737
FXUS66 KPQR 041806 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1006 AM PST Thu Dec 4 2025

...Updated aviation discussion...

.SYNOPSIS...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...Today through Wednesday...December is, on average, the
wettest month of the year here in northwest Oregon and southwest
Washington. So it should not come as a big surprise that the stretch
of mostly dry weather to start the month would not last too long.
Expect varying amounts of rainfall each day through this next week,
with potential for heavier rain early next week, as a series of
weather systems produces multiple rounds of widespread rain across
the Pacific Northwest.

An upper level ridge of high pressure remains over the northeast
Pacific today, but eventually shifts southeast, flattens, and allows
for more zonal flow into the region. A warm front associated with
this initial system will stall near the coast today, spreading light
rain across much of the area. Due to the northwest orientaion of
winds, the Willamette Valley will likely be rain shadowed and see
lower rain accumulations through this evening. The flow turns a bit
more westerly on Friday, and an associated plume of deeper moisture
will attend a cold front approaching the coast. IVT values are
expected to peak around 500-700 kg/m/s, bringing a second round of
widespread, heavier rain through Friday afternoon. Breezy southwest
winds will also be possible with the frontal passage on Friday, with
gusts most likely peaking around 20-30 mph.

Showers expected to linger through Saturday as onshore 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 Thursday through Sunday
are most likely to be around 1.0-2.0 inches for interior lowlands,
2.0-4.0 inches at the coast, and 3.0-6.0 inches for the Coast Range
and 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 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 00z GEFS mean is around 650 kg/m/s
with +1 standard deviation around 800 kg/m/s, and the 12z Euro EPS
mean is around 750 kg/m/s with +1 standard deviation around 850
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 or landslides 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 10-30% 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. Of note, HEFS guidance also indicates a 8-15% chance of
exceeding Major flood stage at rivers such as the Nasselle, Willapa,
and Wilson near Tillamook.

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.  DH/03

&&

.AVIATION...Radar imagery as of late Thursday morning depicts
light rain across northwest Oregon as a warm front lifts over the
region. This is leading to a mix of LIFR/IFR/MVFR CIGs and VIS
across all terminals. As the frontal system progresses inland, there
is high confidence (>80% chance) that CIGs will lower to LIFR/IFR
along the coast by 21-23z Thu. For Willamette Valley terminals, CIGs
will trend toward low-end MVFR this afternoon and potentially drop
to IFR after 06z Fri (40-60% chance). Another frontal system will
move in early tomorrow morning after 12-15z Fri, maintaining these
low CIGs. Guidance suggests potentially heavier rain with this
second system, so there could be further VIS reductions to IFR or
lower for most terminals Friday morning. Winds generally southerly
to southwesterly, remaining under 10 kt through the TAF period.

KPDX AND APPROACHES...Warm front will maintain MVFR CIGs and
IFR/MVFR VIS through the evening, with CIGs lowering to IFR tonight
after 06-09z Fri as another frontal system approaches. Southeasterly
winds turning more southwesterly overnight around 4-6 kt.     -10

&&

.MARINE...Active weather returns as winds turn southerly this
morning ahead of the first in a series of fronts. Winds are
expected to increase to around 15-20 kt later this morning, with
gusts up to 25 kt across the coastal waters north of Cape Falcon
through early afternoon. Winds turn westerly behind the frontal
passage, lingering around 10-15 kt through tonight. West to
southwest winds then gradually increase on Friday to around 20-30
kt as the next front moves through. Have issued Small Craft
Advisories to account for these increased winds, today for the
northern zones, and on Friday for all coastal marine zones. Seas
around 5 to 6 ft today are expected to increase later Friday as a
northwest swell pushes into the waters through Saturday. Wave
heights likely build into the lower teens by Saturday morning,
while there is around a 30% chance that seas exceed 15 ft.

The parade of fronts continue through the weekend and into early
next week. There is around a 30-40% of wind gusts reaching 35 kt
on Sunday, while chances for Gales is higher (50-60%) later
Monday. Seas are likely to subside back under 10 ft by Sunday,
before building again Monday. /DH

&&

.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 this morning to 3 PM PST this
     afternoon for ORZ101.

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

PZ...Small Craft Advisory from 2 PM this afternoon to 6 PM PST this
     evening for PZZ210.

     Small Craft Advisory from 8 AM this morning to 2 PM PST this
     afternoon for PZZ251-271.

     Small Craft Advisory from 4 AM Friday to 4 AM PST Saturday for
     PZZ251>253-271>273.

&&

$$

www.weather.gov/portland

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