430
FXUS66 KPQR 211815
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1115 AM PDT Mon Sep 21 2026

Updated aviation discussion.

&&

.SYNOPSIS...Temperatures trend cooler through the workweek as
broad upper- level troughing develops over the Gulf of Alaska.
Dry weather with areas of morning fog and low clouds giving way
to afternoon will continue through Wednesday. The broad upper-
level trough over the Gulf of Alaska will deepen and progress
towards western Washington and Oregon late Thursday into Friday,
bringing increasing chances for widespread rain. Trending dry
over the weekend in the wake of this trough.

&&

.SHORT TERM...Now through Wednesday night...The short term
forecast is highlighted by continued dry weather, near normal to
slightly below normal temperatures, and areas of morning fog
and low clouds giving way to increasing afternoon sun. Expect
daily high temperatures in the low to mid 70s over inland
valleys and 60s at the coast. Morning lows will generally range
anywhere from the mid 40s to mid 50s, warmest over urban areas
and coolest over rural areas.

In regards to the large scale weather pattern, model ensemble
guidance remains in good agreement for increasing southwest flow
aloft ahead of broad upper level troughing over the Gulf of Alaska.
This trough will generally not be amplified enough yet to bring rain
back into northwest Oregon or southwest Washington through Wednesday
night, as the main weather system associated with this trough will be
impacting the British Columbia coast. The only potential exception to
that is Tuesday night through Wednesday morning along the south
Washington and far north Oregon coast when there is a very low chance
(5-10%) for light rain with an initial weak wave and attendant
decaying surface front.

Despite the lack of precipitation, 500 mb heights and temperatures
aloft will be on a gradual decrease as the trough begins to deepen
towards the middle of the week. This will allow temperatures to cool
slightly each day Monday through Wednesday, however only by a degree
or two each day. -23

.LONG TERM...Thursday through Sunday night...Troughing over the
Gulf of Alaska will approach the region in earnest late Thursday into
Friday as the trough becomes more amplified and progresses eastward
towards Washington and Oregon. As such, the 24-hour probability for
precipitation remains fairly high during that time, ranging between
55-65%. The main reason probabilities are not higher than that is due
to a subset of ensemble guidance that remains dry, mainly with the
GEPS as this ensemble system shows a higher likelihood of the trough
deamplifying after moving inland with the bulk of the precipitation
falling farther to the north over northwest Washington. While this is
not the most likely outcome, it is worth mentioning as it is still
within the realm of possibilities. Those with outdoor plans late
Thursday or Friday should monitor the forecast over the coming days
to see if rain chances trend up or down. If rain does fall, do not
expect any flooding impacts as rain amounts would most likely range
somewhere between 0.1-0.6 inches, highest in the mountains and lowest
over the Willamette Valley. The wettest model solutions produce
0.5-1.2 inches of rain, however chances for rain amounts this high
are only at 10% as suggested by the NBM 90th percentile. Even these
rain amounts would not be enough to produce flooding given the
extended duration.

Upper-level heights look to rebound quickly in the wake of the
shortwave trough, with high confidence for dry weather and slightly
warmer temperatures on Saturday given chances for rain fall below
10%. However, high pressure building in behind this trough Friday
night into the weekend will bring clearing skies and calm winds
during the overnight hours. This will bring an ideal set-up for
radiational cooling and chillier overnight low temperatures. Aside
from the high Cascades, overnight lows will be coldest in the Upper
Hood River Valley where frost development is possible Friday night,
Saturday night, and Sunday night. The NBM is showing a 30-50% chance
for morning lows below 36-37 degrees Friday night and Saturday night
near Parkdale and a 5-10% chance near Odell. Those with outdoor
vegetation that is susceptible to frost damage should monitor the
forecast over the coming days. -23

&&

.AVIATION...Through 18Z Tuesday...As of 18Z Monday, low marine
stratus had dissipated along the coast but remains in place
through the interior lowlands with ceilings ranging from 600-1500
feet, highest around the Portland metro and lowest in the
southern Willamette Valley. Stratus will continue to erode as
daytime heating increases and northerly winds help mix out the
cloud cover between 19-21Z for most locations, exact timing
remains uncertain. VFR conditions return to all terminals by this
afternoon with northerly to northwesterly winds 5-10 kt.
Conditions this evening are expected to be similar to the past few
evenings with stratus developing between 00-06z Tuesday and
spreading through the coast range gaps and along the Columbia
river into the metro and Willamette Valley between 08-12z Tuesday.

KPDX AND APPROACHES...At 18Z Monday, low stratus persists between
1000-1500 feet with VFR conditions returning by 20Z as skies
clear back out. Northerly winds around 5 kt expected. Stratus
develops and pushes eastward again late tonight into Tuesday
morning. -19

&&

.MARINE...High pressure will maintain predominately northerly
winds across the waters through Thursday. An exception would be
Tuesday afternoon to Wednesday morning, when winds may turn more
westerly for the waters north of Cape Falcon with the passage
of a weak front. Winds become more northwesterly and increase
slightly in strength over all of the coastal waters Thursday
night into Friday with the passage of another front. This front
will be relatively stronger than the frontal passage earlier in
the week, with widespread rain likely. Wind gusts should
generally stay under 20 kt through Friday, even with the two
frontal passages.

Seas are expected to increase to 6 to 8 ft Monday evening into
Monday night with the arrival of a long period west-northwestwerly
swell. Seas then remain near that range through Friday, but are
not expected to be particularly steep at any point in time due to
the dominant wave periods of 10 to 13 seconds. -23

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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