060
FXUS66 KPQR 050535
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
935 PM PST Thu Dec 4 2025

.SYNOPSIS...A series of frontal systems will bring a period of
wet weather Thursday through at least the middle of next week
across southwest Washington and northwest Oregon. Ensemble
guidance suggests a potentially impactful atmospheric river
event next Monday into midweek, though uncertainty on exact
impacts remains moderate to high.

&&

.DISCUSSION...Thursday afternoon through Wednesday...Radar
imagery Thursday afternoon depicts rain moving southeast across
SW Washington and NW Oregon as the first of many frontal systems
pushes into the PacNW. Observations indicate a front has
remained stalled just west of the coast through the morning and
early afternoon hours with rain mainly focused in a band north
of Astoria through Clackamas County. The front is beginning to
slowly move southeast, allowing rain to begin spreading farther
south into the area, continuing through the night. Due to the
northwest orientation of winds with this front, the central and
southern Willamette valley as well as Lane County likely won`t
see much rain with this first round. Another frontal system
will move through the region tomorrow, accompanied by an influx
of additional moisture, with ensemble guidance indicating IVT
values peaking around 500-600 kg/m/s. The upper level flow
becomes more zonal with this system tomorrow, allowing for more
rain in the Willamette Valley and Lane County than the first
system. Total forecast rain amounts for this evening through
Friday night top out around 0.3-0.6 inches for the interior
lowlands and 1.5-3.0 inches for the coast, Coast Range, and
Cascades, with locally up to 4.0 inches possible over the
Cascades. Breezy southwest winds will also be possible with
the frontal passage on Friday, with gusts most likely peaking
around 20-30 mph.

Showers are expected to linger through Saturday as zonal flow
continues. Then, another frontal system moves through the area
on Sunday. Ensembles indicate IVT values could peak around
300-500 kg/m/s with this frontal system, which would produce
another round of widespread rainfall. Total rainfall amounts
Saturday through Sunday are most likely to be around 0.4-1.0
inch for the interior lowlands, 0.75-2.0 inches at the coast and
Coast Range, and 1.0-3.0 inches for the Cascades except for
0.4-0.8 inches over the Lane County Cascades. But, there
remains a fair amount of uncertainty in both the low and high
end potential. Impacts are not expected with rain amounts
through Sunday, unless rain rates exceed 0.2-0.3 inches for
multiple hours over certain watersheds leading to rivers that
are prone to flooding quickly, such as the Grays at Rosburg and
Willapa River. This will be something to monitor during heaviest
periods of rain.

The main period of concern continues to focus on Monday into
late next week as ensemble guidance continues to indicate the
potential for an impactful atmospheric river event. Over the
past 48 hours, GEFS and Euro Ensemble guidance have
consistently suggested two rounds of elevated IVT plumes
pushing into the PacNW next week, one generally on Monday and
another sometime Tuesday into Wednesday, with IVT values likely
remaining above at least 250 kg/m/s in between, likely producing
a continuous rain event with two rounds of increased
precipitation. However, significant uncertainty still remains
in the timing of elevated IVT values, location of the moisture
plumes along the WA and OR coast, and exact peaks of IVT values.
For the first peak, the 12z GEFS mean is around 650 kg/m/s with
+1 standard deviation around 850 kg/m/s, and the 12z Euro EPS
mean is around 800 kg/m/s with +1 standard deviation around 900
kg/m/s. The second round still has much more variability with
both ensembles indicating the mean IVT values around 500-600
kg/m/s with +1 standard deviation around 750-800 kg/m/s. What
does this mean? All of these factors have fluctuated between
forecast runs over the past 24 hours and will likely continue to
fluctuate until we get closer to the event, though ultimately,
confidence is increasing slightly in this being a multi-day
atmospheric river event leading to at least minor flooding
impacts.

The main period of concern for river flooding would be late
Monday into Thursday as it takes time after rain begins for
rivers to rise. HEFS guidance has introduced chances for a
number of rivers to rise to Moderate flood stage (10-25%) and
Major flood stage (8-15% chance) for rivers that drain from the
Coast Range, Willapa Hills, and north Oregon and south
Washington Cascades. The probabilities for specific river
points can be found at the National Water Prediction Service
website. Signals are also increasing for the potential of areal
and small stream flood impacts for the Northern Willamette
Valley north into the Washington lowlands (5-10% chance) as well
as the Coast Range, Willapa Hills, and Cascades (30-50%
chance), including the potential for landslides due to saturated
soils. Additionally, there are signals for breezy winds with
the frontal passage on Monday, although the ensemble spread
remains very wide leading to low certainty in the peak wind
forecast. If winds do end up even around 30-40 mph, saturated
soils from nearly a week of rain could cause some downed trees,
leading to impacts such as localized power outages. This is an
event to keep your eye on, especially for those who live in
flood prone regions. -03/DH

&&

.AVIATION...Rain shower coverage has diminished this evening,
particularly inland while coastal terminals report more
persistent rain, drizzle, or mist. This general pattern will
continue through much of the overnight period while chances for
IFR cigs increase across the region. Terminals along the coast
and near the Portland area/northern Willamette Valley are
already seeing IFR cigs for the most part, with confidence
increasing in continued IFR cigs through the next 3-6 hours. In
other portions of the Willamette Valley, low-end MVFR cigs are
favored (50-70% confidence) to degrade to IFR generally by 09z
Fri. Along the coast, LIFR cigs are very likely (80-100%) to
continue through much of the period. Winds largely out of the
south to southeast at 5-10 kt overnight.

Another round of steady rain looks to arrive by 12z Fri along
the northern OR coast, reaching the Willamette Valley by
15-18z Fri. This steady rain should see vis restricted to MVFR,
although there is a 40-50% chance in IFR vis for inland
terminals. Along the coast, continued IFR vis is favored.
Mechanical mixing from rain may also aid in lifting cigs to low-
end MVFR inland. Chances for MVFR or better vis/cigs increase
after 00-03z Sat across the region. Southwest winds around 10 kt
may gust to 20 kt inland while coastal terminals gust to 20-25
kt after 15-18z Fri.

KPDX AND APPROACHES...High confidence in continued IFR cigs
overnight, while vis may improve to VFR/MVFR with mist as rain
shower coverage has decreased this evening. Additional steady
rain after 15-18z Fri will see vis restricted to MVFR (60%
chance) or IFR (40% chance) while cigs lift to around 1 kft.
Southerly to southwesterly winds around 10 kt with gusts to 20
kt expected through much of the day Friday. Chances for cigs
lifting to VFR increase late in the period, after 03z Sat. -36

&&

.MARINE...Winds are forecast to gradually weaken to 10-15 kt
and turn more westerly this afternoon and evening as the current
front progresses inland. This lull will be short-lived as
another frontal system will push across the waters tomorrow,
returning southwesterly winds of 15-20 kt with gusts to 25-30
kt. Small Craft Advisories are in effect across all waters
including the Columbia River Bar beginning 4 AM Friday, lasting
through early Thursday morning. Seas of 4-5 ft this afternoon
will build to around 7-9 ft at 10-11 sec 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 (70-80% chance) build above 10
ft by Saturday morning. There is also a 30-50% chance that seas
build above 13 ft, with the highest chances north of Cape
Lookout and beyond 10 NM.

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
briefly subside below 10 ft on Saturday night and Sunday, before
building again on Monday with the next system. -10

&&

.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 to 3 PM PST Friday for ORZ101.

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

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

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

PZ...Small Craft Advisory from 4 AM Friday to 4 AM PST Saturday for
     PZZ210-251>253-271>273.

&&

$$

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