NWS Portland, OR wrote its last area forecast discussion before the first event at Aug 21, 8:45 PM PST, 3 hours 15 minutes earlier.
Area forecast discussion, Aug 21, 8:45 PM PST, 3 hours 15 minutes before
Area Forecast Discussion
National Weather Service Portland OR
945 PM PDT Thu Aug 21 2025
.SYNOPSIS...High pressure will continue building over the
region 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
through early to mid next week. Dry and breezy conditions on
Friday will lead to near critical fire weather conditions in the
eastern Portland/Vancouver metro and Columbia River Gorge.
&&
.DISCUSSION...Thursday night through Wednesday night...A
significant warmup begins tomorrow, marking the beginning of a
prolonged period of hot and dry weather with record high
temperatures likely to occur, except at the coast where
temperatures will be much cooler. This pattern shift is in
response to an upper level ridge of high pressure centered over
the Four Corners region, which will be building northward into
the Pacific Northwest this weekend. 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 result in
offshore flow, causing an increase in north to northeast winds
for the Willamette Valley later 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 and along
exposed Cascade ridgetops. This will lead to elevated fire
weather concerns in some areas, which is discussed below in the
fire weather discussion.
Confidence remains high extreme heat will impact areas away from
the coast Friday through at least Monday. Forecast high
temperatures on Friday and Saturday have warmed another degree
or two for most locations, which still looks to be the hottest
two days. Expect high temperatures between 96-103F both days for
inland valleys. Even the Coast Range lowlands and Cascade
valleys below 3500 feet are expected to reach 95-100F. Overall
model spread remains minimal, suggesting confidence is high
temperatures within this range will materialize. In addition,
overnight lows will be running warm with many locations
struggling to dip below the upper 60s to around 70F, resulting
in a Moderate to Major HeatRisk for inland areas. The warmest
overnight lows are expected in the Columbia River Gorge where
lows in the low to mid 70s are in the forecast due to breezy
easterly winds through the night. NBM temperature guidance seems
reasonable given the set-up in place and the transition to low-
level offshore flow. Note dry and breezy conditions on Friday
will lead to near critical fire weather conditions in the
eastern Portland/Vancouver metro, Columbia River Gorge and
adjacent Cascade ridgetops. Please see the fire weather
discussion below for details.
Although model spread increases early next week, forecast
confidence continues to increase for high temperatures above
95 degrees on Monday, and possibly into Tuesday. Probabilities
for this have increased to 65-90% on Sunday, remain near 55-75%
on Monday, but have increased to 55-75% on Tuesday (except
25-35% in the Cowlitz Valley and lower Columbia). Probabilities
for highs above 95 have increased considerably on Wednesday as
well, now ranging between 35-55%, except 20-30% in the Cowlitz
Valley and less than 10% in the lower Columbia. These
probabilities indicate there is still considerable uncertainty
regarding the exact duration of this heat event and when it will
end.
Despite uncertainty regarding the end date of this heat event,
confidence remains high enough in the short term to warrant the
continuation of an Extreme Heat Warning for the Portland/Vancouver
metro area through the central/southern Willamette Valley, Oregon
Cascade foothills, Columbia River Gorge, lower Columbia, Cowlitz
Valley, and Upper Hood River Valley. A Heat Advisory remains in
effect for the Cascades below 3500 feet, the Coast Range
valleys, and lower elevations in the Willapa Hills. Both the
Warning and Advisory are in effect from 11 AM Friday through 5
AM Tuesday. If you have extended outdoor plans, especially ones
that include strenuous activity, it is important to stay extra
hydrated to avoid heat-related illness. Stay in the shade as
much as possible, and take frequent breaks.
Another thing that comes into play Sunday into 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 Sunday afternoon in eastern Lane County 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
remains 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...High pressure continues to build across the region,
leading to widespread VFR conditions beneath largely clear skies.
North to northwest winds ease overnight to 5 kt or less at all
terminals. There remains a 50-60% chance of restricted vis from
fog/mist along the coast, most likely at KONP from 12-18z Fri.
Northwest winds will build along the coast to 10-15 kt with gusts
around 20 kt by 18-20z Fri. Inland, winds will build out of the
north to northeast to around 10 kt after 16-18z Fri, with easterly
gusts of 20-25 kt west of the Columbia River Gorge. Additionally,
hot temperatures are forecast, so pilots should be aware of high
density altitude and reduced aircraft performance as a result.
PDX AND APPROACHES...VFR conditions beneath clear skies expected.
Northwest winds ease to around 5 kt tonight, then build out of the
east to northeast to around 10 kt by Friday afternoon. -Picard
&&
.MARINE...Very little change in the forecast pattern through
early next week. 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 extended the Small Craft
Advisories through Friday evening. 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. /Hartsock
&&
.FIRE WEATHER...Expect hot and dry conditions through at least
Tuesday for areas away from the coast. A Red Flag Warning for
fire weather zone 685 (Southern Willamette Valley) has expired
as winds have fallen below criteria this evening.
Breezy easterly winds will develop Friday morning and continue
through Friday evening for the eastern Portland/Vancouver metro,
and through Friday night for the Columbia River Gorge and adjacent
Cascade ridgetops. Winds will be strongest on exposed ridgetops
where wind gusts will most likely peak between 25-35 mph.
Elsewhere, expect wind gusts between 15-20 mph, except up to 30
mph in the western Columbia River Gorge. The breezy east winds
will occur simultaneously with low relative humidity values of
20-30% or less, resulting in near critical fire weather
conditions. Use extra caution with potential ignition sources,
especially in grassy areas. Outdoor burning is not recommended.
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 Sunday 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-
105-126>128.
Extreme Heat Warning from 11 AM Friday to 5 AM PDT Tuesday for
ORZ108>125.
WA...Heat Advisory from 11 AM Friday to 5 AM PDT Tuesday for WAZ202-
208-211.
Extreme Heat Warning from 11 AM Friday to 5 AM PDT Tuesday for
WAZ204>207-209-210.
PZ...Small Craft Advisory until 8 PM PDT Friday for PZZ251>253.
Small Craft Advisory until 5 AM PDT Saturday for PZZ271>273.
&&
www.weather.gov/portland
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