511
FXUS66 KPQR 290534 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1034 PM PDT Fri Aug 28 2026

Updated Aviation Discussion...

.SYNOPSIS...Going through the weekend into next week Western
Oregon and Southwest Washington remain under the influence of a
progressive westerly flow pattern with little to no notable
weather impacts. Expect a weak weather disturbance to arrive
late tonight into Saturday morning ushering in occasional light
precipitation mainly along the coast and at higher terrain
features. Then after a temporary return to drier/warmer weather
Sunday into Monday, model guidance continues to trend towards
more appreciable widespread rainfall around the middle of next
week. However, beyond the broader favorable set-up for rainfall,
details regarding the exact timing/amounts are uncertain.

&&

.DISCUSSION...Today through Thursday...Through the first half of
the weekend upper-level troughing remains the dominant feature
over the far ENE Pacific while helping to keep persistent
onshore flow in place. Expect the axis of this upper-level trough
feature to progress overhead Saturday morning coupled with a
weak cold-frontal boundary which likely leads to a period of
light rainfall/shower chances for a portion of the region.
Overall the best relative chance for light rain (20-50%)
appears to be along the coast, coast range/Willapa Hills, and
Southwest Washington, particularly in the Cascades and Cascade
foothills, where orographic forcing is present. Can`t
completely rule out the Portland Metro receiving a shower in
the morning too ( 15-25% chance) but confidence is high any
rainfall amounts stay rather light (<0.05in). The additional
onshore flow also maintains fairly prolific cloud cover across
the inland valleys into the western slopes of the Cascades
Saturday helping to keep daytime temperatures 5-10 degrees lower
than normal.

By Sunday, the upper-level trough axis shifts eastward of the
Cascades followed by a shortwave ridge building over the region
for the start of the week. This transition likely results in
high temperatures jumping a degree or two day-to-day on Sunday
followed by further warming on Monday with daytime highs
reaching into the upper 70s to low 80s across the interior, 60s
along the coast and into the 70s for the Cascades. It`s worth
highlighting most models depict a weak near surface thermal
trough developing during Sunday/early Monday before quickly
shifting eastward. This would result in a period of slightly
breezier northerly winds across the central and southern
Willamette Valley Sunday afternoon followed by light easterly
winds in the Cascade/Cascades foothills Sunday night into Monday
morning. Beyond the aforementioned winds, slightly degraded
overnight RH recoveries due to the light easterly/offshore flow,
and a 12-18hr window for haze/smoke to drift towards the
northern Willamette Valley from local wildfires can be
anticipated. At least all of these impacts appear minimal and
rather temporary as winds switch back westerly by Monday
afternoon into Tuesday raising RHs and pushing smoke/haze back
towards the Cascade crest-line.

Looking into the middle of next week, ensemble guidance
continues to heavily favor the development of an early fall-
like cooler and wetter weather pattern although confidence in
the exact details are only low to moderate at this time. This
model uncertainty is almost solely derived from the
placement/amplitude of the upper level low progression diving
southward from the Gulf of Alaska with some ensembles increasing
rainfall as early as Tuesday morning (weaker trough feature),
while others hold it off until later Tuesday and Wednesday
(stronger trough). With that said, at this point almost all
deterministic models and their respective ensembles show at
least a wetting rain (>0.25in) in the late Tuesday through late
Thursday night timeframe. Exploring the latest NBM probabilistic
guidance points to a 60-80% chance for greater than 0.5in of
rainfall across the entire region, and even a 40-60% chance
(slightly higher at the coast) for greater than 1.0in of
rainfall over that same 48 hr timeframe - very promising
overall. While this isn`t shaping up to be a "fire season
ender" unless amounts trend much higher, the forecast rainfall
will definitely work to heavily moderate fire weather related
concerns. We`ll take what we can get this time of year. -99

&&

.AVIATION...Through 06Z Sun...Expect west to northwesterly flow
through the TAF period as the upper level trough passes overhead.
Along with this trough, a weak front will move across the airspace
Sat morning, bringing chances of light rain to most terminals
north of KSLE between 12-18Z Sat. Expect lower clouds during the
aforementioned time, with guidance suggesting a 20-40% chance of
MVFR CIGs. Coastal terminals have higher chances of deteriorated
conditions, with a 40-70% chance of MVFR CIGs between 09-17Z Sat
at KAST, and a 40-60% chance of IFR and 20-40% chance of LIFR
conditions between 06-16Z Sat. Other than these times, expect
mostly VFR conditions, with the exception of a 20-30% chance MVFR
CIGs will hold on at coastal terminals through the TAF period.
Otherwise, north to northwest winds will increase in the
afternoon to around 8-11 kt before easing around 03-05Z Sun.

KPDX AND APPROACHES...VFR conditions through the TAF period, with
the exception of a 20-40% chance of MVFR CIGs and low chances of
rain showers between 12-18Z Sat. Northwest winds increase to
around 8 kt between 22Z Sat and 04Z Sun.
~12

&&

.MARINE...High pressure offshore will maintain predominately north
to northwest winds across the coastal waters through the weekend
and into early next week. A weak front pushes across the northern
waters late tonight into Saturday morning with little to no
impact. Winds are generally expected to remain around 15 kt or
less. Predominately wind driven seas remain at around 3 to 5 ft.

The pattern likely becomes more active by the middle of next week
as a stronger frontal system approaches the region later Tuesday
into Wednesday, followed by low pressure moving southeast from
the Gulf of Alaska. There is at least a 50% chance that this
system brings low end gale force wind gusts in excess of 34 kt,
as well as resulting in seas building towards 10 ft. /DH

&&

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

$$

www.weather.gov/portland

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