797
FXUS66 KPQR 080530 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
930 PM PST Sun Dec 7 2025

Updated aviation, marine, and hydrology discussions.

.SYNOPSIS...
A steady, moisture-rich pattern continues through
midweek as two strong atmospheric river surges arrive Monday and
again Tuesday night to Wednesday. Conditions become increasingly
impactful Monday and Tuesday as deep subtropical moisture and
stronger winds move inland. River rises and localized flooding will
need close attention through the week. Gusty winds are also expected
and could bring down trees and cause power outages.

&&

.DISCUSSION...Today through Saturday...Scattered showers perish
across the area as a weak disturbance continues to move across the
region, a quiet precursor to the more impactful weather system that
moves in overnight.

The overall pattern remains basically the same from previous
forecasts with slight changes in rainfall totals and where impacts
are expected. The synoptic set-up for the week starts with a ridge
of high pressure centered over the Pacific between Hawaii and
southern California. Clockwise circulation will advect sub-tropical
moisture northward toward the PacNW in several round between Monday
and Wednesday night. Mid level flow will predominantly be zonal as
the top of the ridge extends northward along the West Coast. At the
same time, several troughs will support surface lows and cold fronts
that will provide lift over the region. The first slug of AR
moisture enters the picture late tonight into Monday with IVT values
along the coast peaking between 750-1000 kg/ms and values inland
peaking between 500-750 kg/ms. This round of rain will be falling on
already saturated soil, WPC has maintained the Slight/Marginal ERO
risk over the CWA. The slight risk covers all of SW WA, down the
coast range in OR and along the Cascades down to around Santiam
pass. A couple of changes to note with this first round are the
slight northward shift in the highest IVT values, leading to changes
in the total QPF Monday through Tuesday. Totals across the southern
Willamette Valley and Lane County Cascades and Coast Range have
dropped slightly while totals over southwest Washington areas
generally north of Salem have increased slightly. While this has not
caused major changes in expected impacts, several river forecasts
have seen increases in flooding potential. More details are included
in the hydrology section below.

The second round of high IVT values is expected to impact the region
Tuesday night into Wednesday. IVT Values look similar to the first
round, maybe a touch lower which will lead to slightly lower
rainfall totals. Rainfall is still expected to be heavy with this
second round, regardless of whether it reaches the same level as the
fist round. Expect continued urban and river flooding issues at
least through Wednesday.

Winds will be a concern Monday through Wednesday, with gusts up to
at least 30 mph inland and up to at least 40 mph at the coast. Given
the soils are already saturate and heavy rain with gusty winds is
expected, a Wind Advisory has been issued for potential impacts. The
advisory covers the OR and WA Coast Range, Willamette Valley from
Salem northward through the Portland/Vancouver Metro, I-5 corridor
in WA, and the Cascade Foothills north of Salem. Saturated soils
will see persistent rain and eventually gusty winds, resulting in
downed trees and possibly power outages. Confidence in impacts
occurring is high but confidence in exact location and extent of
impacts is low.

Snow levels will remain well above 6000 ft through midweek due to
the warm subtropical air mass, ensuring that nearly all
precipitation in the Cascades falls as rain. By Thursday and Friday,
the moisture plume weakens and transitions toward a showery pattern.
Conditions will gradually ease, but rivers and soils will remain
sensitive due to cumulative rainfall. -19/12


&&

.AVIATION...The warm frontal boundary continues to shift towards
the region overnight. Will see an increase of cloud cover and more
widespread MVFR conditions. Showers will begin to intensify by 15Z
Mon which will persist for a prolonged period of time. With the
heavy rain and reduced CIGs, expecting widespread MVFR conditions
if not IFR along the coast. With the frontal system will see
strong south to southwesterly winds. Gusts as high as 40 kt
possible along the coast with increasing chances for gusts around
30 kt within the Willamette Valley. The Columbia River Gorge and
foothills will see less wind though will remain elevated. These
winds will be coupled through the atmosphere so approaches too
will be impacted. At around 2000 ft, wind speeds will be southwest
with speeds up to 50 kt. Some models are suggesting gusts as high as
65 kt but those would be isolated. Therefore, have included the
mention of LLWS in some area TAFs.


KPDX AND APPROACHES...VFR conditions prevail until around 09-12z
when MVFR ceilings are expected to return. Scattered showers are
possible through the night. A frontal system will bring
increasing rain after 14z Monday with lowered visibilities. South
winds will increase with gusts up to 25-30 kts after 14z. -27/03

&&

.MARINE...Winds have decreased substantially this afternoon behind
this morning`s frontal system, and will continue to decrease into
the evening. There will be a brief period of benign conditions
tonight with wind gusts below 20 kts and seas under 8-9 ft.

A stronger frontal system is set to arrive early Monday, bringing
gale force wind gusts up to 35-45 kt, strongest over the inner
waters and near the Columbia River Bar. A Gale Warning is in
effect from 3 AM Monday to 1 AM Tuesday as probabilities for gale
force gusts are up to 80-90%. With the increasing winds, seas will
become steep and hazardous Monday afternoon into Monday night.
Expect seas to peak somewhere between 14 to 17 ft.

