103
FXUS66 KPQR 011015
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
315 AM PDT Mon Sep 1 2025

.SYNOPSIS...Seasonable temperatures continue through Labor Day
with morning marine clouds giving way to breaks of afternoon
sun. Building high pressure will yield a warming trend with the
potential for valley high temperatures reaching the 90s and
Moderate HeatRisk Tuesday through Thursday across inland areas.
Chances of scattered showers and isolated thunderstorms increase
Tuesday and Wednesday afternoon and evening, with dry and
unstable conditions leading to elevated fire weather concerns.

&&

.SHORT TERM...Now through Wednesday Night...Latest water vapor
satellite imagery depicts an upper-level low centered west of
the Olympic Peninsula early this morning. As this low begins to
slowly pull away from the coast through today, there will be one
final day of pleasant and seasonable weather across the region
for Labor Day. Another incursion of marine clouds this morning
will break up and erode by the late morning or early afternoon,
giving way to largely clear skies aside from passing high clouds
which will shift offshore by tonight. Expect afternoon high
temperatures in the low to mid 80s across most inland valleys,
with 70s in the Lower Columbia and Cowlitz Valleys, Cascades,
and Coast Range, and 60s in the Willapa Hills and along the
coast. Overnight lows will be in the low 50s to low 60s tonight.

As the upper low retrogrades to the west and pulls away from the
coast, ridging will build northward from the Great Basin into
the Interior Northwest for Tuesday and Wednesday. An additional
compact upper low will track northward, moving onshore in
northern California by Tuesday evening and into southwestern
Oregon as a shortwave trough by Wednesday morning. Support for
ascent and coincident elevated instability ahead of this
approaching wave will generate 15-25% chances for showers and
thunderstorms originating over the Cascades, most likely in the
central Oregon Cascades of Lane and Linn Counties. Anomalous
southeasterly flow aloft will see storms and/or outflow clouds
propagating downstream over the remainder of the forecast area.
These high-based thunderstorms could also result in dry
lightning as any rain evaporates into a dry sub-cloud layer,
although the column will moisten into Wednesday as the monsoonal
flow encroaches from the southeast. This combination of
thunderstorms, hot and dry surface conditions, and receptive
fuels will elevate fire weather concerns; see the Fire Weather
discussion below for additional information.

As mid-level temperatures reach 23-25C overhead, surface
temperatures will rise into the low 90s across the Willamette
Valley and mid 90s through the Columbia Gorge to the Hood River
Valley each of Tuesday and Wednesday afternoons. Elsewhere,
cooler but still above-normal temperatures largely in the 70s
and 80s are expected. There remains one important caveat in the
temperature forecast though: the location and extent of
potential convective debris clouds are difficult to pinpoint,
and increased cloud cover as well as occasional showers could
result in temperatures failing to reach these most-likely
forecast highs. With overnight lows only falling to the mid 60s
in the central and northern Willamette Valley and upper 60s
along the I-84 corridor, the combination of hot daytime and
elevated nighttime temperatures will result in fairly widespread
Moderate HeatRisk. Major HeatRisk will be limited to the
Columbia Gorge and Hood River Valley, and a Heat Advisory has
therefore been issued from noon Tuesday through 9 PM Wednesday
in these areas. As upper-level ridging begins to deamplify on
Wednesday night, temperatures should trend cooler. -Picard


.LONG TERM...Thursday through Sunday...By the latter half of the
week, long-range ensemble guidance favors a pattern shift as
upper-level troughing develops over the northeastern Pacific.
While details remain low confidence, this pattern would support
temperatures cooling back toward seasonal norms and increasing
chances for showers across the region, especially along the
coast and in the Cascades. The latest Climate Prediction Center
outlooks for September 6-10 also highlight this warm but
potentially wetter period in the Pacific Northwest. -Picard

&&

.AVIATION...Early this morning marine stratus has filled in along
the coastline with MVFR conditions at the majority of coastal
sites. As of 09-10z we`re also beginning to see low clouds fill
into the Portland/Vancouver Metro as well with KTTD falling to
MVFR and KPDX likely soon to follow. Probabilities for MVFR CIGs
peak inland 14-17z with the best chances for degraded MVFR flight
conditions KUAO northward, slightly lower chances at KEUG but
we`ll still need to watch the central/southern Willamette Valley
closely during this period. Any low clouds that do form over
inland valleys this morning should be short-lived, most likely
scattering out between 17-20z. While inland sites then remain VFR
through Monday night, coastal terminal likely see MVFR to IFR CIGs
return this evening. Winds will generally remain light at around
5-10 kt or less through the TAF period.

PDX AND APPROACHES...VFR conditions will most likely (80-90%) give
way to a period of MVFR cigs starting 10-12z Monday - KTTD has
already seen CIGs drop to BKN 1.8kft. Except low clouds to scatter
out and lift back to VFR towards ~19z Monday. Light northwest
winds around 5 kt continue. -Schuldt

&&

.MARINE...A rather typical summer-time weather pattern persists
through the next week leading to minimal marine concerns overall.
Currently, a weak area of low pressure sits just off the coast of
Washington helping to keep winds fairly light but also allowing
marine stratus to be rather prolific. Headed into Tuesday and
Wednesday however, a ridge of surface high pressure is expected to
build over the waters increasing northerly winds. Winds will be
strongest in the afternoon and evening hours Tuesday through
Thursday, especially across the outer waters and southward. Peak
wind gusts during the mid-week period get close to small craft
conditions (14-20 knot gusts) but not much higher. At least model
guidance continues to decrease the probability to meet small
craft criteria (>21 knots) with only around a 10-25% chance during
the afternoon and evenings hours Tuesday through Thursday. Seas
hold below ~4 ft through Tuesday morning, with an increase in wind
waves building seas to 5-6 ft at 7-9 seconds late Tuesday through
the end of the week. -Schuldt

&&

.FIRE WEATHER...A compact low pressure system is expected to
move north from California into Oregon Tuesday into Wednesday,
which will pose a threat for thunderstorm development. Anomalous
southeasterly flow aloft ahead of the low, along with synoptic
support for ascent and increasing elevated instability could
favor convection along the Cascades, most likely in the
Willamette National Forest in Lane and Linn Counties, with
storms and/or outflow clouds propagating downstream over the
remainder of the forecast area. Model soundings also show
abundant dry air beneath any elevated convection, which could
allow for thunderstorms with dry lightning through the
afternoon and evening on Tuesday and Wednesday. Even in wet
thunderstorms, lightning strikes could reach well outside of any
narrow rain core. With the relative humidity falling to
afternoon minimums below 25-30% along the Cascades and receptive
fuels remaining in place, there is an increased risk of new
fire starts from dry lightning. Additionally, gusty thunderstorm
outflow could result in unanticipated erratic fire spread,
while high instability could also lead to the growth of fires
already on the landscape. There does remain uncertainty in the
location and intensity of the low however, and therefore also
whether thunderstorms will be limited to Tuesday or continue
into Wednesday. Given this uncertainty, a Fire Weather Watch
remains in effect only for Tuesday from 2-8 PM. -HEC/Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory from noon Tuesday to 9 PM PDT Wednesday for
     ORZ120>122.
     Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ689-690.

WA...Heat Advisory from noon Tuesday to 9 PM PDT Wednesday for
     WAZ209-210.

PZ...None.

&&

$$

www.weather.gov/portland

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