836
FXUS66 KPQR 300445 CCA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
944 PM PDT Mon Sep 29 2025

...Updated aviation discussion...

&&

.SYNOPSIS...A fall like weather pattern is expected to continue
through much of the week as we transition from September into
October. Expect periods of rain through at least Thursday as a
series of fronts brings unsettled weather and cooler temperatures.
Breezy southerly winds expected on Tuesday, followed by rain showers
and chances for thunderstorms as a low pressure system remains
offshore. Dry weather likely returns for the weekend while
temperatures remain seasonably cool.

&&

.DISCUSSION...Tonight through Sunday...A Pacific front moving across
the region this afternoon is bringing a round of light rain across
much of the area. Many coastal locations have seen total rainfall
amounts of around 0.40-0.80 inch thus far, while most inland
locations will likely see around 0.10 to 0.25 inch of rain through
this evening. Expect conditions to turn dry again tonight through at
least Tuesday morning as the shortwave upper trough lifts east of
the Cascades and surface winds turn offshore. Temperatures will be
running near to slightly below normal through most of the week with
highs generally in the mid to upper 60s inland.

A more potent frontal system is expected to push another round of
rain onshore by Tuesday afternoon along with breezy southerly winds
through the evening. Water vapor satellite imagery this afternoon
shows a low pressures system developing near 140W/48N, which is
expected to strengthen to around 977mb overnight. Fortunately, there
is strong ensemble agreement that the surface low remains well
offshore and to the north, potentially nearing Vancouver Island late
Wednesday or Thursday, as it gradually weakens. There is still some
timing uncertainty with the front on Tuesday, but generally expect
an initial band of more moderate rain to push across the area late
Tuesday morning through the afternoon. Expect southerly wind gusts
along the coast, especially beaches and headlands, approaching 40-50
mph, with just a 10-20% chance of high wind gusts exceeding 58 mph
as an occluding cold front approaches the coast. Inland areas will
mainly see wind gusts of 30 mph or less. While guidance suggests
there is only a 5-25% chance of wind gusts up to 40 mph, with leaves
still on the trees, there is a small chance for a few broken tree
limbs or garbage cans falling over through Tue night.

Behind the initial frontal band, precipitation turns showery through
at least Thursday as the low pressure system remains offshore. With
the cooler air aloft and proximity to low pressure, will see
increasing chances for a few showers to grow deep enough to produce
some thunder. The main chances (15-25%) for thunderstorms likely
remains along the coast on Tuesday, before spreading inland with
around a 20% chance of thunder Wednesday afternoon. Southerly winds
remain somewhat elevated through Wednesday with gusts generally up
to 35 mph along the coast and 25 mph inland. Total rainfall amounts
will largely be dependent on where showers set up, but generally
expect around 0.25-0.50 inch of rain inland, and up to 1.5 inch
along the coast and higher terrain through Thu night.

By Friday, most ensemble members are showing drier weather returning
into the weekend. WPC 500 mb cluster analysis shows relatively good
agreement of the upper low dropping into northern California late
Friday. /Hartsock

&&

.AVIATION...Shower activity has ended across most of northwest
Oregon as the front has progressed further east. Tonight, expect
mostly dry conditions with predominately low-end VFR CIGs across
most terminals (except KONP) as the region enters a brief lull
before the next frontal system. Winds will weaken overnight and come
out of the south/southeast, generally remaining under 10 kt. The
next warm front arrives Tuesday morning, with rain beginning along
the coast by 15-18z Tue and across the Willamette Valley by 18-21z
Tue. The trailing cold front arrives Tuesday afternoon, with
moderate to high confidence (60-80% chance) in a return of MVFR CIGs
along the coast after 21z Tue. For inland locations, there is lower
confidence (20-30% chance) for MVFR CIGs Tuesday afternoon.
Southerly winds increase across the area beginning 15-18z Tue, with
gusts peaking in the afternoon around 35-40 kt along the coast and
25-30 kt inland.

Widespread rain transitions to showers behind the front after 00z
Wed, with a 15-25% chance for thunderstorms along the coast. Any
passing thunderstorms can bring gusty, erratic winds and heavy rain
which could briefly reduce visibility to IFR/MVFR thresholds.

KPDX AND APPROACHES...Predominately low-end VFR conditions with a 15-
30% chance for MVFR CIGs through 18z Tue. Winds turn more east-
southeasterly tonight, around 4-6 kt. Rain returns after 18-21z Tue
with the next front, with winds turning southerly and increasing to
10-15 kt with gusts up to 25 kt.        -Alviz

&&

.MARINE...An initial cold front has moved onshore this afternoon,
however southerly winds of 8-15 kt continue to support seas of
8-11 ft. While winds ease further through this evening, seas
may not fall below 10 ft, and the Small Craft Advisory has
therefore been extended through tonight ahead of the next system.

As a surface low deepens west of Vancouver Island over by Tuesday
afternoon and another, more potent frontal boundary approaches the
coast, southerly winds will increase dramatically tonight through
Tuesday. At their peak through Tuesday afternoon, speeds will
reach 30-40 kt with gusts to 45 kt across the waters. Isolated
storm-force gusts of 48 kt or more cannot be ruled out, but are
not expected to be persistent or widespread. In response, seas
will also build quickly to 15-18 ft by Tuesday afternoon, although
chances for 20 ft or greater seas have fallen to less than 5%.
While winds are expected to fall below gale-force on Tuesday
night, seas elevated above 10-15 ft are expected to linger through
much of Thursday. Additional Small Craft Advisories therefore
remain in place beginning late this evening until Gale Warnings
come into effect by early Tuesday morning through late Tuesday
evening. Hazardous Seas Watches have been issued for lingering
elevated seas midweek.

Behind the departing front, winds are expected to steadily weaken
below 15 kt by Thursday evening while seas similarly ease to 6-8
ft by Friday. As the deep low fills and moves inland by this
weekend, it will likely be replaced by building surface high
pressure over the northeastern Pacific which will see winds return
to seasonable northerly flow.      -Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Tuesday for PZZ210-251>253.

     Gale Warning from 11 AM Tuesday to 2 AM PDT Wednesday for PZZ210-
     251>253.

     Hazardous Seas Watch from late Tuesday night through late
     Wednesday night for PZZ251>253.

     Small Craft Advisory until 5 AM PDT Tuesday for PZZ271>273.

     Gale Warning from 5 AM to 11 PM PDT Tuesday for PZZ271>273.

     Hazardous Seas Watch from Tuesday evening through late Wednesday
     night for PZZ271>273.
&&

$$

www.weather.gov/portland

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