540
FXUS66 KPQR 090357
AFDPQR

Area Forecast Discussion...Updated Aviation
National Weather Service Portland OR
857 PM PDT Fri Aug 8 2025

.SYNOPSIS...It`s going to get hot! A robust ridge of high
pressure slated to develop over the far eastern Pacific will
usher in a sharp warming trend with temperatures peaking Sunday
and Monday - Extreme Heat Warnings and Heat Advisories are in
effect during this period. Conditions are favored to return
closer to normal by the middle of next week, however there
remains lingering uncertainty in exact details of this
transition.

&&

.SHORT TERM...Now through Saturday Night...Late this afternoon a
ridge of high pressure over the eastern Pacific continues to
amplify overhead which as lead to mostly clear skies across
western Oregon and southwest Washington, in addition to a
noticeable increase in temperatures compared to past days.
This`ll be the trend moving forward on Saturday with ample
sunshine beyond patchy marine stratus/fog along the coastline
and some occasional high clouds. Due to the increasing
influence of the ridging aloft, high temperatures Saturday top
out near to just above normal in the mid to upper 80s across the
inland valleys with mid 60s to low 70s along the coast. All in
all, not a bad day to start the weekend, although keep in mind
it only gets hotter the back half of the weekend into early next
week. -Schuldt


.LONG TERM...Sunday through Thursday night...Going forward both
deterministic and ensemble guidance are in general agreement
through Monday all showing high-amplitude upper ridging and
building surface high pressure. As heights aloft continue to
rise overhead, 850-hPa temperatures will reach around 20-25C on
Sunday which, combined with a slight offshore component to low-
level winds, will push surface temperatures 12-15 degrees above
normal for early to mid August. High confidence (80-95%) in
maximum daytime temperatures reach up into at least the mid to
upper 90s across the lower inland valleys - highest central and
south Willamette Valley. Sunday is also the first day many
inland valley locations appear to be a stones-throw away from
triple digits. Chances to reach 100 degrees on Sunday afternoon
sit around 10-25% in the Lower Columbia and Cowlitz Valleys,
25-45% in the Portland/Vancouver Metro and Columbia Gorge, and
60-70% in the Willamette Valley. It`s definitely worth
highlighting that due to the lack of robust onshore flow, a
warm airmass aloft, and dew points in 60s, very mild overnight
temperatures are anticipated Sunday night into Monday morning
with many spots remaining above ~65 degrees overnight. This
would exacerbate heat related impacts and is what`s pushing
HeatRisk into the major category for much of the
Portland/Vancouver metro through the Willamette Valley into the
Cascade Foothills and Columbia Gorge. Mild overnight
temperatures are one of the facets driving the decision to
upgrade the Extreme Heat Watch to an Extreme Heat Warning for
the aforementioned areas.

There still remains a little bit of of uncertainty headed into
Monday in regards to just how hot temperatures will get, if
anything the latest guidance is bullish on hotter temperatures
compared to this time yesterday (Thursday). On the synoptic
scale, global ensembles show a weak shortwave eroding heights
aloft on the downstream flank of the ridge, however the location
and amplitude of this feature remain moderately confident. A
stronger shortwave would increase northwesterly onshore flow,
which would see the largest effect along the coast and up the
Columbia River into the Portland area, where temperatures may
cool slightly. Farther to the south through the Willamette
Valley, this robust shortwave scenario would only minimally
affect the forecast - highs still rise near 100 degrees.
Conversely, given the very warm start expected Monday morning,
even a weaker shortwave (currently the most likely scenario)
would see net warmer surface temperatures across the region
despite slight cooling aloft. Therefore, temperatures Monday
will most likely be slightly warmer than Sunday, but the spread
in potential outcomes is still more apparent across northern
areas given the uncertainty around marginally increased onshore
flow. These factors result in 30-50% chance to exceed 100
degrees in the Lower Columbia and Cowlitz Valleys, 40-65% in the
Columbia Gorge and Portland/Vancouver Metro, and 75-85% in the
Willamette Valley. It`s also worth mentioning for the central
and southern Willamette Valley there`ll be a period of
critically low relative humidity (<25%) and marginally breezy
north winds (>15 mph) due to the placement of the near surface
thermal trough Sunday afternoon/early evening. Current
probabilities for Red Flag conditions across the previously
mentioned areas only sit at 30-40% but it`s a feature worth
monitoring.

Regardless of a few degrees of differences in potential
afternoon highs on Monday, the combination of hot daytime
temperatures and lack of sufficient overnight cooling can raise
the cumulative heat stress and elevate the risk for heat
illness, resulting in areas of Major HeatRisk most prevalent
across inland valleys. All residents, but especially those
without access to adequate cooling or hydration, should make
plans for this period of hazardous heat.

Ensemble guidance continues to diverge with respect to the
breakdown of upper ridging through the midweek period. In the
mean, the ridge retrogrades westward away from the region with
heights aloft decreasing locally, yielding a cooling trend
through the workweek. Some members favor a more pronounced upper
trough developing over western Canada, which would see more
robust cooling, resulting in substantial uncertainty in
temperatures particularly on Tuesday. The latest model guidance
has slightly delayed this progression of this trough resulting
in a trend towards higher temperatures on Tuesday (low to mid
90s inland). Farther out into the workweek, the signal for
steady easing of temperatures toward seasonal norms becomes more
robust and even a slight chance for rainfall near the coast.
-Schuldt/Picard

&&

.AVIATION...High pressure creating generally VFR conditions
throughout the period. Exception will be marine stratus at the
northern coast overnight (KAST), with a 40% chance of MVFR cigs
there between 10-18z Sat. Other terminals look unlikely to develop
any cloud cover, and should just remain clear sky VFR throughout
the period. Slightly breezier northerly gusts up to 20 kts by
daytime hours Saturday.

PDX AND APPROACHES...Clear sky VFR throughout the period. North-
northwesterly gusts up to 20 kts after 20z Sat. /JLiu

&&

.MARINE...High pressure across the waters will maintain north
winds across the waters through at least Wednesday. Pressure
gradients will tighten this weekend as high pressure builds over
the NE Pacific and the winds will increase. The winds will be
strongest in the afternoon and evenings and will be hazardous to
small crafts from Saturday through at least Monday. The wind gusts
will relax in the mornings, especially closer to shore. Have
extended the Small Craft Advisory through Monday morning for waters
south of Cape Falcon. The waters north of Cape Falcon may need to
be added to the Advisory Sunday afternoon depending on where he
High pressure settles over the Pacific. Seas will be mostly wind-
driven and around 3-5 ft at 10-11 sec through Saturday. A fresh
NW swell builds to 7 ft at 11-12 sec Sunday, and close to 10 ft on
Monday. There is a 60-80% chance of wave heights exceeding 8 ft
by Monday, with the highest chances over the outer waters. ~TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory from noon Sunday to 11 PM PDT Monday for ORZ104-
     105-108.

     Extreme Heat Warning from noon Sunday to 5 AM PDT Tuesday for
     ORZ109>125.

WA...Heat Advisory from noon Sunday to 11 PM PDT Monday for WAZ202-
     204-208.

     Extreme Heat Warning from noon Sunday to 5 AM PDT Tuesday for
     WAZ205>207-209-210.

PZ...Small Craft Advisory from 3 PM Saturday to 8 AM PDT Monday for
     PZZ252-253-272-273.
&&

$$

www.weather.gov/portland

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