294
FXUS66 KPQR 202100
AFDPQR

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

.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 18Z Monday...Satellite imagery and terminal
observations as of late Sunday morning depicts predominately
IFR/LIFR stratus across the Willamette Valley and coast as high
pressure keeps CIGs low. Stratus is beginning to gradually
dissipate and should clear out for most terminals by 20-21z
Sun as daytime heating progresses. An exception would be the
central Oregon coast including KONP, where IFR stratus may
linger throughout the day. VFR conditions prevail across the
Valley through late tonight.

This evening after 00-03z Mon, there is high confidence (>80%)
for a return of IFR/MVFR stratus along the coast. Similar to
previous mornings, will see a return of stratus across the
Valley between 12-18z Mon with a 30-50% chance for MVFR CIGs at
any given hour. Unlike today, high pressure aloft will move
eastward tomorrow, so CIGs may not be as low tomorrow (10-20%
chance for IFR at any given hour across the Valley). However,
given that we`ll get decent onshore flow from an approaching
trough, stratus tomorrow morning could linger longer into the
afternoon. North/northwesterly winds today under 10 kt for most
terminals except 10-15 kt with gusts up to 25 kt at KONP.

KPDX AND APPROACHES...IFR CIGs and MVFR VIS through 19-20z Sun.
Afterwards, VFR conditions prevail through late tonight. MVFR
stratus returns after 12z Mon, with a 10-20% chance for IFR CIGs
at any given hour between 12-18z Mon. Northwesterly winds 4-6
kt today, weakening overnight. -10

&&

.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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