306
FXUS66 KPQR 210552
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1052 PM PDT Sun Sep 20 2026

.UPDATED AVIATION DISCUSSION.

&&

.SYNOPSIS...Temperatures trend cooler through the workweek as
upper-level troughing develops over the Gulf of Alaska. Dry
weather with morning clouds and afternoon sun continues through
midweek, before rain chances increase late in the week as
troughing moves over the Pacific Northwest.

&&

.SHORT TERM...Now through Tuesday...Guidance remains in good
agreement that a broad but low-amplitude ridge will steadily
shift eastward through the middle of the week while being
replaced by an upper-level trough digging southward over the
Gulf of Alaska. While onshore flow will continue through this
period, the lack of moisture within southwesterly flow aloft
will keep the forecast dry. As heights and temperatures aloft
both trend downward, surface temperatures will similarly cool a
few degrees each day, falling below seasonal norms by Tuesday.
Longer nights and light winds will favor periods of early
morning fog within sheltered valleys each day, with a typical
fall pattern of low clouds pushing inland overnight and clearing
through the day otherwise expected. -36


.LONG TERM...Tuesday Night through Sunday...Troughing over the
Gulf of Alaska will approach the region in earnest late in the
workweek. Tuesday night into Wednesday, an initial weak wave
will bring very low chances, 5-10%, for light rain to the
southern Washington and far northern Oregon coasts, but the
majority of rainfall is likely to remain farther north on the
Olympic Peninsula with dry weather otherwise continuing in
northwest Oregon and southwest Washington. Even without any
rain, temperatures will continue to trend cooler as onshore flow
strengthens and cloud cover increases.

Model consensus in the progression of the upper-level pattern
has increased, yielding improved confidence that the upper
trough will move overhead from Thursday afternoon through Friday
morning. Rainfall chances have therefore increased during that
time, with a 40-65% chance of rainfall amounts exceeding a
quarter of an inch in 24 hours across the region, highest over
the western slopes of local terrain and lowest on the eastern
slopes. Upper-level heights look to rebound quickly in the wake
of the shortwave trough, with 15% or less chances for rainfall
and a slight warming trend Friday into next weekend. Cool air
persisting in the mid-levels while skies trend clearer behind
the departing trough will raise the likelihood of overnight
lows falling into the mid 30s and supporting frost development
within the Upper Hood River Valley. At this time, the chances
for morning lows next weekend at or below 36 degrees range from
10% near Odell to 40% near Parkdale.      -36


&&

.AVIATION...Through 06Z Tuesday...Late this evening satellite
imagery and observations show building coastal low stratus
expected to fill in northward over the next several hours. Similar
to previous mornings, we`ll see a return of patchy fog and areas
of low stratus across the Valley between 12-18z Mon with a 40-60%
chance for MVFR/IFR CIGs at any given hour. However, high
pressure aloft which has been compressing the moisture layer
towards the surface will be progressing eastward by Monday
morning so there is lower confidence in truly widespread fog and
thus degraded VIS outside of spots like KEUG. Expect clearing by
the midday into the early afternoon hours as any inland fog/low
stratus breaks up followed by VFR conditions. Winds remain fairly
light, generally less than 10 knots at all sites.

KPDX AND APPROACHES...Expect VFR conditions with clear skies
prevail through late tonight. Low MVFR to near IFR stratus
returns 12-17z Mon, with a 35-40% chance for IFR and 15% chance
for LIFR CIGs/VIS at any given hour during this time frame. High
confidence in VFR conditions returning midday into the afternoon.
-99

&&

.MARINE...High pressure over the waters will maintain northerly
winds this week. Northerly winds peak in strength this
afternoon and evening with high confidence for maximum wind
gusts of 20-23 kt over the waters, strongest 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 still not anticipated for this afternoon.
Northerly winds are expected to linger through most of the week
with wind gusts generally staying under 20 kt. An exception
would be Tuesday afternoon to Wednesday morning, when winds may
turn more westerly for the waters north of Cape Falcon due to a
weak front.

Significant wave heights will remain around 3-5 ft through
tonight, mainly driven by a persistent west-northwesterly swell
with a dominant wave period of 8-9 seconds. A long period
northwesterly swell is set to arrive Monday afternoon, building
significant wave heights to 6-8 ft. However, seas will not be
particularly steep as the dominant wave period with this swell
will be around 12-13 seconds.      -10

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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