698
FXUS66 KPQR 241524 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
823 AM PDT Sun Aug 24 2025

...Updated public discussion...

.SYNOPSIS...High pressure will lead to high temperatures,
morning fog along the coast, and low humidity. Temperatures will
remain elevated today, but slightly cooler than the last two
days. Monsoonal moisture will move into the region, generally
staying to the east. The Storm Prediction Center has the
Cascades under a general thunderstorm risk, and there is around
a 15% chance of lightning through the Willamette National
Forest. Temperatures slowly decrease daily through the remainder
of the week with chances for thunderstorms persisting through
Tue afternoon. Large scale pattern shift on Friday.

&&

.MORNING UPDATE...Radar imagery as of 8 AM Sunday is showing
convective activity moving into the Linn and Lane County Cascades
from the south due to increased monsoonal moisture. A few lightning
strikes have been observed already with passing thunderstorms. This
came earlier than expected today, so the forecast has been updated
to reflect a 10-20% chance for thunderstorms across the Linn and
Lane County Cascades beginning now and lasting through the evening.
The latest public discussion continues below.        -Alviz

&&

.SHORT TERM...Now through Tuesday Night...Conditions remain hot
today as high pressure lingers over the area. Highest
temperatures continue to be within the Willamette Valley, Clark
County Lowlands, and the valleys of the Cascade Foothills.
Daytime highs have around a 30% chance of exceeding 100 deg F
within the Willamette Valley, with the highest probability
along the I-5 corridor between Portland and Salem. One
difference between the heat of this last Fri/Sat and today is
that there is a slight westerly flow with monsoonal moisture
coming in which will initiate the "cooling" trend.

Weak lows forming over northern California and eastern
Washington, and shortwave troughs aloft, will usher in more
moisture starting this afternoon. Based on the 850mb-250mb
profile, the saturated air will just make it west of the
Cascades today, and intensify through Tue. While moisture is
incoming, it is not going to manifest as stratiform rain.
Instead, it will bring in just enough moisture to increase our
chances for thunderstorms. Based on the overall flow most of
the storms (if they do form), will occur over the Willamette
Natl Forest Cascades. Chances sit at around 15% at this time,
with them initiating during the warmest time of day - around 4PM
PDT. With that said, there is still ample dry air in the region
at the surface, and an inversion in place aloft at around 3500
ft. This cap will inhibit significant thunderstorm formation.

This forecast is a rinse and repeat with highs each day falling
by a few degrees. The only changing feature will be the
compounding moisture each day. On Monday, the Storm Prediction
Center (SPC) has more of the forecast area under a general
Thunderstorm risk. While storms cannot be ruled out, overall the
flow is not as favorable. If it was truly southerly or a little
more southwesterly then the chances would increase. Instead,
the flow at 850 mb is closer to east to southeast. Could have
some bleed over storms from east of the Cascades along the
Cascade crest though on Mon and Tue afternoon/evening. One
component to watch for though, especially on Tue, is a
shortwave trough aloft combined with an enhanced band of
vorticity. This added lift could provide just enough "oomph" to
trigger storms. Based on the ECMWF ensembles there is a
climatologically abnormal signal for instability on Tue. With
all that being said, PWATs are very high with ensembles showing
anywhere from 1-1.25 inches. These high values would coincide
with wet thunderstorms.-Muessle


.LONG TERM...Wednesday through Saturday...Wednesday will be a
transitional day as the low pressure that formed over eastern
Washington weakens and a broad area of low pressure moves down
from the Gulf of Alaska. Based on ensemble low-locations, there
is a lack of consistency and a low center. This leads us to a
lower confidence forecast in the long term in regards to the
specifics. In general though expect seasonable high temperatures
and ample moisture within the area. Humidity will likely rise
during the afternoons. As is common with the first system after
a prolonged period of high pressure, it will act as the
sacrificial lamb causing the ridge to weaken.

While some models are suggesting showers, given the still low
humidity and trend towards systems weakening, if there is
precipitation it will lean towards virga where the
precipitation would evaporate before reaching the surface.
Areas with orographic lift like the Cascades have the highest
probability of rain.

The next potential rain day would be Friday as a stronger system
arrives. This pattern is a bit more chaotic but is leaning on
the wetter side. -Muessle

&&

.AVIATION...Upper-level ridging and high clouds continue
continue through the TAF period, maintaining VFR conditions inland
and overnight LIFR/IFR fog/stratus along the coast. There is high
confidence for LIFR/IFR conditions at coastal terminals to
continue at least until 18Z Sunday, and low to moderate confidence
for these conditions to continue after 18Z Sunday. Specifically,
there is a 10-20% chance for MVFR conditions between 18Z Sunday
and 00Z Monday for KAST and a 60-80% chance for KONP.
Additionally, KONP has a 30-50% chance for LIFR/IFR conditions
between 18Z Sunday and 00Z Monday. By 00-03Z Monday, coastal
terminals will return to LIFR/IFR conditions as fog and/or stratus
redevelops. Otherwise, expect northerly to northwesterly winds
under 10 kt at all terminals through the period.

Additional hot temperatures are expected today; pilots should
continue to be aware of high density altitude and reduced aircraft
performance as a result.

PDX AND APPROACHES...High clouds, VFR conditions, and
northwesterly winds below 10 kt expected through the period. -Hall

&&

.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
in the outer waters each afternoon and evening as pressure
gradients tighten. Could see Small Craft conditions today, but
confidence remains low as winds look weakened 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 7-9
seconds. The next chance for a pattern change arrives late next
week with the potential of a low pressure system approaching the
coastal waters. ~Hall/DH

&&

.CLIMATE...
Daily record high temperatures:

Sunday, August 24

Portland   99 (1982)
Vancouver 100 (1988)
Hillsboro 101 (1958)
McMinnville 100 (1916, 1926)
Salem  99 (1958)
Eugene  96 (1958, 1982, 2010)

Monday, August 25

Portland  95 (2016)
Vancouver  96 (1982, 2010)
Hillsboro  96 (1982, 2010, 2016)
McMinnville  98 (1916, 1996)
Salem  97 (1967, 2016)
Eugene  97 (2010, 2016)

&&

.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...None.

&&

$$

www.weather.gov/portland

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