360
FXUS66 KPQR 082121
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
221 PM PDT Mon Sep 8 2025

.SYNOPSIS...
A closed low offshore is bringing cool and showery weather to
the region today and Tuesday. Conditions gradually trend drier
late in the week as the system shifts inland and weakens.
Confidence is growing that another Fall-like storm will arrive
late Saturday into Sunday, bringing cooler temperatures and a
round of widespread rain to western Oregon and Washington.

&&

.SHORT TERM...Now through Wednesday Night...Cool and unsettled
weather remains the theme through midweek as a closed upper-
level low continues to edge toward the coast. Scattered showers
have already spread into much of northwest Oregon this
afternoon, with most of the Willamette Valley, Cascades, and
foothills expecting to see activity this evening. The main
exception continues to be the south WA and north OR coast and
the Willapa Hills, where coverage has been limited so far.

Thunder potential is fairly moderate overall for the southern
Willamette Valley and Lane and Linn County Cascades, with Lane
County Cascades standing out with the best combination of
ingredients. Storm probabilities there run around 35-50% into
early evening, and some could produce brief heavy rainfall with
a 20-30% chance of exceeding 0.5 inches per hour between 4-8 PM
today. This raises some localized debris flow concerns in burn
areas, though storm motion should prevent repeated impacts on
the same location. The chance of more impactful rainfall rates
(~1 inch per hour or more) is low, around 5-10%.

Showers will persist tonight into Tuesday but become less
widespread. Isolated thunder remains possible Tuesday afternoon
(25-35%), with heavier cells capable of localized downpours.
Totals will vary widely - some areas may only pick up 0.05 to 0.10
inches, while others could receive 0.25 to 0.50 inches, especially
in the Cascades. By Wednesday, most of the lowlands trend
drier, but the Cascades maintain a 50-70% chance of afternoon
showers and even a 10-20% chance of thunder in higher elevations
of the Cascades. Temperatures stay on the cool side of normal,
with highs in the low to mid 70s inland and 60s along the coast.


.LONG TERM...Thursday through Sunday night...The upper low
drifts east across the Great Basin Thursday and Friday before
eventually settling into Montana and Wyoming for the weekend. As
it departs, shortwave ridging builds in and helps the region dry
out late week. That said, there is still some potential for
lingering showers, especially along the Cascades Thursday where
the lows influence may hang on a bit longer.

As for the weekend, ensemble guidance has shown increasing
agreement on another system arriving late Saturday into Sunday.
Confidence is moderate to high that this feature will bring
cooler weather and widespread rain across western Oregon and
Washington. NBM probabilities for 24 hour rain range from
50-70%, with lower chances on Saturday and higher chances on
Sunday. Looks like there is a 25-35% chance of a quarter inch of
rain on Saturday and a 40-50% chance on Sunday. Either way, the
weekend system looks to deliver a much more Fall-like pattern
across the region.
~Hall

&&

.AVIATION...A low pressure system offshore of the Oregon coast
will maintain showers across the region through at least Tuesday
night. There is a 20-30% chance for thunderstorms this afternoon
and evening, and another 20-30% chance for thunderstorms late
Tuesday morning through Tuesday afternoon. Lightning, erratic
winds, and heavy rain are the main threats with the
thunderstorms. There may be breaks in the rain and clouds along
the coast late tonight into Tuesday morning where calm winds,
clearing, and residual moisture may lead to patchy fog. Low
pressure systems usually provide enough atmospheric mixing to
prevent fog, but the low may be weak enough and south enough that
mixing along the coast may be minimal.

KPDX AND APPROACHES...There is a 20% chance for thunderstorms
through 06z Monday with lightning, erratic winds, and heavy
showers possible. The chance for thunderstorms returns 18z
Tuesday. Otherwise, Showers and light winds will dominate for the
next 24 hours. ~TJ

&&

.MARINE...Light winds with seas less than 4 feet tonight and
Tuesday as weak low pressure holds across the coastal waters. High
pressure builds Tuesday night and Wednesday returning north to
northwest winds. Northerly winds persist through Thursday. Winds
today through Thursday remain under 10 kt with minimal impacts.
Seas remain around 2-3 ft at 12-14 sec through Tuesday, building
to 4-5 ft at 11-12 sec Wednesday to Thursday with a west-
northwesterly swell.

By Friday into the weekend, guidance is suggesting another low
pressure system approaching the Pacific Northwest. A front
associated with this low will weaken as it moves across the
waters Saturday and Sunday. The winds will be south ahead of the
front on Saturday, then northerly with and behind the front on
Sunday. There is a 10-15% chance for wind gusts of 21 kt (small
craft conditions) across the waters on Saturday. Seas will be 5-7
feet with 12 second wave periods this weekend. ~TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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