637
FXUS66 KPQR 181730
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1030 AM PDT Mon Aug 18 2025

.SYNOPSIS...Troughing over the Pacific Northwest early this
week gives way to high pressure building into the region from
the Desert SW late this week. Persistent onshore flow will help
to moderate daytime high temperatures through the middle of the
week. However, the building high pressure late this week will
produce warmer temperatures as well as weak offshore flow.

&&

.SHORT TERM...Now through Wednesday...Satellite imagery and
observations early Monday morning indicate mid to high level
cloud cover over NW Oregon and SW Washington as upper level
longwave troughing remains over the eastern Pacific and the
PacNW. A perturbation along the trough moving through the PacNW
early this morning is producing the cloud cover, which is
expected to become partly cloudy for most locations throughout
this afternoon. Daytime temperatures today are forecast to be
just below to right around daytime normals with upper 70s to low
80s for inland valleys and 60s along the coast. Ensemble
guidance is in good agreement that this longwave troughing will
remain over the region for the first half of this week,
producing weak onshore flow and stratus in the overnight through
morning hours tonight into Wednesday. Otherwise, cloud cover
inland is expected to clear by the late morning hours each day,
allowing high temperatures to warm into the low 80s for inland
valleys Tuesday and Wednesday. -HEC

.LONG TERM...Thursday through Sunday...By Thursday, ensemble
guidance indicates upper level high pressure over the Desert SW
will begin expanding north and west into California and Oregon,
and will continue expanding Friday into Saturday. This will
allow temperatures to rise Thursday into Saturday. With guidance
indicating 850mb temperatures peaking around 21-24 deg C Friday
and Saturday, expecting these to be the warmest days with
surface high temperatures in the low to mid 90s for inland
valleys. NBM indicates a 75-95% chance of temperatures reaching
90 degrees for most of the Willamette Valley and SW Washington
lowlands on Friday. For Saturday, similar probabilities for the
Willamette Valley south of the Portland/Vancouver metro area,
but probabilities lower for the Portland/Vancouver metro area
and SW Washington lowlands to around 40-60%. Probabilities for
reaching 95 degrees are around 35-50% on Friday and 20-40% on
Saturday. Low temperatures Friday night and Saturday night are
also forecast to remain in the 60s for most inland locations
except for the southern Willamette Valley where temperatures
are forecast to fall into the upper 50s. These warm days and
mild overnight temperatures are causing widespread Moderate Heat
Risk Friday and Saturday for most inland locations. This type of
heat impacts those who are vulnerable to the heat, including
those with outdoor plans and those without access to adequate
heating.

By Sunday, ensemble guidance indicates the high pressure begins
shifting southeast out of the PacNW, which would allow for
inland temperatures to begin lowering back into the 80s.
However, some ensemble members want to keep the high pressure
over the region into Sunday, and because of this NBM indicates
a 25-50% chance of high temperatures reaching 90 degrees again.
However, the probability of temperatures reaching 95 degrees is
only 5-20%, which leads to moderate confidence in temperatures
beginning to lower on Sunday. -HEC

&&

.AVIATION...VFR conditions expected at all terminals through
this afternoon. Along the coast, marine stratus will yield
MVFR cigs overnight after 06z Tue, although there remains at
least a 30-40% chance in IFR cigs, most likely at KONP and lower
to the north. Inland, chances for MVFR cigs are lower, only
10-20%, most likely near the Portland metro and along the
Cascade foothills. Otherwise sct-bkn strato-cu at 6-7 kft will
continue into this afternoon and evening, with lowering cloud
bases overnight. Winds continue today at 5-10 kt out of the
southwest to northwest, then ease to 5 kt or less tonight after
06-12z Tue.

PDX AND APPROACHES...VFR flying conditions with sct-bkn strato-cu
at 6-7 kft will continue through this afternoon with decreasing
cloud coverage. Low chances, 10-20%, of stratus resulting in
MVFR cigs tonight, but best chances from 12-15z Tue and to the
east closer to the Cascades. Light southwest winds this morning
will increase to 5-10 kt out of the northwest by 19-21z Mon,
then ease to less than 5 kt tonight. -Picard

&&

.MARINE...Benign conditions are expected to persist through
much of the week. Light southerly to southwesterly winds and
seas 4 to 5 feet driven largely by a subsiding west-
northwesterly swell at 9-10 seconds. High pressure building over
the northeastern Pacific will see winds shift out of the
northwest to north by Tuesday night, with a 60-80% chance of
gusts exceeding 20 kt for the inner waters on Wednesday south of
Cape Falcon and 20-60% chances to the north. The amplifying
ridge will further increase chances for 20 kt gusts to 60-90% or
more across all waters on Thursday and Friday. -Batz/Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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