444
FXUS66 KPQR 210924
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
224 AM PDT Thu Aug 21 2025

.SYNOPSIS...High pressure will build over the region Thursday
through the weekend, bringing a prolonged stretch of hot and dry
weather with Moderate to Major HeatRisk for inland valleys
Friday through Monday with high temperatures of 95-103F.
High confidence high temperatures of 90F or warmer continue
Tuesday and that above normal temperatures in the upper 80s to
low 90s continue into Wednesday. Dry and breezy conditions over
the southern Willamette Valley Thursday afternoon and early
evening will increase fire weather concerns.

&&

.DISCUSSION...Thursday through Wednesday...A significant
pattern change begins today for NW Oregon and SW Washington
with hot and dry conditions developing across the region for the
next week. This pattern shift is in response to an upper level
ridge of high pressure originally centered over the Four
Corners region building northward into the Pacific Northwest.
Meanwhile, a thermally induced surface trough in southwest
Oregon will shift northward over the Willamette Valley Thursday
night into Friday with high pressure east of the Cascades. This
set-up will produce offshore flow, causing an increase in north
to northeast winds for the Willamette Valley this afternoon and
evening and easterly winds in the Cascades and Columbia River
Gorge tonight into Friday morning. Max wind gusts are still
forecast to reach 20-25 mph, except 30-35 mph in the western
Gorge. This will lead to elevated fire weather concerns in some
areas, which is discussed below in the fire weather discussion.

High temperatures across inland valleys are expected to
increase rapidly from the upper 80s/low 90s today to upper 90s
to around 103 degrees for Friday and Saturday. Even Coast Range
and Cascade elevations below 3500 feet are expected to reach
temperatures in the mid to upper 90s. Confidence remains high
that Friday and Saturday will be the hottest days of the heat
event with probability of temperatures exceeding 100 degrees
around 55-80% both days, mainly from the Portland/Vancouver
metro area south to Corvallis. Confidence is increasing that
temperatures above 95 degrees will continue into at least Monday
and possibly into Tuesday. Probabilities for this have
increased to 60-85% on Sunday, 55-75% on Monday, and 40-60% on
Tuesday. There`s also a 15-35% chance that these temperatures
could continue into Wednesday. These probabilities indicate the
relatively low confidence there still is in how long this heat
event could last. NBM deterministic high temperature forecast
has also increased slightly for each day from Friday through
Wednesday, reflecting the increasing probabilities of hot
temperatures. Additionally, overnight low temperatures play a
significant part in this heat event. Most inland locations will
see low temperatures remaining above 60 degrees Friday night
through Tuesday night. Locations from Portland through Salem as
well as the Columbia River Gorge, Upper Hood River Valley, and the
Cascades and foothills below 3500 feet have a 55-80% chance of
overnight temperatures remaining above 65 degrees with some
locations in the Columbia River Gorge likely to remain at or
above 70 degrees.

These conditions combined result in widespread Moderate HeatRisk
across the majority of NW Oregon and SW Washington, excluding
the coast where high temperatures will remain in the 60s to 70s.
Additionally, Major HeatRisk has expanded in coverage Friday
through Monday for portions of: the Willamette Valley north of
Salem into far SW Washington, the Cascade foothills below 3500
feet, the Columbia River Gorge, and Upper Hood River Valley.
Because of this, an Extreme Heat Warning is now in effect for
the Portland/Vancouver metro area through the southern
Willamette Valley, Oregon Cascade foothills, Columbia River
Gorge, and Upper Hood River Valley. A Heat Advisory is in effect
for the SW Washington lowlands, the Cascades below 3500 feet,
the Coast Range valleys, and the Willapa Hill lowlands. Both the
Warning and Advisory are in effect from 11 AM Friday through 5
AM Tuesday.

The main reason Major HeatRisk is not more widespread is due to the
overnight low temperatures most nights remaining in the low to mid
60s for many locations. However, an increase in those overnight
temperatures even 1-3 degrees could result in Major HeatRisk
expanding in coverage, as we have seen happen with each forecast
package. Either way, daytime temperatures will be very warm and
will impact those who don`t have access to adequate cooling,
including those spending time outdoors. If you have extended
outdoor plans, especially ones that include strenuous activity,
it is important to stay extra hydrated to avoid heat-related
illness.

