795
FXUS66 KPQR 250419
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
820 PM PST Mon Nov 24 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.

&&

.SHORT TERM...Now through Thursday night...Current satellite
and radar observations depict an area of shower activity
advancing through southwest Washington and Western Oregon late
this afternoon. This is all thanks to a weak mid-level shortwave
and accompanying frontal boundary. Confidence is high this
feature shifts out the region this evening followed by the
return of dry conditions overnight. With at least partial
clearing behind the front (already evident on Satellite),
lighter winds, and ample surface moisture, fog will once again
be a concern headed into the sunrise hours on Tuesday. Certainly
can`t rule out patches of local dense fog as well, especially
for more protected spots on the lee-side of the coast range
towards the I-5 corridor, and Tualatin Valley, where high
resolution models like the REFS project a 50-70% chance for
<0.25 mile visibility. It wouldn`t hurt to factor in foggy
conditions into your morning commute on Tuesday - make sure to
slow down and use your headlights.

Our attention then turns to our next weather system the second
half of Tuesday in the form of a warm-frontal boundary
ushering in a slug of warmer moist air and widespread
precipitation into the Pacific Northwest. It`s worth pointing
out snow-levels will initially begin around pass-level
(3,000-4,000ft) on Tuesday before rapidly jumping to
6,000-8,000ft Tuesday evening/night. Model soundings do 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 above-freezing
temperatures and just rain takes hold.

Otherwise, an elongated band of rainfall is expected to sit over
over Western Oregon and Southwest Washington Tuesday night
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 before swinging northward
again later Wednesday in response to a developing low over the
eastern Pacific. This may come into play for a few of our more
responsive rivers in the coast range and Willapa Hills
depending on where the warm-front and moisture stall, but the
chances for minor river flooding appear low (<15%) - still
worth keeping an eye on.

Then Wednesday night into Thursday the developing low off the
coast finally races onshore into either Western Washington
and/or far southwest British Columbia resulting in another surge
of moisture associated with a cold-front before we break into
post-frontal showers late Thursday/Thursday night. Throughout
this multi-day event, (Tuesday through Thursday afternoon)
anticipating rainfall amounts of 2-4 inches for the coast and
Coast range/Willapa Hills, 1-2 inches North Willamette Valley
into the SW WA lowlands, 0.30-0.90 inch central/South Willamette
Valley, and 2-4.5 inches for the Cascades (highest SW WA). -99


.LONG TERM...Friday through Monday...Late week 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
weakens 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. Exploring the Ensemble cluster analysis for Monday in
particular reveals roughly 35-40% of ensemble members between
the EPS, GEFS, and GEPS favor a scenario in which a trough digs
southward just east of the Cascades allowing 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, the other 60-65% of ensemble members 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`ll 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 over the
weekend. -99

&&

.AVIATION...Northwest flow aloft as weak high pressure moves over
the area tonight. Predominately VFR conditions this evening but
still expect areas of fog and low clouds to redevelop along the
coast and across the Willamette Valley overnight. By 08-09z
Tuesday, probabilities for surface visibilities below 1 SM and/or
LIFR/IFR cigs increase to 40-60% for all inland terminals, except
10% at KTTD due to the development of light east winds from the
Columbia River Gorge. Fog and LIFR/IFR cigs will likely begin
gradually lifting towards 13-16z Tuesday as widespread stratiform
rain begins pushing into the region from the west. Once rain
begins Tuesday morning, expect rain to persist through the day at
all terminals with cigs trending towards a mix of MVFR/VFR
thresholds, except IFR/MVFR at the coast and in southwest
Washington.

KPDX AND APPROACHES...Predominately VFR expected through 08-10z
Tuesday. Fog and/or low stratus with LIFR to IFR cigs is then
expected to develop thereafter (60% chance). By 14-15z Tuesday,
fog will likely lift with cigs gradually trending towards MVFR or
even low-end VFR thresholds as persistent rain develops. Once
rain begins Tuesday morning, expect rain to persist through the
day. Cigs and surface visibilities trend lower again around 00z
Wednesday onward with a 60% chance for cigs below 2000-3000 ft.

&&

.MARINE...Buoy observations from early Monday morning showed seas
ranging between 7 to 8 ft with a dominant wave period around 13 to
14 seconds. Winds were out of the south to southwest at 5 to 15
kt across the coastal waters, but will become more westerly while
weakening later Monday afternoon into the early evening. By late
Monday evening, light offshore develops, bringing a transition to
easterly winds over the waters. Offshore flow will be short-lived,
as winds will veer to the south early to mid Tuesday morning
ahead of the next frontal passage.

The aforementioned front will move over the waters Tuesday
afternoon through early Wednesday morning, bringing southerly wind
gusts up to 30 kt with a 15-30% chance for gale force wind gusts
up to 35 kt over the northern and central waters, except around a
50% chance over northern outer waters beyond 20-30 NM offshore.
Not expecting gale force wind gusts to be frequent enough or
widespread enough to justify the issuance of a Gale Warning.
Therefore, a Small Craft Advisory has been issued instead.

A stronger frontal system arrives Wednesday night into Thursday,
bringing southerly wind gusts up to at least 30 kt with a 45-55%
chance for gale force wind gusts over 34 kt for all marine zones.
Seas should peak near at least 14-15 ft on Thursday. There is a
10% chance seas will peak around 16-19 ft, with a 1-5% chance seas
will reach 20-22 ft. This is due to uncertainty regarding the
exact strength and track of a 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

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 10 AM Tuesday to 1 AM PST Wednesday
     for PZZ210.

     Small Craft Advisory from 10 AM Tuesday to 10 AM PST Wednesday
     for PZZ251-252-273.

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

     Small Craft Advisory from 4 AM Tuesday to 10 AM PST Wednesday
     for PZZ271-272.
&&

$$

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