878
FXUS66 KPQR 232230
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
330 PM PDT Sat Aug 23 2025

.SYNOPSIS...It`s still hot! The region remains stuck underneath a
robust ridge of high pressure allowing well above normal
temperatures (mid-90s to near 100) to persist into early next
week across the interior valleys. Expect Moderate to Major
HeatRisk for all inland locations through at least Monday as a
result - little relief during the overnight hours, especially
urban locations. Moving forward through the rest of the week, a
slow cooling trend finally moderates high temperatures back
into the 80s around the Thursday/Friday timeframe. However,
thunderstorms across the Cascades, particularly in the Lane
County Cascades, are a growing concern for late Sunday onward,
although confidence in the exact details is still only low to
moderate.

&&

.DISCUSSION...This afternoon through Friday...High temperatures
are expected to peak over the next several hours as our heatwave
across the interior of western Oregon and southwest Washington
continues. Satellite imagery depicts the bulk of the high cloud
cover holding to our east as of 1500 with little in the way of
additional high clouds up-stream to our south potentially
working to inhibit the remainder of our daytime heating. While
the axis of the surface thermal trough has remained situated
near the base of the Cascade foothills/eastern Willamette Valley
limiting the westward extent of the truly warmest air
downsloping off the Cascades, we`re still on track for many
locations to reach the vaunted 100 degree mark, especially in
the Portland metro. Thus, a number of daily high temperatures
records are in jeopardy yet again this afternoon - those
records, and the records for tomorrow (Sunday) are listed in the
Climate section below.

Moving forward, the strong upper level high pressure causing
the well-above normal temperatures is anticipated to weakening
slightly Monday into Tuesday while the axis of the thermal
trough shifts eastward allowing weak onshore flow to percolate
inland. This will allow daytime temperatures to lower slightly
each day, most notable in the Coast Range, Willapa Hills, and
Cascades where daytime temperatures will fall back into the 70s
to 80s. However, inland valleys are still expected to remain
hot. NBM still indicates high confidence (70-95% chance) of
temperatures remaining above 90 degrees both days. However,
compared to the previous forecast confidence is now only
moderate for temperatures to reach or exceed 95 degrees with a
40-70% chance, noticeably lower on Tuesday and for the
Portland/Vancouver metro area northward. The ECMWF AIFS
deterministic and ensemble have been big proponents of knocking
high temperatures down into the 90-94 degree range Tuesday the
last few forecast cycles and it appears numerical models are now
following suit. Should this trend continue, the Extreme Heat
Warning may need to be ended a day early. This overall trend
persists into Wednesday with inland valleys in the upper 80s to
low 90s. It`s worth noting one facet of the forecast which may
be contributing to the decrease in temperatures the last few
forecast cycles is an increase in cloud cover aloft as
convective debris streams overhead helping to put a partial
damper on insolation, mainly Monday onward (more on
convection/thunderstorms a little later on).

In any case overnight temperatures are expected to remain on
the warm side every night through Tuesday night. Most inland
locations will remain above 60-68 degrees, warmer north of
Salem, with the Columbia River Gorge remaining above 70 degrees.
Thus, widespread Moderate to Major HeatRisk continues through
at least Monday. The combination of hot daytime temperatures
plus warm overnight lows make it very difficult for buildings
without air conditioning to sufficiently cool, exacerbating heat
impacts in a long-duration heat event such as this one. Those
vulnerable to heat should be extra cautious.

Beyond the heat, a surge of monsoonal moisture will move into
Oregon from the desert southwest/California Sunday into next
week. While much of this instability is capped over western
Oregon, enough local terrain forcing and mid-level moisture
could (15-25% chance) be available over the Lane County
Cascades for convective initiation late Sunday afternoon. The
latest high resolution guidance still favors the crestline
eastward into central Oregon thanks to W-SW winds aloft steering
activity out of our area. These steering winds likely shift
straight southerly on Monday which is a cause for concern, but
it appears rather weak, suggesting any storms that do develop
would move very slowly. Given the high PWAT values in place
(>1"), heavy rain would be possible with any thunderstorm that
develops. As of right now, confidence remains low regarding the
exact location and coverage of storms. However, the trend is for
potential storm placement to advance northward along the
Cascades with time. Several deterministic models show a subtle
mid to upper-level vort max progressing south to north across
Oregon into Washington in the Tuesday/Wednesday timeframe which
could be a trigger for additional activity depending on the
exact timing/placement. We`ll have to watch this latter feature
closely as it does present some outlying scenarios of high
based convection working into the I-5 corridor.

Ensemble guidance indicates the lingering high pressure finally
begins breaking down in earnest Thursday and Friday as
troughing develops in the eastern Pacific. Temperatures will
fall back into the 80s inland, if not Thursday then by Friday.
Onshore flow is expected to return along with marine stratus.
There are early indicates of the potential for rain to return,
though chances are very low (10-20%) as there are significant
differences in ensemble members for the general progression of
the trough. Either way, the region will finally see relief from
the heat by late week. -Schuldt/HEC

&&

.AVIATION...An upper level ridge remains over the region with
southwest flow aloft and scattered high clouds streaming northward
across the area. VFR conditions will persist inland through the
TAF period. VFR conditions at KONP are not expected to last very
long, as there is at least a 50-60% chance of IFR to LIFR stratus
returning after 02z Sunday. VFR conditions remain along the
northern coast but there are increasing chances (30-40%) for
conditions to deteriorate at KAST to low MVFR/IFR after 10z
Sunday. Surface winds generally expected to remain under 10 kt.

Additionally, hot temperatures are expected today and Sunday, so
pilots should be aware of high density altitude and reduced
aircraft performance as a result.

PDX AND APPROACHES...VFR with scattered high clouds streaming
northward. Generally northwest winds expected below 10 kt.
Temperatures are expected to heat up again today, so pilots should
be aware of high density altitude and reduced aircraft
performance as a result. /Hartsock

&&

.MARINE...Very little change in the forecast pattern through early
next week. High pressure remains offshore over the northeast
Pacific through the weekend. This will maintain northerly winds
across the coastal waters, with strongest winds diurnally-driven
each afternoon and evening as pressure gradients tighten. Gusts to
25 kt expected this afternoon and early evening. Small Craft
Advisory conditions are expected across the outer coastal waters,
until this evening. Could see Small Craft conditions continue into
tomorrow, but will likely weaken enough to stay out of most of
the outer water zones. Seas generally remain wind driven, at
around 4 to 6 ft with a dominate period of around 8-9 seconds. The
next chance for a pattern change arrives late next week with the
potential of a low pressure system approaching the coastal waters.
/DH

&&

.CLIMATE...
Daily record high temperatures:

Saturday, August 23

Portland 101 (1988)
Vancouver  95 (2000)
Hillsboro  97 (1996)
McMinnville 103 (1988)
Salem 102 (1988)
Eugene  98 (1911)

Sunday, August 24

Portland   99 (1982)
Vancouver 100 (1988)
Hillsboro 101 (1958)
McMinnville 100 (1926)
Salem  99 (1958)
Eugene  96 (2010)

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory until 5 AM PDT Tuesday for ORZ104-105-126>128.
     Heat Advisory until 5 AM PDT Monday for ORZ106-107.
     Extreme Heat Warning until 11 PM PDT Tuesday for ORZ108>125.
WA...Heat Advisory until 5 AM PDT Tuesday for WAZ202-211.
     Heat Advisory until 5 AM PDT Monday for WAZ203.
     Extreme Heat Warning until 11 PM PDT Tuesday for WAZ204>210.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for
     PZZ271>273.
&&

$$

www.weather.gov/portland

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