202
FXUS66 KPQR 251756 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
956 AM PST Tue Nov 25 2025

Updated aviation discussion.

.SYNOPSIS...A series of weather systems are expected to bring
widespread wet conditions to much the region on Tuesday through
Thursday with rising snow-levels helping to mitigate any
significant travel concerns over the Oregon Cascades passes
leading into Thanksgiving. That said, guidance continues to show
a trend towards a cooler and somewhat drier weather pattern
late weekend into early next week. Due to high model uncertainty
beyond Friday, the exact impact of this pattern change is
rather nebulous, however, the potential for our coldest
temperatures of the season are likely within reach by Monday.

&&

.DISCUSSION...Tuesday through Wednesday...Fog and low stratus
are being observed across much of the NW Oregon and SW
Washington interior lowlands early Tuesday morning with
satellite imagery indicating mid to high level clouds streaming
over the area ahead of an incoming frontal system. Surface
temperature observations show the central and southern
Willamette Valley have fallen into the mid 30s with mid to low
30s being observed across the foothills. Expect some locally
frosty conditions early this morning as temperatures could fall
another couple of degrees early this morning. A weather system
is slated to move inland from the Pacific later this morning
through tonight, ushering in warmer moist air and widespread
precipitation. As temperatures will still be on the colder side
when the front initially moves in, snow-levels will initially
begin around pass- level (3,000-4,000ft) before rapidly jumping
to 6,000-8,000ft this evening. Model soundings continue to show
a brief period of warm-air overrunning during this transition
coupled with a weak easterly wind component near the surface in
the Cascades, so after some snowfall, a few hours of a wintry
mix can`t be ruled near the passes before temperatures fully
warm to above freezing and transition precipitation to solely
rain.

Otherwise, an elongated band of rainfall is expected to sit over
over Western Oregon and Southwest Washington tonight through
Wednesday. Given the extended duration of this moisture
streaming overhead and a high probability (80%+) for IVT values
greater than 250kg/m/s in both the EPS and GEFS, this set-up
would technically be classified as an "atmospheric river",
albeit a weak one. There remains some uncertainty as to how the
axis of this moisture and the heaviest relative rainfall
fluctuates longitudinally, particularly on Wednesday, with most
models showing a shift southward into central Oregon before
swinging northward again later Wednesday in response to a
developing low over the eastern Pacific. Heaviest rainfall
amounts are likely to be north of Lane County with 0.5-1.25
inches across the interior lowlands and 1.25-2.5 inches across
the coast, Coast Range, and Cascades with localized amounts up
to 3-3.5 inches. It`s difficult to pinpoint where those
localized higher rainfall amounts will be and will depend on
where the front stalls. Some of the more responsive rivers in
the coast range and Willapa Hills may be more impacted from the
rain depending on where the front and moisture stall, but the
chances for minor river flooding appear low (<15%) - still worth
keeping an eye on.

For the developing low off the coast bringing the next weather
system, the 00z and 06z ensemble runs for the EC, GEFS, and GEPS
have both slowed down the low`s progression inland and shifted
the trajectory farther south into the Washington/Oregon coast.
There still remains uncertainty on exactly where along the coast
the low will move inland, but the majority of the ensemble
members now suggest this will be somewhere between central
Washington and northern Oregon instead of British Columbia as
previous guidance suggested. Additionally, guidance has pushed
back the onset of precipitation to be closer to sometime on
Thursday morning instead of Wednesday night with the low pushing
inland later on Thursday. The 01z NBM, which the current
forecast is based on, hasn`t fully picked up on this transition,
but if this trend continues with the 12z and beyond ensemble
runs, expect the NBM to begin shifting to this scenario.
Ultimately, there`s not much change in the overall impacts with
this system except for potentially slightly increased winds
depending on exactly where the low sets up. Current probability
for 30+ mph winds are 20-40% for interior lowlands and 70-80%
along the coast. Probability for 40+ mph winds drop to less than
10% across the interior lowlands and 30-45% along the coast.
Rain amount forecast could fluctuate over the next few model
runs, but overall not expecting a significant amount of rain,
generally 0.15-0.25 inch for the central and southern Willamette
Valley, 0.3-0.75 inch for the northern Willamette Valley into
the SW Washington lowlands, 0.5-1.5 inches along the coast and
Coast Range, and 0.5-2 inches along the Cascades. Snow levels
are expected to remain above pass level until early Friday, but
by then conditions are expected to become showery with limited
precipitation accumulation.