Winds decrease from north to south late Monday night into Tuesday
morning with gusts falling below 20 kts everywhere except for
portions of zones PZZ273 and PZZ253 briefly on Tuesday morning.
Then another frontal system moves through the waters Tuesday
afternoon into Wednesday, causing increasing southwesterly winds
with gusts up to 25-30 kt likely. There`s around a 20% chance of
occasional gale force wind gusts over 34 kts across the waters
Tuesday night, though it is not expected to have widespread gale
force gusts for a long enough period of time at this moment. Seas
also decrease slightly Tuesday into Wednesday but remain above 10
ft. Seas and winds subside late in the week. -23/03

&&

.BEACH HAZARDS...
Tidal overflow flooding around high tide is
likely on Monday, December 8 and possible on Tuesday, December 9.
Despite total tide forecasts decreasing over the coming days,
river levels will be rapidly rising due to a prolonged period of
heavy rain Monday through Wednesday. Confidence has increased that
several rivers along the Oregon Coast north of Lincoln City and
the SW Washington Coast will reach at least 80% of flood flow by
high tide Monday afternoon, when total tide forecasts are expected
to peak near or slightly above 9.5 ft. Therefore, a Coastal Flood
Advisory for tidal overflow has been issued from 2 PM to 6 PM PST
Monday. Uncertainty remains on whether a Coastal Flood Advisory
will be needed on Tuesday because even though there is high
confidence that coastal rivers will be above 80% of flood flow
during high tide, the total tide forecast is below 9.5 ft at most
tidal points. Will continue to monitor the tidal forecast for
Tuesday.

Additionally, there is an elevated threat for sneaker waves late
Wednesday through Thursday along the SW Washington coast south
through the central Oregon coast. The swell period increases to
14-15 seconds with swell heights around 9-11 ft with both
decreasing late Thursday night. This will create energetic waves
that will present a moderate to high risk for sneaker waves,
waves which can run significantly farther up a beach than normal,
including over rocks and jetties. Sneaker waves can suddenly knock
beachgoers off if their feet and quickly pull them into the
frigid ocean which may lead to serious injury or drowning. Caution
should be used when in or near the water. Always keep an eye on
the waves, and be especially watchful of children. -03

&&

.HYDROLOGY...A series of frontal systems will keep conditions
very wet through the weekend and much of next week. The most
notable period will be late Monday through late Wednesday, when
a prolonged atmospheric river will bring significant rainfall
and the potential for widespread river flooding across southwest
Washington and northwest Oregon. There is a 10% chance for rain
amounts up to 10 inches  in the Cascades and Coast Range, up to
8-10 inches at the coast, and up to 7.5-8.5 inches in the
Willamette Valley, Portland/Vancouver metro and Cowlitz Valley.
While rain amounts this high are unlikely to occur, they
represent a reasonable worst case scenario. This outcome would
result in widespread major flooding.

Although rain will likely become lighter Thursday into Friday,
the threat of flooding will linger along slow responding rivers.
Rivers draining the Coast Range and Willapa Hills have a 30-60%
chance of reaching moderate flood stage and a 20-40% chance of
reaching major flood stage. Additionally, many Willamette River
tributaries that drain from the Coast Range and Cascades have
anywhere from a 40-70% chance of reaching minor flood stage and
a 20-50% chance of reaching moderate flood stage. Flooding of
the Willamette mainstem rivers is unlikely to occur (less than
10%). Probabilities for specific river points can be found at
the National Water Prediction Service website.

There is also increasing confidence in the potential for
flooding along small creeks and streams as well as urban
flooding, including roadway flooding. Periods of heavy rain
will also increase the risk for landslides in areas of steep
terrain and debris flows over recently burned areas. People,
structures, and roads located below steep slopes, in canyons,
and near the mouths of canyons may be at serious risk from
rapidly moving landslides.

A Flood Watch remains for all of southwest Washington and
northwest Oregon from 4 AM Monday to 4 AM Friday. Considering
increased confidence and probabilities for flooding of rivers
and urban areas as the forecasted event approaches, a Flood
Warning has been issued for the following rivers.

Wilson River near Tillamook affecting Tillamook County, from
Monday evening to late Tuesday morning. There is a 75% chance of
reaching moderate flooding and a 50% chance of reaching major
flooding. Grays River at Covered Bridge near Rosburg affecting
Wahkiakum County, from Monday evening to early Thursday morning.
There is a 70% chance of reaching moderate flooding and a 50%
chance of reaching major flooding. Johnson Creek at Sycamore
affecting Clackamas and Multnomah Counties, from Monday evening
to late Wednesday evening. There is a 70% chance of reaching
moderate flooding, and a 60% chance of reaching major flooding.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory from 2 PM to 6 PM PST Monday for ORZ101-
     102.
     Wind Advisory from 7 AM Monday to 4 AM PST Tuesday for
     ORZ104>115-123.
WA...Coastal Flood Advisory from 2 PM to 6 PM PST Monday for WAZ201.
     Wind Advisory from 7 AM Monday to 4 AM PST Tuesday for
     WAZ202>208.
PZ...Gale Warning from 3 AM Monday to 1 AM PST Tuesday for PZZ210-
     251-271.
     Gale Warning from 3 AM Monday to 7 AM PST Tuesday for PZZ252-
     253-272-273.
&&

$$

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