Another thing that comes into play this weekend or early next
week is the potential for monsoonal moisture moving into the
area from the south, bringing increasing chances for showers and
thunderstorms in the central Oregon Cascades. This could begin
as early as Saturday afternoon or Sunday afternoon when an upper
level impulse will push northward towards the Cascades. Note
that steering flow looks to be very weak, suggesting any storms
that do develop would move very slowly. Given the high PWAT
values in place (over 1"), heavy rain would be possible with any
thunderstorm that develops. As of right now, confidence is very
low regarding the exact location, timing, and coverage of
storms. NBM PoPs are generally around 15% or less, but have the
potential to increase with future forecast updates. Even if
thunderstorms develop east of the Cascade crest, there is the
potential for convective cloud debris to move west of the
Cascades and impact high temperatures, depending on the
coverage and timing. -TK/HEC

&&

.AVIATION...Building high pressure over the region will support
VFR conditions through the TAF period. Light and variable winds
through the overnight and early morning hours will increase from
the N/NW after 18z. Sustained winds of 10-15 kts expected with
gusts 15-20 kt for inland terminals and 20-25 kt along the coast.

PDX AND APPROACHES...VFR conditions expected through the duration
of the TAF period. Light winds overnight will increase from the NW
after 18z to between 8-12 kt with gusts 15-18 kt. -Batz

&&

.MARINE...High pressure will remain offshore over the northeast
Pacific through the remainder of the week and 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 25-30 kt are expected this
afternoon and evening before dropping to 20-25 kt tonight.
Conditions on Friday are expected to be similar with peak wind
gusts slightly lower than Thursday, likely around 25 kt. So, have
maintained the Small Craft Advisories through Friday afternoon.
Could see wind-driven Small Craft Advisory conditions persist into
the weekend but mainly over the outer waters as the pressure
gradient shifts offshore. Seas generally remain wind driven, at
around 4 to 6 ft with a dominate period of around 8-9 seconds.
-Batz/Hartsock

&&

.FIRE WEATHER...Expect hot and dry conditions today through at
least Monday for areas away from the coast. Critical fire
weather conditions are expected to occur over the southern
Willamette Valley this afternoon and evening due to dry and
windy conditions. During that time, expect northerly winds
sustained around 15 mph with gusts up to 25 mph. Relative
humidity values will fall as low as 15-25%. Main concern is for
rapid fire spread with any grass fire that may ignite. A Red
Flag Warning remains in effect for fire weather zone 685 from 2
to 8 PM PDT Thursday to cover this threat.

Modest easterly winds develop across the Cascade ridgetops and
western Columbia River Gorge tonight into Friday, weakening
beginning late Friday afternoon. Wind gusts up to 35 mph will
be possible in the western Gorge during that time. While winds
of this magnitude would normally warrant a headline given the
dry conditions in place, have decided not to issue any fire
weather headlines for the Columbia River Gorge due to the round
of wetting rain that fell August 16th. Would be more concerned
if the breezy winds were set to occur near or after the end of
the upcoming heat event, as fuels would have several more days
of hot weather to dry out.

Strong nighttime thermal belts will produce poor overnight
relative humidity recoveries across the higher terrain Thursday
night through at least Saturday night, especially for elevations
above 3000 ft where maximum relative humidity values in the 20s
and 30s are forecast. Note that relative humidity values most
likely won`t follow a typical diurnal curve, as humidity values
are likely to peak in the evening before dropping rapidly
overnight.

There is also a 10-15% chance of thunderstorms over the Willamette
N.F. near the Cascade crest Saturday through early next week during
the afternoon/evening hours each day. Note that any
thunderstorm that does develop will most likely be slow moving
and wet, which would help decrease the potential for widespread
new fire starts. -TK/HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory from 11 AM Friday to 5 AM PDT Tuesday for ORZ104-
     105-108-126>128.

     Extreme Heat Warning from 11 AM Friday to 5 AM PDT Tuesday for
     ORZ109>125.

     Red Flag Warning from 2 PM this afternoon to 8 PM PDT this
     evening for ORZ685.

WA...Heat Advisory from 11 AM Friday to 5 AM PDT Tuesday for WAZ202-
     204-208-211.

     Extreme Heat Warning from 11 AM Friday to 5 AM PDT Tuesday for
     WAZ205>207-209-210.

PZ...Small Craft Advisory from 2 PM this afternoon to 5 PM PDT
     Friday for PZZ251-271.

     Small Craft Advisory until 5 PM PDT Friday for PZZ252-253-272-
     273.

&&

$$

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