125
FXUS66 KPQR 280403 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
903 PM PDT Wed Aug 27 2025

Updated aviation discussion and PQR WATCHES/WARNINGS/ADVISORIES.

.SYNOPSIS...An upper level low will maintain weak isolated
showers and a 15-30% chance for thunderstorms along the
Cascades through this evening. Warm and dry conditions Thursday
to Friday, until another active low aloft over the northeast
Pacific approaches the Pacific Northwest. This will bring
stronger onshore flow and moisture, cooling temps a few degrees
over the weekend.

&&

.SHORT TERM...Now through Friday Night...Synoptic analysis as
of early Wednesday afternoon depicts a vertically stacked
700-300 mb low centered over the north Oregon and south
Washington coast. This is bringing weak, wrap-around showers
along the northern and western periphery of the low over the
marine waters, coast, and areas north of Cowlitz/Skamania
County. Meanwhile, drier air is being advected in along the
southern and eastern periphery of the low into interior
northwest Oregon. Most of northwest Oregon is mostly sunny,
allowing to sufficient surface heating and instability; however,
there is currently around 100 J/kg of CIN west of the high
Cascades that`s preventing any thunderstorms from popping up.

It`s a different story for the high Cascades, where they are
above the capping inversion and isolated thunderstorms are
developing along the crest of the Linn and Lane County
Cascades (as of 3 PM). A 15-30% chance for thunderstorms
continues through this afternoon for the Cascades. Main threats
from passing thunderstorms include lightning, gusty outflow wind
gusts up to 40 mph, and heavy downpours. The increased
instability and chances for thunderstorms will also bring fire
weather concerns, however, more details on that in the Fire
Weather section below. Today looks like our last day for any
threat of thunderstorms as the aforementioned upper low moves
further north tomorrow and the mid to upper level atmosphere
dries up.

Other than thunderstorms, temperatures are still remaining warm
with highs forecast in the upper 80s across interior valleys
today through Friday. Upper level shortwave ridging re-builds,
maintaining warm and dry conditions. Onshore flow continues,
so concerns for extreme heat are negligible. Overnight lows will
gradually cool down these next couple nights into the 50s and
low 60s for most places, while the Portland/Vancouver Metro and
Columbia River Gorge cool into the mid 60s. Moderate HeatRisk
still remains for these locations through Friday. Mild
conditions continue along the coast with marine layer clouds
and highs in the 60s. With lower pressure aloft, this could keep
clouds heights higher than previous days, preventing marine fog
and visibility reductions. -Alviz

&&

.LONG TERM...Saturday through Monday...Minimal changes to the
weekend forecast. Another upper level low over the northeast
Pacific will approach the Pacific Northwest. This low will
likely bring cooler daytime temperatures and increased moisture.
NBM chances for 85 degrees or warmer across the Willamette
Valley are around 20-40% on Saturday and 10-35% on Sunday, with
the highest chances in the southern Willamette Valley. The
stronger onshore flow further north would help keep temps
cooler.

There is still some uncertainty with how much precipitation we
would receive from this low, but it still looks like the jet
stream is projecting a jet streak moving directly overhead on
Saturday afternoon with stronger southwesterly flow. This type
of scenario could produce another round of thunderstorms.
However, the issue is that this low is moving further north
toward far northwestern Washington and British Columbia, and
there may not be enough moisture for thunderstorms or rain in
general. Coming out of prolonged high pressure with a generally
very dry air mass, it would take a much stronger low with a lot
more moisture to really get us some widespread rain. NBM chances
for measurable rainfall (0.01" or greater) over 48 hours ending
5 AM Monday are 25-45% along the coast, 15-25% across the
Portland Metro, southwest Washington lowlands, Columbia River
Gorge, and Upper Hood River Valley, and 5-10% south of Portland.

The decaying low will push northward leaving dry air trailing,
however, there is uncertainty with the progression of the low
heading into early next week. Ensemble clusters have a few
scenarios: the low retrograding back into the northeast Pacific
but not re-strengthening, the low retrograding and then
strengthening, or the low completely going away. It`s pretty
messy right now, but in general it appears that we`ll start the
next week remaining warm and dry for now. -Alviz/Muessle

&&

.AVIATION...Light rain showers associated with an upper level low
are moving northward out of the area this evening and were pushing
into northwest WA as of 04z Thu. Expect dry conditions for
southwest WA and northwest OR through 06z Friday with mostly
clear skies inland, except at KEUG where low marine stratus will
likely develop (40-50% chance) towards 13-14z Thu. Meanwhile,
coastal areas will remain under low marine stratus with LIFR to
IFR cigs through most of the 06z TAF period. Winds are generally
light and variable around 5 mph or less, but will increase out of
the west around 02z Friday as a sea breeze pushes inland.

PDX AND APPROACHES...Expect VFR flight conditions to continue
through 06z Friday with mostly clear skies. Light northwest winds
this evening will become light and variable less than 5 mph
overnight. -TK

&&

.MARINE...Benign conditions are expected into early next week
with winds below 10 kt and seas 2-4 feet. Winds become southerly
on Thursday in response to weak low pressure over the coastal
waters drifting northward, however wind gusts should stay below
10-15 kt. Both winds and seas are forecast to stay below small
craft advisory criteria for the foreseeable future. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

www.weather.gov/portland

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