406
FXUS66 KPQR 042304
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
303 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...Radar imagery and terminal observations as of early
Thursday afternoon depicts a mix of LIFR/IFR/MVFR CIGs and VIS
across all terminals as a warm front lifts over the region and
brings rain and mist. High confidence (>90% chance) that IFR/MVFR
CIGs and VIS continue through the evening as this system progresses
inland. Another frontal system will move in early tomorrow morning
after 12-15z Fri, maintaining these low CIGs as the low-level
atmosphere remains saturated. Guidance suggests potentially heavier
rain with this second system, so there could be brief VIS reductions
to IFR or lower for any terminal Friday morning. Winds generally
southerly to southwesterly, remaining under 10 kt through late
tonight. Southerly winds increase to 10-15 kt Friday morning for
most terminals with gusts up to 20-25 kt along the coast.

KPDX AND APPROACHES...High confidence for IFR/MVFR CIGs and VIS
through Friday morning as back-to-back frontal systems impact the
area. East-southeasterly winds 4-6 kt increase and turn more
southerly 8-10 kt after 12-15z Fri.     -10

&&

.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 until 6 PM PST this evening for PZZ210.

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

$$

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