The rest of Friday into the weekend and beyond, model
uncertainty increases significantly as both deterministic and
ensemble systems struggle to resolve the placement of the upper-
level longwave pattern. Guidance is split as to whether another
shortwave trough off the coast of British Columbia dives
southward into the Pacific Northwest for Saturday, or if a high
pressure ridge over the eastern Pacific strengthens into the
Pacific Northwest and shunts this feature well to our east. At
the very least all models show the development of broad
northerly flow over the weekend which favors a cooling trend by
Sunday and Monday. The scenario in which a trough digs
southward just east of the Cascades would allow for a colder
airmass to seep into the region through the Columbia Gorge and
Cascade Gaps punching temperatures well below normal (overnight
lows in the mid 20s to near 30 across the lowlands). On the flip
side, scenario would keep the amplifying trough near the
Rockies with a more predominate ridge feature overhead,
resulting in less of an easterly wind influence. In any case,
the NBM probability for subfreezing low temperatures across the
lowlands is decent (generally 45-75%) by Monday morning which
will give us a fighting shot for the lowest temperatures of the
season thus far. For reference our lowest overnight minimum
temperatures from Oct through Nov are as follows: Portland 36F,
Salem 32F, Hillsboro 32F, and Eugene 30F. All in all, the
overall model spread is large during this period with any
impacts beyond the decrease in temperatures tough to nail down -
forecast confidence is low Friday onward.

Definitely keep an eye on the forecast as we go through this
week, especially if you have holiday travel plans. -99/03

&&

.AVIATION...Areas of fog and low stratus have begun lifting as of
1730z Tuesday, resulting in mainly VFR flight conditions aside
from isolated pockets of LIFR cigs lingering, impacting KEUG and
KUAO. That said, expect cigs to lift to VFR thresholds at these
two terminals by 19-20z.

Widespread stratiform rain will begin spreading across all of
northwest Oregon and southwest Washington this morning at the
coast and this afternoon inland. Rain has already begun at KAST.
Once rain becomes steady, expect widespread surface visibilities
between 3-5 SM through tonight. Cigs will gradually lower between
06-12z Wednesday from north to south, with probabilities for cigs
below 3000 ft increasing to 70-90% and probabilities for cigs
below 1000 ft increasing to 10-30%, except 80% at the coast where
IFR cigs are more likely to occur.

KPDX AND APPROACHES...VFR cigs/visibilities will likely continue
through at least 00z Wednesday before cigs lower to MVFR
thresholds or lower Wednesday evening as steady rain develops.
Probabilities for cigs below 3000 ft rapidly increase to 80% at
03z Wednesday. Probabilities for cigs below 1000 ft increase to
30% at that time as well. -23

&&

.MARINE...A frontal system is approaching the waters Tuesday
morning and will move over the waters this afternoon through
early Wednesday morning. Southerly wind gusts will increase ahead
of and along the front with widespread gusts up to 30-34 kt
expected. There is a 10-15% chance for occasional gale force wind
gusts over 34 kt over the northern and central waters, though not
frequent or widespread enough to justify the issuance of a Gale
Warning. Therefore, a Small Craft Advisory remains in effect. Seas
will rise to 9-11 ft at 12 seconds by Tuesday night, continuing
through early Wednesday morning, so the Small Craft Advisory
continues into Wednesday morning for the seas.

Another frontal system approaches the waters sometime Wednesday
night into Thursday, though there are still uncertainties in exact
timing. Another round of increased southerly wind gusts up to at
least 30 kt is expected. There`s a 45-60% chance for maximum wind
gusts to peak at gale force wind gusts over 34 kt for all marine
zones, though 3 hourly probabilities drop to 10-20%, indicating
widespread gale force gusts are unlikely. Seas are expected to
rise again, peak near at least 14-15 ft on Thursday. There is a
15-25% chance seas will peak around 18-20 ft, with a 5-15% chance
seas will reach 20-22 ft. This is due to the uncertainty
regarding the exact strength and track of the closed surface low
and the frontal system accompanying the low. Regardless of the
outcome for this system, it appears winds and seas will lower
significantly on Friday, especially late in the day. Winds and
seas over the upcoming weekend are currently forecast to stay
below small craft advisory thresholds with a period of offshore
flow. -23/03

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 1 AM PST Wednesday for PZZ210.

     Small Craft Advisory until 10 AM PST Wednesday for PZZ251-252-
     273.

     Small Craft Advisory from 4 PM this afternoon to 10 AM PST
     Wednesday for PZZ253.

     Small Craft Advisory until 10 AM PST Wednesday for PZZ271-272.
&&

$$

www.weather.gov/portland

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