846
FXUS66 KPQR 030535
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
935 PM PST Tue Dec 2 2025

.SYNOPSIS...High pressure nudges into the region later tonight
with dry conditions expected on Wednesday. Widespread fog and
frost return tonight through tomorrow morning. A series of
fronts will bring a period of wet weather Thursday through the
weekend across southwest Washington and northwest Oregon with
potentially more impactful rain next week.

&&

.DISCUSSION...Tuesday afternoon through Monday...Satellite
imagery Tuesday afternoon shows low to mid-level clouds across
much of NW Oregon and SW Washington, except for some clearing
occurring in the central and southern Willamette Valley as
northwest flow aloft increases. The few light showers that
linger over the high terrain this afternoon should dissipate by
this evening as high pressure builds overhead and into the
eastern Pacific, with dry weather continuing into Wednesday.
Clearing is expected to continue through the evening hours
across the interior lowlands, and CAMs (HREF and REFS) indicate
this leads to 70-90% chance for low stratus and 50-70% chance
for fog to form late tonight into Wednesday morning across this
area. Additionally, temperatures will be on the colder side
again tonight, falling into the mid to upper 30s across much of
the region except for 40s right along the coast line and 20s
over the Cascades.

Our extended cool and mostly dry weather pattern comes to an end
Thursday as a series of upper level shortwaves and attendant
frontal systems are slated to impact the PacNW into next week.
The parent low- pressure system associated with the frontal
systems impacting the region Thursday into Friday is currently
centered around the Aleutian Islands. This low is funneling
tropical moisture north along the west side of the upper level
ridge, which will then travel over the ridge then south into the
PacNW by Thursday. As the first front moves through the region
Thursday from north to south, IVT values are expected to
increase above 250 kg/m/s. However, the interior valleys will
likely be rainshadowed with not as much rain accumulating,
though the higher elevations and the coast north of Lane County
will likely see steady rain. However, the ridging will begin to
flatten Thursday night into Friday, allowing for more zonal,
westerly flow into the region. Friday is also expected to be
when IVT values peak in the area around 500-750 kg/m/s. These
components together will allow for less rainshadowing in the
interior lowlands with continued steady rain over the terrain
and coast through Friday. As the IVT stream continues into
Saturday, the ridge continues to become zonal, with similar
conditions to Friday. Latest forecast indicates rain totals
from Thursday to Sunday will be around 1-1.5 inches in the
interior lowlands and 1.5-3.5 inches along the coast, Coast
Range, and Cascades. The locations with highest probabilities of
exceeding 3 inches of rain are localized to the Coast Range in
Tillamook County (40-60%) and far north Oregon and SW Washington
Cascades (50-60%). By Sunday, upper level flow is expected to
be completely zonal over the region with another front slated to
move through. This will bring another round of widespread rain.
Overall, widespread flooding impacts are not expected with this
round of rain, though flashier rivers in the Coast Range and
Willapa Hills such as the Grays River near Rosburg and Willapa
River at Willapa could see significant rises if hourly rainfall
rates exceed 0.25-0.3 inches for multiple hours in a row. Will
have more details about potential rainfall rates as we get
closer to this event.

By Monday into the middle of next week, more uncertainty is
introduced in the forecast as ensemble members struggle to
resolve specifics in the pattern. Ultimately, the GEFS and Euro
ensembles both suggest the potential for another atmospheric
river late Monday into Wednesday. However, there is significant
spread in the members between where the strongest band of
moisture will funnel between WA and OR along with the maximum
strength of the IVT values. One thing to note is soils will
already be saturated and river levels elevated from the days of
rain leading up to this event. Depending on the strength of this
this event, flooding could definitely be a concern. HREF
guidance for many rivers indicate a 10-25% chance of certain
rivers reaching action stage and a 5-10% chance for minor flood
stage, mainly focused on the Dec 9-10 time period. Additionally,
there are signals for breezy winds of some sort this the frontal
system associated with this event, although the spread is also
very wide, and if winds do end up even around 30-40 mph,
saturated soils could cause some localized impacts due to downed
trees. Keep in mind there`s still the potential that this event
could be more similar to the lower impact event of this weekend.
It is worth keeping an eye on the forecast as we get closer to
this event. -03

&&

.AVIATION...Largely VFR conditions in place as of 05z Wed,
however post-frontal clearing overnight will allow for
widespread fog and low stratus inland by 10-12z Wed. IFR/LIFR
conditions are likely, 45-80% chances, at inland terminals. Vis
favored to improve back to MVFR/VFR by 15-18z Wed, then cigs
improve as low stratus breaks up by 18-22z Wed. Most guidance
tends to clear these conditions too early this time of year, and
while corrections have been attempted to account for this
timing bias, there are 10-30% chances that MVFR cigs in
particular linger past 21-22z Wed, most likely in the southern
Willamette Valley including at KEUG. Calm to light and variable
winds are expected inland, building to 2-5 kt out of the north
to northwest Wednesday afternoon. Along the coast, light
offshore flow will limit chances for widespread fog, however
favored east-southeasterly flow at KAST could still see periods
of MVFR vis within mist overnight. Otherwise, largely VFR
conditions with easterly winds turning northerly to
northeasterly Wednesday afternoon.

KPDX AND APPROACHES...Current VFR conditions expected to trend
downward through the next 6 hours as fog and low stratus
develops. Chances for 1/2SM or less vis are 50-60% through
much of the overnight period, before vis restrictions improve by
16-17z Wed. Low stratus may linger later into the morning, but
is expected to break up by Wednesday afternoon. Calm to light
and variable winds overnight will increase out of the north-
northwest to 3-5 kt. -36

&&

.MARINE...Northerly winds will weaken through the evening,
dropping below 10 kt overnight. A long period westerly swell
moving into the waters will keep wave heights around 8-10 feet
with a period of 14-16 seconds through tonight. Waves drop to
wound 5 feet by late Wednesday and should remain close to this
level through at least Thursday night.

A more active pattern develops later in the week as a series of
frontal systems is expected to move across the waters beginning
Thursday. Winds turn southerly on Thursday with a 30-50% chance
for wind gusts to reach 21 kt, highest probabilities across the
northern waters. Seas expected to remain below 10 feet through
there is a 10-15% chance of greater than 10 feet Saturday morning.
Expect this parade of fronts to continue through the weekend,
with the strongest of the fronts potentially on Sunday. -19

&&

.BEACH HAZARDS...A long-period westerly swell is expected to
continue along the northwestern Oregon and southwestern
Washington coasts through Wednesday. Waves may run up farther
than normal on beaches, including over rocks, logs, and jetties.
In addition, a period of perigean spring or "king" tides means
these waves will have an even easier time reaching high up onto
beaches, limiting the areas which may be safe from wave action.
A Beach Hazards Statement therefore remains in effect through
10 PM Wednesday. Sneaker waves can create potentially life-
threatening conditions in the surf zone where beachgoers can be
unexpectedly knocked off their feet and pulled out into cold
ocean waters. Caution should be used when in or near the water,
and those with children should be especially watchful. Never
turn your back on the ocean, and be sure to keep an eye on the
waves. Those participating in razor clam digs should
additionally exercise caution.

Additionally, there is a Coastal Flood Advisory for minor tidal
overflow flooding during high tide from 9 AM to 2 PM on Thursday
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. -19/36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Coastal Flood Advisory from 9 AM to 2 PM PST Thursday for
     ORZ101.

WA...Coastal Flood Advisory from 9 AM to 2 PM PST Thursday for
     WAZ201.

PZ...None.

&&

$$

www.weather.gov/portland

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