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058 
FXUS66 KPQR 210450 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
950 PM PDT Wed Aug 20 2025

Updated Aviation Discussion...

.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 Sunday
with high temperatures of 95-101F. Remaining warmer than average
early next week with a 80-90% chance for high temps of 90F or warmer.
Dry and breezy conditions over the southern Willamette Valley
Thursday afternoon and early evening will increase fire weather
concerns.

&&

.DISCUSSION...Wednesday night through Tuesday night...A significant
pattern change begins tomorrow for NW Oregon and SW Washington as a
hot and dry conditions begin to develop across the region. This
pattern shift is in response to an upper level ridge of high pressure
currently centered over the Four Corners region, which will be
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 bring a transition from
low-level onshore flow to offshore flow, causing an increase in north
to northeast winds for the Willamette Valley, Cascades and Columbia
River Gorge (except due east in the western Gorge and high Cascades).
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.

In regards to forecast temperatures, there hasn't been much change
with today's update. The deterministic NBM continues to suggest heat
will peak Friday and Saturday with widespread highs ranging between
95-101F across inland valleys. Overall model spread is minimal on
Friday, suggesting confidence is high to start the heat event. Spread
increases a few degrees on Saturday, with the NBM 90th percentile for
high temps approaching 105F. Spread increases more considerably
Sunday through Tuesday, suggesting confidence is relatively low
regarding the exact duration of this heat event (e.g. will it only
last 2-3 days, or will it last 4-5 days)? The latest NBM run (21z
Aug. 20th) shows NBM 10th-90th percentile values ranging from the
upper 80s/lower 90s to the lower 100s on Sunday and Monday, and mid
80s to around 100 degrees on Tuesday. For now, the most likely
outcome is for high temps somewhere in the 90s on all three days
(80-90% chance). Given the ongoing uncertainty regarding exact
temperatures early next week, have left the current timing of the
Extreme Heat Watch as is, covering Friday afternoon through Sunday
night for inland valleys away from the coast. However, have decided
to expand the Extreme Heat Watch to include the south Washington
Cascade foothills and the remainder of the Oregon Cascade foothills,
as these locations are likely to see temps peak in the 95-100F range
with a Moderate to Major HeatRisk. Note these areas will also have
mild overnight low temps in the 60s, similar to urban areas in the
Willamette Valley. Note the coast is not included in any heat
headlines as temperatures will be much cooler there with highs in the
70s.

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. 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 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
Cascades beginning as early as Saturday afternoon or Sunday afternoon
when an upper level impulse will push northward towards the Cascades.
Note 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 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 impact high
temperatures depending on the coverage and timing. -TK/HEC

&&

.FIRE WEATHER...Expect hot and dry conditions Friday through at least
Sunday for areas away from the coast. Critical fire weather
conditions are expected to occur over the southern Willamette Valley
between 2 and 8 PM Thursday 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 is 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 Thursday night into Friday. 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.

Winds weaken beginning late Friday afternoon. 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
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 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

&&

.AVIATION...With high pressure overhead and mostly clear skies 
through the period, there is low confidence (guidance suggests a 
10-25% chance) for LIFR/IFR stratus along the coast between 12-16Z
Thursday. Otherwise, high confidence for widespread VFR 
conditions to continue through the TAF period. Coastal terminals 
will see mostly northwesterly winds, with wind speeds under 10 kt 
until 18Z Thursday, then becoming 13-18 kt with gusts up to 25 kt.
Inland terminals will generally stay under 10 kt through most of 
the period, but will increase to around 7-10 kt by 20-23Z 
Thursday, with gusts up to 20 kt possible at KPDX, KSLE, and KEUG.

PDX AND APPROACHES...VFR and mostly clear skies through the 
TAF period, with northwesterly winds expected around 7-10 kt with
gusts up to 20 kt by 19-23Z Thursday.
~Hall

&&

.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. Expect winds to gust up to 25 kt by
late this afternoon south of Cape Falcon. Winds ease a bit by
early Thursday morning, then increase again, with gusts up to 25-30
kt, with gusty northerly spreading northward as well. Expect
similar conditions through the end of the week. So, have 
maintained the Small Craft Advisories through Friday afternoon.
Seas generally remain wind driven, at around 4 to 6 ft with a
dominate period of around 8-9 seconds. /Hartsock

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Extreme Heat Watch from Friday morning through late Sunday night 
     for ORZ108>125.

     Red Flag Warning from 2 PM to 8 PM PDT Thursday for ORZ685.

WA...Extreme Heat Watch from Friday morning through late Sunday night 
     for WAZ204>210.

PZ...Small Craft Advisory from 2 PM Thursday 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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