655
FXUS66 KPQR 292139
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
239 PM PDT Sat Aug 29 2026

.SYNOPSIS...Weak low pressure will result in lingering showers
across southwest Washington and far northwest Oregon this
afternoon. Tomorrow through Monday, high pressure will return
dry and warm conditions with mostly sunny skies. Tuesday through
the end of the week, will see a significant pattern change
with cooler temperatures, widespread rain chances, and a 10-20%
chance for thunderstorms. Wednesday and Thursday are expected to
be the wettest days.

&&

.SHORT TERM...Today through Monday night...Radar imagery as of
early Saturday afternoon depicts lingering light showers across
southwest Washington and far northwest Oregon as an upper-level
trough gradually exits the Pacific Northwest. Later this
evening, expect conditions to dry out as high pressure rebuilds.
Will see ridging set-up over the region Sunday to Monday,
returning predominately warm and dry conditions with mostly
sunny skies. Temperatures tomorrow are expected to warm up
slightly with highs in the mid to upper 70s across interior
valleys with 60s along the coast. Monday will be the warmest day
of the week as the ridge reaches its peak amplitude and
supports afternoon highs in the low to mid 80s across interior
valleys.      -10

&&

.LONG TERM...Tuesday through Friday...A significant pattern
change begins on Tuesday as we head toward more fall-like
weather. The long term forecast is characterized by two separate
upper-level troughs, the first one approaching on Tuesday. This
initial trough will swing down from the Gulf of Alaska and track
towards southern Oregon and northern California. This will
result in increased cloud cover, cooler temperatures, and about
a 25-50% chance for light showers on Tuesday. Given the position
of this trough, will also see southerly flow aloft setting up over
our area. Southerly flow aloft is typically a favorable pattern
for thunderstorms as it brings more moist, unstable air from
the south. However, there is uncertainty with exactly how much
instability we would get from this trough, as model soundings on
Tuesday aren`t showing too much of an unstable layer in the
mid-levels of the atmosphere. For now, there is about a 10-15%
chance for thunderstorms across the region on Tuesday.

Wednesday to early Thursday morning will be the more significant
soaker days as the majority of ensemble members are showing a
second, much deeper upper-level trough dropping down into the
Pacific Northwest. The associated surface front swings through
on Wednesday, returning widespread rain, cooler temperatures,
and breezy winds from the south. Chances for exceeding 1 inch
of rain in 48 hours between 5 AM Wednesday to 5 AM Friday are
around 60-80% across northwest Oregon and southwest Washington,
with the highest chances along the coast, Coast Range, and
Cascades. Chances for exceeding 2 inches during this same
timeframe are around 25-45% for these higher terrain features,
and 10-20% along the I-5 corridor.

Thursday afternoon to Friday, the upper-level trough moves
directly overhead and we transition into a post-frontal
environment with lingering showers. Given that there will be
cooler air aloft, this will favor increasing atmospheric
instability and thus chances for thunderstorms. There is around
a 15-20% chance for thunderstorms on Thursday following the
initial frontal passage. Any passing thunderstorms may produce
lightning, heavy rain, erratic winds, and small hail. Overall,
while the upcoming rain will not quite yet be a "fire season
ender," it will definitely slow down fire season and help put a
damper on ongoing wildfires.       -10

&&

.AVIATION...Through 00z Monday...Rain showers are becoming
increasingly isolated as they track eastward across the airspace
as their parent upper-level low moves inland over Washington
State this afternoon. Chances for light rain will quickly diminish
after 00z Sun, and terminal impacts therefore remain unlikely.
Otherwise, largely VFR conditions persist as skies trend clearer
and showers end as the low departs to the east. Additional marine
stratus will yield sct-bkn low clouds after 06-12z Sun, with high
chances for MVFR cigs at coastal terminals and modest chances,
30-60%, at inland sites. Southwest winds of 5-10 kt early this
afternoon will shift out of the northwest to north by 00z Sun,
then ease below 3-5 kt again after 06z Sun.

KPDX AND APPROACHES...VFR conditions favored to persist
throughout the period. Light rain showers quickly diminish in
coverage after 00z Sun, and terminal impacts are not anticipated.
Skies trend clearer this afternoon until marine stratus builds
inland from the coast near 12z Sun yielding 60% chances of MVFR
cigs late in the period. Southwest winds of 5-10 kt will shift out
of the northwest through this afternoon, then ease below 3-5 kt
this evening.        -36

&&

.MARINE...An upper-level low moving inland along the Canada-US
border will be replaced by brief upper-level ridging Sunday
through Tuesday. North to northwest winds will strengthen as the
ridge builds, reaching diurnal peaks of 10-15 kt each of Sunday
and Monday afternoons. By Tuesday, more robust upper-level
troughing digging over the northeast Pacific will see winds shift
out of the south to southwest Tuesday morning. Largely wind-driven
seas through this period will persist at 3-5 ft.

As the aforementioned trough and associated surface frontal
features approach the coast Tuesday into Wednesday, southerly
winds will rise to 20-30 kt, with modest chances (around 50%) for
gale- force gusts, particularly on Wednesday. Seas will similarly
build above 10 ft as winds increase. Both winds and seas are then
expected to ease through the end of the workweek, however
uncertainty during this period remains high.        -36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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