020
FXUS66 KPQR 090350
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
750 PM PST Mon Dec 8 2025

Updated hydrology discussion...

&&

.SYNOPSIS...A strong atmospheric river is underway across the
Pacific Northwest, bringing heavy rainfall, breezy winds, and
widespread hydrologic concerns. Widespread river flooding is
likely across the region, along with urban and small stream
flooding due to persistent heavy rain. Wind may lead to downed
trees and power outages due to saturated soils.

&&

.SHORT TERM...Now through Wednesday...The strong atmospheric
river (AR) event has begun across the Pacific Northwest, with
radar imagery showing moderate to locally heavy rain across
northwest Oregon and southwest Washington as of early Monday
afternoon. Satellite imagery clearly shows a long, narrow band
of moisture extending from the Pacific Northwest out 3000+ miles
offshore, and about 1200 miles northwest of Hawaii. This
atmospheric river is being driven by a sub-tropical ridge
centered just offshore of Southern California and Baja
California (~32.05N/130.94W), and a trough centered south of the
Aleutian Islands of Alaska (~45.79N/164.89W). Moisture is being
advected along the northwestern periphery of the ridge and the
southeastern periphery of the trough, leading to the extended
band of moisture progressing toward the Pacific Northwest.
Ensemble guidance suggests minimal change in the synoptic
pattern over the next 48 hours; therefore, this atmospheric
river event and associated impacts are expected to last through
at least Wednesday.

There are one of two surges of moisture coming into the Pacific
Northwest with this AR. The first is coming in right now, which
is associated with integrated vapor transport (IVT) values of
750-900 kg/ms based off GFS and Euro ensemble guidance. This
will maintain steady rainfall and lead to substantial rainfall
amounts across western Washington and far northwest Oregon
(especially Salem northward). Lower rainfall amounts are
forecast for the southern Willamette Valley (Eugene) as most of
the moisture is being directed further north. Urban and small
stream flooding is possible throughout this AR event, especially
in areas that are low-lying and have poor-drainage. If you will
be commuting during periods of heavy rain, make sure to slow
down, use your headlights, and leave plenty of distance ahead of
you.

Since 12 AM Monday, weather stations have already reported
1.5-2.25 inches of rain across the north Oregon and south
Washington coasts, Willapa Hills, north Oregon Coast Range, and
south Washington Cascades and foothills. These amounts were
achieved by heavier rainfall rates of 0.25-0.50" per hour. Based
off the latest REFS guidance, most ensemble members suggest
that these rainfall rates will continue through late Monday
evening into early Tuesday morning over the aforementioned
areas. Rainfall rates this high will result in rivers and creeks
that drain off the Willapa Hills and Clatsop/Tillamook Counties
to start rising sharply through the evening (see hydrology
section for more details on river flooding). Elsewhere, rainfall
amounts since midnight generally range from 0.25-1", with the
highest amounts further north you go. South of Albany, OR and
across the Lane County Cascades, amounts have been around 0.10"
or less, which is expected given the more northward track of the
AR.

This plume of AR moisture will briefly track southward tonight
into early Tuesday morning, bringing increased rainfall toward
Lane County. Will see a very brief break in rainfall rates
across far northwest Oregon and southwest Washington Tuesday
morning, but that doesn`t mean we`ll be clear of any impacts.
Local rivers tend to lag a few hours following heavy rainfall,
so will most likely see rivers continuing to rise and
potentially hit flood stage even when rain has decreased.
Again, more details in the hydrology section on this.

The second surge of AR moisture will hit western Washington and
far northwest Oregon Tuesday afternoon into Wednesday, bringing
another round of moderate to locally heavy rainfall to these
areas. It will be a similar set-up as today, but this time
ensemble guidance suggests IVT values will be around 600-700
kg/ms. Given the amount of rain that will have fallen before
this second round, local rivers will continue to rise and
concerns for urban and small stream flooding will continue. For
snow and winter sport lovers, unfortunately this AR will be very
warm and snow levels will remain above 8000 feet through
Wednesday.

Wind will also potentially bring impacts during this AR event.
Wind Advisories remain in effect through 4 AM Tuesday across the
central/north Oregon Coast Range, Willapa Hills, Willamette
Valley from Salem northward, and southwest Washington lowlands.
Southerly winds are forecast to gust up to 30-40 mph tonight.
Saturated soils will increase the likelihood that even moderate
gusts could cause uprooting of trees and lead to damage. While
there is moderate to high confidence in potential impacts due to
wind, there is low confidence on exactly where impacts may
occur. If you see a downed power line, move away from it. Do not
drive over downed power lines. If you come into contact with a
downed power line, stay inside the car and call for help. -10


.LONG TERM...Wednesday Night through Sunday...The atmospheric
river event weakens Wednesday afternoon into Thursday as
moisture transport gradually decreases. Light residual showers
continue, with some localized areas of moderate rain showers
over higher terrain across southwest Washington and the north
Oregon Coast Range. By Thursday, the pattern transitions toward
a more typical post- frontal regime with shower activity
gradually decreasing and winds weakening. Friday becomes drier
as high pressure builds over the region. Snow levels will also
gradually lower heading into the weekend, but still remain above
pass-level at around 5500-6500 feet. Despite the weather
becoming more tame late in the week, rivers and soils will
continue responding to the cumulative rainfall load from the
current atmospheric river event. Any slow-responding rivers
within flood stage will need extra time to subside. -10/12

&&

.AVIATION...Messy conditions are expected to persist through the
TAF period and beyond as westerly flow brings moisture laden air
into the region. A steady stream of rain will continue through the
afternoon with a short lull this evening. CIGs inland are
expected to maintain MVFR but could see brief drops to IFR in the
heaviest rainfall. KEUG will be a potential exception as rain is
generally expected to be lighter over the southern Willamette
Valley and may not see persistent VIS or CIG reductions. CIGs
along the coast remain IFR or LIFR through most of the period.
Could see a brief period of improvement for KAST and KPDX/KTTD as
the stream of moisture shifts south late tonight into early
Tuesday morning before lifting back northward Tuesday afternoon.

