393
FXUS66 KPQR 030446 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
946 PM PDT Tue Sep 2 2025

Updated aviation and short term discussions.

.SYNOPSIS...High pressure strengthening inland through midweek
will bring hot weather to much of the region, with valley highs
near 90 degrees and warm overnight lows in the 60s. HeatRisk
will be Moderate in most areas, with pockets of Major HeatRisk
in the Gorge and Hood River Valley. An upper low moving north
from California will trigger scattered showers and isolated
thunderstorms this evening and again Wednesday, mainly near the
Cascades. By the weekend, a shift in the weather pattern will
favor cooler temperatures and an increasing chance for showers.

&&

.SHORT TERM...Now through Thursday Night...The upper low that
kept conditions cooler over the Holiday weekend is drifting west
into the Pacific, while upper ridging expands northward from
the Great Basin. Meanwhile, a compact disturbance offshore of
California is forecast to move inland tonight and into Oregon
early Wednesday, providing the lift necessary for showers and
isolated dry thunderstorms along the Cascades through midweek.

Inland valleys will rise into the upper 80s to low 90s through
Thursday. The coast remains cooler in the 60s to 70s. Warm
nights will provide little relief, with lows generally in the
mid to upper 60s in the Willamette Valley and Cascade foothills,
near 70 in the Gorge, and 50s to low 60s elsewhere. This
combination of hot afternoons and warm nights will generate
widespread Moderate HeatRisk, with localized Major HeatRisk in
the Gorge and Hood River Valley. A Heat Advisory remains in
effect for these areas through Wednesday evening.

The shortwave trough within the flow around the low has enhanced
southeasterly flow aloft with north to westerly winds at the
surface. Have observed dry thunderstorms this evening however
not quite where previously expected. The orographic lift over
the Cascades has forced storms further west than forecast and
thus most of the lightning has been around the Central Oregon
Cascade Foothills into the Lane County coastal area. Most of the
lightning has been cloud-to-cloud (not necessarily reaching the
ground), but several strikes have reached the ground.
Precipitation with these storms has been minimal due to the dry
lower atmosphere. Based on the sounding from Salem this
afternoon, there is a warm air nose/capping inversion present at
around 4000 ft which could inhibit some lower elevation
convection. However, elevated convection is expected to persist
through the night. Cannot rule out a "midnight surprise" with
early morning convection rearing it`s ugly head.


Wednesday will see similar conditions with another round of
thunderstorms expected. Simulated reflectivity shows an more
intense band of showers moving northward after 1500 as the low
becomes centered just south of the forecast area. 850 mb winds
remain easterly and therefore drier air will be present at the
lower elevations and thus dry thunderstorms are possible though
confidence is slightly lower as there is a higher probability of
more cloud cover. There is around a 50-70% chance of overcast
skies through the day which would not bode well for thunderstorm
development. In order to get more solid convection, daytime
heating will be necessary which will be inhibited by cloud
cover.

By Thursday, the low begins to weaken and lift north, leading to
fewer storms and slightly cooler temperatures. Cloud debris or
showers may also limit daytime heating in places. Note,
thunderstorms may still remain possible on Thursday, but
confidence remains low at this time. -Muessle/Hall

.LONG TERM...Friday through Monday...By late week, forecast
ensembles show a shift toward troughing over the northeast
Pacific, ending the hot pattern and steering temperatures back
toward seasonal levels. Cooling is expected to continue into
early next week, with some areas trending below normal. The
broad offshore trough should also bring increasing moisture and
improve chances for showers, particularly along the coast and
Cascades. While details remain uncertain, the overall signal
points toward a cooler and potentially wetter stretch developing
from the weekend into next week.-Hall


&&

.AVIATION...VFR conditions expected for the next 24 hours at
inland sites. Stratus has started to fill in around KONP and will
slowly spreading north along the coast to form at KAST by 07-11z
Wed. Ceilings should return to VFR conditions by 18-20z Wed with
daytime heating. Light winds less than 5 kts expected overnight,
becoming northwesterly at 5 to 10 kts after 18-20z Wed.

Scattered showers and thunderstorms have started to develop as a
weather system passes overhead. Current rain probability is
10-15% at any valley terminal but given the ongoing precipitation
a few terminals may warrant a period of Prob30 overnight.
Wednesday will bring another chance for scattered showers and
thunderstorms, possibly a bit more abundant as a more robust
system moves over the region. 15-30% chance for showers and storms
across much of the area through Thursday 06z.

PDX AND APPROACHES...VFR conditions next 24 hours. Winds remain
light overnight, becoming northwesterly around 5 kt after 18z
Wed. -Batz/HEC

&&

.MARINE...
Fairly benign late-summer like conditions are expected
through the rest of the week into the weekend. A weak surface low
pressure off southwest Vancouver Island gradually moves westward
the next several days with high pressure strengthening across the
coast waters. This progression will tighten surface pressure
gradients, allowing for increasing northerly winds during the
afternoon and evening hours, especially on Wednesday and Thursday.
Peak wind gusts during the mid-week period likely sit in the
15-20 knots range with probabilities for small craft conditions of
gusts over 21 kts only around 10-20%. Seas increase to around 4-6
ft this afternoon due to an increase in wind waves and a fresh
northwesterly swell. Expect seas to hold in this 4-6 ft range
through Friday before backing off slightly over the weekend into
next week. -Schuldt/HEC

&&

.FIRE WEATHER...A low pressure system lifting north from
California into Oregon on Wednesday, and Washington by Thursday,
will bring a heightened risk for dry thunderstorms.
Southeasterly flow aloft will increase lift and instability
while surface conditions remain dry, with relative humidity
bottoming out near 20-30% along the Cascades and 30-40% in the
foothills. This setup will favor thunderstorm development over
the Oregon Cascades this evening, with activity drifting into
foothills and valley locations overnight.

High cloud bases will allow much of the rainfall to evaporate
before reaching the ground (virga), making dry lightning and
gusty, erratic outflow winds possible. While slower-moving
storms could bring localized rain, lightning strikes may extend
well away from rain cores. Given the very receptive fuels, new
fire starts are possible, especially in the Willamette NF where
a Red Flag Warning continues for Zones 689 and 690. Strong
instability may also enhance pyroconvective activity for ongoing
fires.

Through Wednesday, the low weakens as it tracks farther north,
lowering confidence in widespread dry lightning. However,
elevated instability will persist in southeastern Lane County,
supporting the extensions of the Red Flag Warning through 9 PM
Wednesday. To the north, a Red Flag Warning has been issued for
Zones 634, 635, and 688 - including the Mt. Hood and Gifford
Pinchot NFs from 12 PM to 9 PM Wednesday. -Hall

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from noon to 9 PM PDT Wednesday for ORZ688.
     Heat Advisory until 9 PM PDT Wednesday for ORZ120>122.
     Red Flag Warning until 9 PM PDT Wednesday for ORZ689-690.
WA...Red Flag Warning from noon to 9 PM PDT Wednesday for WAZ634-
     635.
     Heat Advisory until 9 PM PDT Wednesday for WAZ209-210.
PZ...None.
&&

$$

www.weather.gov/portland

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