturday. Steady stratiform
rain is expected over the KPDX terminal between 15-19z Friday,
lowering surface visibilities down to 2 to 4 SM much of the time.
South winds increase Friday afternoon with gusts up to 25 kt. -23
&&
.MARINE...A frontal system will move over the waters today, with
westerly winds increase to 15 to 25 kt with gusts up to 30 kt,
strongest over the northern waters. Small Craft Advisories remain
in effect across all waters including the Columbia River Bar
beginning 4 AM Friday, lasting through Saturday evening. Seas of
4-5 ft early this morning will build to around 7-9 ft at 10-11
seconds by Friday evening as a northwesterly swell moves in with
the next system. Will note that a very strong ebb may lead to
higher seas of 10-11 ft in the Main Channel of the Columbia River
Bar around 4-5 PM Friday. For the rest of the waters, seas likely
(80% chance) build above 10 ft by late Saturday morning. There is
also a 25% chance that seas build above 13 ft, with the highest
chances north of Cape Lookout and beyond 10 NM. There is a 1-5%
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 early next
week; however, chances for brief and isolated Gale force wind
gusts are around 40-50% on Sunday and 50-70% on Monday. 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 Friday
and 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.
Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for ORZ101.
WA...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this
afternoon for WAZ201.
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.
&&
$$
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