Winds will be gusty through much of the period. Expect wind gust
of 35-45 kt along the coast up to 25-35 kt within the Willamette
Valley.

KPDX AND APPROACHES...MVFR conditions are expected to persist
through much of tonight. A steady stream of rain will keep low
CIGs and moderately impacted VIS in place. Moderate to heavy rain
through the afternoon will lighten up a bit this evening before
increasing again overnight. Guidance suggests a short period of
improving CIGs early Tuesday morning as the stream of moisture
shifts slightly to the south. Could see a brief period of clouds
lifting above 3000 feet before dropping back below that mark by
the afternoon. Winds will be southerly around 20 kt with gust to
25-35 kt through around 12z Tuesday. -19

&&

.MARINE...Gales continue across all zones as a strong system
passes over the region. Wind gusts 35-45 kt will continue
through early tonight, strongest over the inner waters. The gale
warning remains in effect until 1 AM Tuesday.

A lull is expected late tonight as the frontal boundary drops
southward, decreasing winds from north to south. Wind gusts are
expected to drop below 20 kt everywhere except zones PZZ273 and
PZZ253. Seas will also remain steep during this time so a Small
Craft Advisory will follow the gale warning from 1 AM to 7 PM
Tuesday to cover these elevated conditions. Another front is
expected to pass through the waters late Tuesday into Wednesday
and will push wind gusts closer to 30-35 kt. There is increasing
potential for gales Tuesday evening into the overnight and may
require an upgrade to another Gale Warning. Seas will increase
into the low to mid teens with this second system, reaming above
10 feet through Wednesday night. -19

&&

.BEACH HAZARDS...Tidal overflow flooding around high tide is
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 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 remains in effect until 6 PM PST today for
these coastal zones. It appears the threat for tidal overflow
flooding is low on Tuesday, as total tide forecasts decrease
considerably and are well below the 9.5 foot threshold. That
said, minor tidal overflow flooding cannot be completely ruled
out for a brief period of time around high tide on Tuesday given
river levels will be running quite high at that time. Will
continue to monitor the tidal and river forecasts 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
around 14-15 seconds with swell heights around 9-11 ft, with
swell heights and swell periods 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. -23/03

&&

.HYDROLOGY...A series of frontal systems will maintain very wet
conditions through much of this week. The most notable period
will extend through late Wednesday, when a long-duration
atmospheric river will bring significant rainfall and the
potential for widespread river flooding across southwestern
Washington and northwestern Oregon. There is a 10% chance for
72-hour rainfall of up to 6-8.5 inches in the Cowlitz Valley,
lower Columbia, Portland/Vancouver metro and northern Willamette
Valley, 4-6 inches in the central Willamette Valley, 3-4 inches
in the southern Willamette Valley, 7-11 inches in the Cascades,
Cascade foothills, Coast Range, and the coast, except 3.5-7
inches from Florence to Newport and in areas of higher terrain
in Lane County. While rain amounts this high are unlikely to
occur, they represent a reasonable worst case scenario. This
outcome would result in widespread major flooding. Chances for
high total rainfall amounts as well as brief periods of locally
heavy rainfall in Lane County have continued to trend downward
today. This suggests future river model suites will see similar
downward trends in the likelihood of river flooding.

Although rain will likely become lighter on Thursday, the
threat of flooding will linger along slow-responding rivers.
Rivers draining the Coast Range and Willapa Hills have a 40-70%
chance of reaching moderate flood stage and a 20-50% chance of
reaching major flood stage, with a handful of rivers already
observing flooding, namely the Grays River draining the Willapa
Hills in Major flood and the Wilson River in Tillamook in Minor
flood. Additionally, many Willamette River tributaries that
drain from the Coast Range and Cascades have anywhere from a
40-60% chance of reaching minor flood stage and a 20-40% chance
of reaching moderate flood stage. Flooding of the Willamette
mainstem rivers have a very low chance to occur (5-15%).
Probabilities for specific river points can be found at the
National Water Prediction Service website. Flood Warnings have
been issued for the Naselle River near Naselle and the Grays
River near Rosburg draining the Willapa Hills in southwestern
Washington, the Wilson and Trask Rivers near Tillamook, and
Johnson Creek at Sycamore near Milwaukie and along the southern
edge of the City of Portland.

There is also increasing confidence in the potential for
flooding along small creeks and streams as well as urban
flooding, including roadway flooding. Latest high-resolution
guidance favors a band of heavier precipitation lingering over
southwestern Washington and far northwestern Oregon late this
evening, before sagging southward through the early morning
hours. These potential heavy rainfall rates could make for a
hazardous morning commute along the I-5 corridor from Salem
north through Portland/Vancouver to Kelso/Longview with abundant
water atop area roadways. 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. Therefore, a Flood Watch remains in effect across
all of southwest Washington and northwest Oregon from now until
4 AM Friday.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Wind Advisory until 4 AM PST Tuesday for ORZ104>115-123.

WA...Wind Advisory until 4 AM PST Tuesday for WAZ202>208.

PZ...Gale Warning until 1 AM PST Tuesday for PZZ210-251-271.

     Small Craft Advisory from 1 AM to 7 PM PST Tuesday for PZZ210-
     251-271.

     Gale Warning until 7 AM PST Tuesday for PZZ252-253-272-273.

     Small Craft Advisory from 7 AM to 7 PM PST Tuesday for PZZ252-
     253-272-273.

&&

$$

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