860
FXUS66 KPQR 181000
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
300 AM PDT Fri Sep 18 2026

.SYNOPSIS...Temperatures will be around ten degrees cooler than
yesterday for inland valleys today due to persistent morning cloud
cover and stronger onshore flow. Temperatures warm up a few degrees
this weekend as an upper level shortwave ridge moves over the area.
Trending a bit cooler again next week as an upper level trough slowly
drops southward from the Gulf of Alaska. Dry conditions are heavily
favored through Thursday with a possible pattern shift
toward cooler and wetter conditions by next Friday, September 25th.

&&

.DISCUSSION..Now through next Friday...Satellite observations from
early Friday morning showed a low marine stratus deck along the coast
and Coast Range expanding into the lower Columbia and southern
Willamette Valley. This cloud deck will most likely (60-80% chance)
continue to expand towards sunrise, eventually filling in across the
central and northern Willamette Valley, Cowlitz Valley, and greater
Portland/Vancouver metro area. Morning clouds will eventually give
way to afternoon sun as daytime heating progresses, except at the
coast where skies will likely remain mostly cloudy through the day
(>60% chance). Given the stronger onshore flow in place today and
persistent low clouds in the morning, temperatures will be around ten
degrees cooler than what was observed yesterday for inland valleys.
Expect highs generally in the mid 70s Friday afternoon over the
lowlands, except mid 60s at the coast.

Temperatures will warm a bit over the weekend. This is when model
ensemble guidance depicts good agreement for an upper level shortwave
ridge moving from west to east over the Pacific Northwest. It appears
this ridge will move rather slowly with a closed upper low on the
southwestern periphery of the ridge and another closed low on the
eastern periphery of the ridge. Low-level onshore will favor at least
some degree of morning cloud cover this weekend, however forecast
soundings suggest the marine stratus deck will become more shallow.
This means any cloud cover that does develop inland should dissipate
quickly during the mid to late morning hours. Combined with warming
temperatures aloft, this should allow for highs back into the upper
70s to lower 80s inland and upper 60s to around 70 degrees at the
coast, warmest on Sunday.

The pattern trends relatively cooler next week as 500 mb flow becomes
nearly zonal ahead of broad troughing approaching from the Gulf of
Alaska. As this trough slowly approaches, temperatures will cool
gradually Monday through Wednesday. However, cooling does not look
overly significant as highs in the 70s are still favored inland with
highs staying in the 60s at the coast. Confidence is high dry weather
will continue Monday through Thursday afternoon, as 85-95% of the
total ensemble space from the ENS/GEFS/GEPS shows no QPF at all. The
remaining 5-15% showing very light rain amounts is mainly being
driven by a few GEPS members. It appears a frontal system over the
northeast Pacific will approach western Washington and western Oregon
mid-week, which helps explain the small subset of ensemble guidance
showing at least some light rain. However, the vast majority of
ensemble guidance suggests this frontal system will decay over the
coastal waters before pushing inland, hence the reason dry weather is
the most likely outcome. That being said, chances for cooler and
wetter weather increase more considerably come next Friday, September
25th. This is when roughly 40% of the total ensemble space from the
ENS/GEFS/GEPS combined shows rain moving into the area. The remaining
60% keeps conditions dry. -23

&&

.AVIATION...Through 06Z Saturday...Satellite and surface weather
observations from 10Z Friday depicted an expansive marine stratus
deck extending along the coast and Coast Range, with ceilings
around 1000-1500 ft from KAST to KONP. Confidence remains high
this marine stratus deck will expand into the Willamette Valley
and I-5 corridor by 12-15Z Friday at the latest. There is a
60-80% chance at any given hour between 12-18z Fri for MVFR
ceilings around 1500-3000 ft across inland terminals, except for
a 100% chance at KEUG where stratus has already developed. Stratus
should gradually dissipate after 19-21z Friday across the Valley
as daytime heating progresses, bringing a return to widespread VFR
conditions inland. However, the coast will most likely remain
socked in with MVFR ceilings through the day (>60% chance).

KPDX AND APPROACHES...VFR conditions with clear skies will most
likely give way to MVFR ceilings with an incoming marine stratus
deck between 13-17Z Friday (70-80% chance). Ceilings then trend
back to VFR after 19-21Z Friday as low clouds gradually scatter
out and skies become clear again. Confidence is moderate in
regards to the timing for the morning stratus deck to lift in the
afternoon. -23

&&

.MARINE...High pressure will strengthen over the waters on Friday
and then remain in place through the weekend. Relatively light
winds on Friday will strengthen out of the north Saturday
afternoon and evening with gusts up to 15-20 kt, strongest over
the central and southern waters. Northerly winds peak in strength
Sunday afternoon and evening when there is a 70-90% chance for
maximum wind gusts of 21 kt or slightly stronger across the
waters, with the highest chances across the inner waters out to 10
NM offshore. Given wind gusts of this magnitude are not expected
to be frequent or widespread, Small Craft Advisories are not
anticipated at this time. Significant wave heights will remain
around 4 to 6 ft through the weekend, mainly driven by a persistent
northwesterly swell with a dominant wave period near 8 seconds.

Northerly winds are expected to linger Monday through Wednesday,
with wind gusts generally staying under 20 kt. A long period
west-northwesterly swell is set to arrive either Monday afternoon
or evening, pushing seas up by one or two feet. However, seas will
not be particularly steep as the dominant wave period with this
swell will be around 11-13 seconds. -23

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

www.weather.gov/portland

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