936
FXUS66 KPQR 052039
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
138 PM PDT Sat Sep 5 2026

.SYNOPSIS...Cooler than average temperatures will continue tomorrow
with chances with showers and isolated thunderstorms as a closed
upper level low remains in place over the region. Trending drier
Monday through Wednesday with temperatures gradually warming each day
as low-level offshore flow develops. The forecast becomes more
uncertain Thursday and Friday, with the potential for weather
conditions to range anywhere from cool and showery to warm and dry
depending on the evolution of the large scale weather pattern.

&&

.SHORT TERM...Now through Monday night...An observed 500 mb height
analysis from 5am PDT Saturday depicted a 562 dam closed low pressure
system centered directly over northwestern Oregon and western
Washington. This is the same low that has brought cool and showery
weather conditions to the area the past few days. The latest model
ensemble guidance is in good agreement this low will remain in place
over the region through Sunday afternoon before finally moving east
of the Cascades Sunday evening/night into eastern Oregon, eastern
Washington, and western Idaho. This low will bring continued chances
for showers and isolated thunderstorms Saturday night through Sunday
afternoon across most of northwestern Oregon and southwestern
Washington (except at the immediate coast) before chances decrease
from west to east Sunday evening/night as the low progresses
eastward. With the excess cloud cover and occasional rain on Sunday,
temperatures will remain cooler than average for early September with
highs mainly in the mid 60s to lower 70s, coolest at the coast and in
the Cascade foothills. Rain amounts on Sunday will be noticeably
higher for many inland valley locations tomorrow when compared to
Saturday. Although mainly dry conditions are favored at the coast,
relatively wetter conditions are expected inland over the Willamette
Valley, greater Portland/Vancouver metro area, Columbia River Gorge,
Cascades/foothills, and Upper Hood River Valley. These areas have a
20-40% chance for 0.25 inches of rain or more between 11pm PDT
Saturday and 11pm PDT Sunday, except 45-55% in the Cascade foothills
and 55-85% in the Cascades. The additional rain will continue to
dampen fire season and keep wildfire smoke out of the area.

Although the aforementioned low will be well to the east of the
Cascades on Monday, a shortwave trough rotating counter-clockwise
around the low is set to move over Washington and possibly far
northern Oregon. Model ensemble guidance suggests this feature will
most likely produce very little to no rain at all, and the NBM backs
this up with probabilities only ranging between 1-15% for 0.01 inches
or more between 5am PDT Monday and 5am PDT Tuesday. -23

.LONG TERM...Tuesday through Friday night...Temperatures will trend
noticeably warmer Tuesday into Wednesday with high confidence for dry
weather as low-level offshore flow begins to develop. On Tuesday,
afternoon temperatures will warm into at least the mid to upper 70s,
except lower 70s at the coast. Come Wednesday, widespread highs in
the 80s are expected. Even the coast has a chance to reach 80 degrees
or warmer on Wednesday if offshore winds last into the afternoon
hours (1-15% chance, except a 30-50% chance for inland coastal
communities such as Astoria and Tillamook).

Come Thursday and Friday, uncertainty increases substantially in
regards to the large scale weather pattern and whether or not we will
remain warm and dry or transition back to a cool and showery weather
pattern. On Thursday, a cluster analysis for 500 mb heights and
anomalies shows around 40% of the total ensemble space from the
GEFS/ENS/GEPS produces weak upper level troughing over B.C., western
WA and far northwest OR. The remaining 60% of the total ensemble
space depicts some degree of ridging over the area. The current
deterministic forecast aligns with the latter outcome, suggesting the
continuation of dry weather with highs still in the 80s for inland
valleys, albeit several degrees cooler than Wednesday. By Friday,
clusters come into better agreement for upper level troughing (around
85% of the total ensemble space shows some degree of troughing over
or near the area). However, there are significant differences in the
exact amplitude and location of this trough, which will impact
precipitation chances and temperatures. As such, temperature spread
on the NBM 1D Viewer is very large both days, ranging from 70-90
degrees on Thursday and 65-85 degrees on Friday. Friday currently has
a higher chance for conditions to trend cooler and wetter compared to
Thursday, however the outcome is still uncertain. Anyone with outdoor
plans late in the week should monitor the forecast for updates over
the next several days. -23

&&

.AVIATION...Through 00z Monday...A low pressure system continues
to meander overhead, bringing continued moist conditions with
light winds and persistent low chances for rain showers. VFR
conditions are nonetheless favored across the airspace. Showers
are most likely in areas of terrain, particularly the Cascades,
with 15-25% chances for an isolated thunderstorm which could
produce locally heavy rain, gusty and erratic winds, and small
hail through this evening. Terminal impacts from any light rain
showers are unlikely, and would be very brief if they do occur.
Otherwise, VFR cigs/vis continue with light winds of 5 kt or less
across the area. Chances for MVFR cigs and additional rain
showers increase after 06-12z Sun as low stratus coverage
increases along the coast, lower Columbia and Cowlitz Valleys, and
the southern Willamette Valley, and rain showers spread westward
from the Cascades toward the I-5 corridor.

KPDX AND APPROACHES...VFR conditions expected through at least 12z
Sun. Light northwest winds below 5 kt become variable after 06z
Sun. Low chances, <10%, for rain showers persist through much of
the day, but increase to 25-35% after 12z Sun. Low cloud coverage
also increases overnight, with 15-25% chances of MVFR cigs after
12-15z Sun. -36

&&

.MARINE...Surface low pressure centered on the Oregon coast will
continue to weaken as its parent upper-level low meanders
overhead. Weak high pressure beginning to build offshore will see
light northerly winds of less than 15 kt develop. Rather benign
conditions then continue through at least the middle of next week
with surface high pressure strengthening as the stagnant upper
low ejects eastward, favoring stronger north winds. Seas continue
at 3-6 ft with a dominant northwest swell at 8-10 seconds. -36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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