173
FXUS66 KPQR 040530
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
930 PM PST Wed Dec 3 2025

.SYNOPSIS...Today is the last dry day before a series of fronts
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 is very high.

&&

.DISCUSSION...Wednesday afternoon through Tuesday...Today is the
last dry day in at least the next week before a series of
weather systems associated with Pacific moisture produce
multiple rounds of widespread rain over NW Oregon and SW
Washington. Although today remains dry, satellite imagery
indicates mid- to high-level clouds streaming over the region
Wednesday afternoon ahead of the first frontal system. These
clouds are keeping temperatures on the cooler side with 2 PM PST
temperature observations in the low to mid 40s across much of
the interior except for the 30s over the Cascades. Areas of
patchy fog and frost are possible for interior valleys once
again late tonight through early tomorrow morning before rain
begins spreading inland.

Upper level high pressure over the region and into the eastern
Pacific will become more zonal over the next couple of days as
the first weather system moves over the ridge. A warm front
associated with this weather system will approach the coast from
the northwest late tonight, then stall just off of the coast
through tomorrow. This will bring the first round of widespread
rain, beginning around midnight to 3AM along the northern coast
then quickly spreading inland through the early morning hours
with showers continuing through the night. Due to the northwest
orientation of winds, the Willamette Valley will likely be rain
shadowed and see lower rain accumulations. By Friday morning,
the cold front begins moving closer to the coast, pushing the
warm front inland. This cold front will bring a stronger surge
of moisture with IVT values peaking around 500-700 kg/m/s, and
round two of widespread heavier rain impacts the region through
Friday afternoon. The flow will be more westerly at this point,
allowing more precipitation to fall in the Willamette Valley.
The precipitation amounts through Saturday night are expected to
be around 0.4-0.6 inches for the central and southern Willamette
Valley, 0.6-1 inch from the Portland area north through the I-5
corridor, 0.9-2 inches along the coast and Coast Range, and
0.9-3 inches along the Cascades. Some elevated winds are likely
as the cold front passes through on Friday with a 20-40% chance
of wind gusts of at least 30 mph for inland valleys and a 50-75%
chance along the coast. There`s a 20-50% chance of gusts of 40
mph or higher along the Cascades.

Guidance suggests the cold front might stall and weaken over the
PacNW into Saturday, with showers lingering through Saturday.
Then, another frontal system is slated to move through the area
on Sunday. Ensembles indicate IVT values could peak around
400-500 kg/m/s with this frontal system which would produce
another surge of widespread rainfall. A bit uncertainty exists
in rain amounts during this 48 hour period. Looking at the 25th
to 75th percentiles, the interior lowlands could receive
anywhere from 0.3-1 inch of rain, lowest amounts in the southern
Willamette Valley. The coast, Coast Range, and Cascades could
see anywhere from 0.5-2.5 inches, lowest in Lane and Lincoln
counties. Impacts are not expected with rain amounts Thursday
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 at Willapa. This will be something to monitor
during heaviest periods of rain.

The main period of concern for the forecast period is next week
as ensemble guidance continues to indicate the potential for an
impactful atmospheric river event Monday through Wednesday.
Significant uncertainty still remains in the length of elevated
IVT values, location of the moisture along the WA and OR coast,
and exact peak of IVT values (the 12z GEFS mean is around 550
kg/m/s with +1 standard deviation around 800 kg/m/s and the 12z
Euro EPS mean is around 700 kg/m/s with +1 standard deviation
around 900 kg/m/s). These factors have jumped around between
forecast runs over the past 24 hours and will likely continue to
fluctuate until we get closer to the event. There is the
potential that this ends up being another moderate atmospheric
river event, similar to the preceding events. If this is the
case, impacts are likely to be minimal once again. However, if a
strong to extreme atmospheric river event materializes, there
is the potential for widespread river flooding and/or wind
damage as soils will already be saturated and river levels
higher from the rain from the 4 days prior. The main period for
river flood concerns is Tuesday into Wednesday as it takes time
after rain begins for rivers to rise. HEFS guidance indicates a
10-25% chance of rivers reaching Minor flooding for Coast Range
and Coastal rivers and 5-20% chance for Willamette River
tributaries, mainly focused on Tuesday into Wednesday, possibly
beyond depending on the rain forecast and/or response times of
the rivers. Additionally, there are signals for breezy winds
with the frontal passage, although the ensemble spread remains
very wide leading to low certainty. If winds do end up even
around 30-40 mph, saturated soils could cause some localized
impacts due to downed trees. Keep an eye on this forecast,
especially if you live in flood prone regions. -03

&&

.AVIATION...Largely VFR flying conditions remain in place across
the region, with intermittent MVFR cigs at terminals along the
Lower Columbia and in the Portland area head of an approaching
frontal system. Abundant low-level moisture will favor
widespread MVFR cigs inland as rain begins by 12-15z Thu. Ahead
of the onset of rainfall, the southern Willamette Valley may see
enough clearing to yield areas of IFR vis/cigs within fog and
mist after 09-12z Thu, including at KEUG. As rain continues
through the morning, there are 50-60% chances for IFR cigs for
most inland terminals, however cigs look to lift into the
afternoon while chances for vis restrictions increase after
18-21z Thu. Calm to light and variable winds overnight will
increase out of the south to southeast at around 5 kt.

Along the coast, there is higher confidence in persistent VFR
conditions are expected overnight with light offshore flow,
before cigs trend downward to MVFR as rain begins by 09-12z
Thu along the southern WA and northern OR coast, and by 12-15z
along the central and southern OR coast. While MVFR conditions
are most likely after 12z Thu, there is a 10-20% chance of IFR
cigs Thursday afternoon as rain continues. Winds will turn out
of the south-southwest and increase to around 10 kt by 18z Thu.

KPDX AND APPROACHES...Mixed VFR/MVFR cigs expected ahead of
precipitation onset by 12z Thu with calm to light and variable
winds. 40-60% chance of IFR cigs through the early morning,
trending toward MVFR by Thursday afternoon with southerly to
southeasterly winds around 5 kt. -36

&&

.MARINE...High pressure over the waters today will maintain
northerly winds 10 kt or less through tonight. Seas around 6 to
7 ft continue to subside as well.

Active weather returns as winds turn southerly on Thursday ahead
of the first in a series of fronts. Chances for frequent wind gusts
greater than 21 kt remain below 15% on Thursday, with winds
turning more westerly behind the front. However, another slightly
stronger front moving through on Friday will bring a 50-70% chance
for wind gusts greater than 21 kt, with the highest chances north
of Cape Foulweather. Will likely need to issue a Small Craft
Advisory for Friday with the upcoming forecast package. The only
Small Craft Advisories in effect right now are for the Columbia
River Bar this afternoon and Thursday afternoon due to very strong
ebbs bringing seas in the Main Channel up to 9-10 ft.

A series of fronts continue through the weekend and into early
next week. There is a 50-60% chance for isolated gale force wind
gusts on Monday, but chances for frequent and widespread Gales are
only 15-30%. Seas remain around 6-8 ft at 11-12 seconds through
Friday, building on Saturday due to an increasing west-northwesterly
swell. There is a 60-80% chance of seas exceeding 10 feet by late
Friday night into Saturday morning. -10

&&

.BEACH HAZARDS...There is a Coastal Flood Advisory for minor
tidal overflow flooding during high tide from 9 AM to 3 PM
Thursday and Friday for the South Washington Coast and the 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. Remain
out of the water to avoid hazardous swimming conditions. -10

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

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

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

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

PZ...Small Craft Advisory from 2 PM to 6 PM PST Thursday for PZZ210.

&&

$$

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