656
FXUS66 KPQR 310915
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
215 AM PDT Sun Aug 31 2025

.SYNOPSIS...A cutoff low lingering offshore of northwestern
Oregon and southwestern Washington will bring continued
seasonable temperatures through the holiday weekend with
morning incursions of marine clouds giving way to breaks of
afternoon sun. Building high pressure will yield a warming trend
early in the week, with the potential for high temperatures
reaching the 90s and Moderate to Major HeatRisk Tuesday through
at least Thursday across inland areas. Chances of scattered
showers and isolated thunderstorms increase midweek.

&&

.SHORT TERM...Now through Monday Night...Latest water vapor
satellite imagery depicts one area of low pressure continuing to
meander near the coast of northwestern Oregon and western
Washington, and a second, deeper low centered well to the west
of the San Francisco Bay region. Little change in the location
of these features is expected through the next two days,
bringing continued near to slightly above normal temperatures
with afternoon highs today and Monday likely in the low to mid
80s throughout the Willamette Valley and Columbia Gorge, 70s in
the Cascades, Coast Range, and Lower Columbia Valleys, and 60s
along the coast. Morning incursions of marine stratus up the
Columbia River and through Coast Range gaps will break up from
the late morning through the afternoon beneath persistent
passing high clouds. Numerical guidance often struggles to
maintain low clouds late enough into the day and can also
overestimate insolation through multiple cloud decks, so if
stubborn low clouds persist later in the day, temperatures could
easily underachieve. Overnight lows will remain comfortable in
the 50s to low 60s through Monday night. -Picard


.LONG TERM...Tuesday through Saturday...Through the workweek,
the aforementioned cutoff lows over the northeastern Pacific
will begin to interact, with the nearby low retrograding
westward while the more distant low begins to fill. In the wake
of the retreating low, narrow upper-level ridging will build
out of the Great Basin across the interior Pacific Northwest,
while yet another weak but very important upper low will track
northward and inland over northern California and ultimately
western Oregon by Friday. This pattern will yield a warming
trend across the region, with medium-range forecasts coming
into better agreement on temperatures Tuesday and Wednesday,
however there remains increased uncertainty in temperatures
through the latter half of the week.

On Tuesday and Wednesday, slightly cooler temperatures have been
maintained, with afternoon highs most likely in the low 90s
through the Willamette Valley, 80s in the Lower Columbia and
Cowlitz Valleys, Cascades, and Coast Range, and 60s to 70s
near the coast. The Columbia Gorge and Hood River Valley are
favored to see the hottest temperatures in the mid to upper 90s.
Chances to reach 100 degrees on Wednesday have fallen to 15% or
less across the region, with the exception of 30-60% heading
east through the Columbia Gorge to the Hood River Valley.
Overnight lows largely in the 50s to low 60s will help to
minimize the HeatRisk to Minor or Moderate across most of the
region, however warmer lows in the upper 60s through the Gorge
and Hood River Valley maintain the coverage of Moderate to Major
HeatRisk in those communities.

Some of this increased forecast temperature confidence is due
to developing consensus on the position and intensity of the
approaching upper low, but its arrival also raises the specter
for chances of Cascade convection and attendant fire weather
concerns. Anomalous southeasterly flow aloft ahead of the low,
along with synoptic support for ascent and increasing elevated
instability could favor convection along the Cascades, with
storms and/or outflow clouds propagating downstream over the
remainder of the forecast area. Model soundings also show
abundant dry air beneath any elevated convection, which could
allow for dry lightning through the afternoon and evening on
Tuesday and Wednesday. Even with increasing moisture, column
moistening would still result in periods of virga and lightning
strikes could also reach well outside of any narrow rain cores.
With the relative humidity falling to afternoon minimums below
25-30% along the Cascades and receptive fuels remaining in
place, those with fire weather interests should continue to
follow the forecast through the coming days.

Thursday and beyond, spread among long-range ensemble members
increases with regard to the placement and intensity of the
upper low as it passes overhead, with this uncertainty
manifesting as decreased confidence in forecast temperatures.
Afternoon highs will most likely remain in the low to mid 90s
across interior valleys on Thursday, but chances to reach 100
degrees increase to 5-25% from the Portland/Vancouver Metro
south through the Willamette Valley, and remain higher, 20-50%,
through the Gorge and Hood River Valley. Temperatures begin to
trend cooler into Friday and Saturday, yielding largely Minor
HeatRisk across the region by the weekend. -Picard

&&

.AVIATION...Early this morning a marine layer is attempting to
deepen along the coast while a weak upper-level low pressure
system sits in the far northeastern Pacific. Based off current
satellite trends and model guidance, degraded flight conditions
due to the developing stratus will stay focused around and after
~12-13z for coastal sites. It still remains to be seen whether
low clouds will be able to push inland with now only a 30-45%
chance for low-end MVFR to IFR CIGs between ~13-18z across the
Portland metro and north Willamette Valley terminals. KSLE and
KEUG only have a 15-25% chance for MVFR CIGs/VIS or below, and
most likely remain VFR. Overall confidence in the set-up is only
low to moderate thanks to scattered high cloud cover over the
region which can act as an inhibitor to low cloud formation,
especially inland. VFR conditions likely prevail at inland
terminals during the afternoon and evening hours Sunday.

PDX AND APPROACHES...Generally VFR conditions are expected to
prevail through the forecast period. Still, we`ll need to watch
the 13-17z time period with a ~40-45% chance for MVFR to IFR
CIGs. Any stratus that forms likely clears to VFR by 18-20z
Sunday. Light winds of 5-10 kt or less are expected through the
period. -Schuldt

&&

.MARINE...Overall fairly benign late summer-time weather
conditions continue through next week. Expect a weak area of low
pressure just off the coast to help maintain rather light winds
across the coastal waters through Monday and Monday night. From
there, confidence is high a ridge of surface high pressure builds
over the waters Tuesday and Wednesday increasing northerly winds.
The winds will be strongest in the afternoons and evenings,
especially to the south where the surface pressure gradient will
be the strongest. Peak wind gusts during this early to mid week
period get close to small craft conditions (>21 knots gusts),
mainly across the outer waters, although guidance keeps the
probability to meet criteria only around 20-35%. Seas hold below
4 ft through Monday, with an increase in wind waves building
seas to 5-6 ft Tuesday through Thursday. -Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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