833
FXUS66 KPQR 071814
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1014 AM PST Sun Dec 7 2025

Updated aviation discussion.

.SYNOPSIS...
A steady, moisture-rich pattern continues through midweek as two
strong atmospheric river surges arrive Monday and again Tuesday
night to Wednesday. Today will see comparatively lighter
rainfall, but 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.

&&

.DISCUSSION...Today through Friday...Light to moderate rainfall
will continue at intervals today under upper level zonal flow.
A passing disturbance this evening will maintain steady but
manageable rain rates, with amounts today lower than what is
expected early in the week. Rainfall totals through tonight are
projected to reach 0.2 to 0.4 inch across the interior lowlands,
0.4 to 0.8 inch along the coast, 0.8 to 2.0 inch in the Coast
Range, and 1 to 2.5 inches in the Cascades. These totals alone
are not expected to produce widespread impacts, but they will
further saturate soils ahead of the first major surge of
moisture on Monday.

Sunday night into Monday marks the transition to a
significantly wetter regime as the first atmospheric river
strengthens offshore and moves inland. IVT values Monday are
expected to rise sharply, reaching 600 to 800 kg/ms in many
ensemble members. Confidence is high that this surge will
produce widespread heavy rain beginning Monday morning, with
notably heavier rain amounts beginning Monday evening and
continuing overnight into early Tuesday. Rainfall amounts for
Monday continue to increase each model cycle. Current
expectations from 4 AM Monday to 4 AM Tuesday place 1.25 to 2.50
inches across the interior lowlands, with the highest amounts
centered over southwest Washington, where 1.9 to 2.2 inches are
likely in the Portland-Vancouver corridor. Coastal areas will
see 3 to 4.5 inches, the Coast Range 4.5 to 7 inches, and the
Cascades 4 to 7 inches. The Lane County Cascades will be lower,
generally 0.75 to 1.75 inches. These amounts will occur on
already softened ground, prompting notable river responses and
elevated hydrologic concerns (see hydrology section below for
more details).

Winds will likely be a significant impact on Monday through
Wednesday, notably on Monday. Confidence is high that inland
locations will experience gusts of at least 30 mph on Monday,
while coastal communities and headlands will see at least 40
mph. There is a 5 to 10% chance of advisory-level southwesterly
gusts of 45 mph or greater within the Willamette Valley on
Monday. Along the coast and the Coast Range, there is a 40 to
60% probability of gusts reaching or exceeding 50 mph. Should
gusts of this magnitude occur, scattered downed trees and power
outages will be possible. Given that soils are already wet and
will continue to saturate further, root stability will be
compromised, raising the risk of tree damage and associated
impacts.

The second atmospheric river surge arrives Tuesday night into
Wednesday and will closely follow the substantial rainfall
produced on Monday. IVT values with this second wave generally
center around 700 to 800 kg/ms, with ensemble solutions allowing
for a high-end scenario of 850 to 950 kg/ms and a lower
scenario of 550 to 650 kg/ms. The duration of this second round
may be longer than the first surge, maintaining high confidence
in additional heavy rainfall. Rainfall on Tuesday will be
somewhat moderated during the day, but heavier rates redevelop
Tuesday night and persist through Wednesday. Overall from 4 AM
Tuesday to 4 AM Thursday, expect 3.5 to 5 inches across the
interior lowlands, with the highest amounts over southwest
Washington and lowest amounts in the southern Willamette Valley.
The Portland/Vancouver corridor will likely receive 4.5 to 5
inches. Coastal areas will see 5.5 to 8.25 inches (highest
amounts in Tillamook), the Coast Range 8.25 to 12 inches, and
the Cascades 7.5 to 11 inches. Similar to Monday, the Lane
County Cascades will be lower, generally 4 to 7.5 inches.
Combined totals from both surges will be substantial and will
almost certainly produce localized flooding somewhere in the
forecast area. Rivers draining the Coast Range remain the most
sensitive to rapid rises.

Winds will stay elevated Tuesday and Wednesday, though Monday
remains the peak wind day. Inland gusts around 30 mph and
coastal gusts around 40 mph are expected during this period,
with the potential for isolated stronger gusts depending on the
timing of each frontal passage. Again, due to saturated soils
and compromised root stability, scattered downed trees and power
outages will be possible.

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.

Forecast confidence is moderate regarding the timing and
intensity of Mondays surge and moderate for the Tuesday to
Wednesday period due to spread within the ensemble clusters.
Overall, the early to midweek timeframe remains the primary
focus for hydrologic and wind-related impacts. See the hydrology
section below for discussion related to the Flood
Warnings/Watches and river forecasts.
~12

&&

.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
72-hour rain amounts (from 4 AM Monday through 4 AM Thursday) up
to one foot 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, and are only 1-3 inches higher than the
official forecast. This worst case scenario would result in
widespread moderate to major flooding.

Although rain will likely become lighter Thursday into Friday,
the threat of flooding will linger along slow responding rivers.
Most rivers draining the Coast Range and Willapa Hills have a
30-75% chance of reaching moderate flood stage and a 20-50%
chance of reaching major flood stage. Additionally, several Willamette
River tributaries that drain from the Coast Range and Cascades
have anywhere from a 60-80% chance of reaching minor flood
stage and a 20-40% 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, which do
vary quite a bit from river to river.

There is also increasing confidence in the potential for
flooding along small creeks and streams as well as urban
flooding, including roadway flooding. Water pooling on roadways
will increase the risk of hydroplaning and car accidents.
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, which
currently have the highest chance of reaching at least minor
flood stage within the next 36-48 hours:

Wilson River near Tillamook affecting Tillamook County, from
Monday evening to late Tuesday morning. This river has 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. This river has 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. This
creek has a 70% chance of reaching moderate flooding, and a 60%
chance of reaching major flooding.


&&

.AVIATION...Predominately MVFR conditions expected across the region
through Sunday as persistent stratiform rain continues to spread
across the region. There is also a 15-25% chance for periods of IFR
cigs Sunday morning and afternoon, except 50-60% at KAST and 80-90%
at KONP. By 00z Monday, expect persistent stratiform rain to
transition over to light rain showers. Conditions could improve to
low-end VFR or high end MVFR across the Willamette Valley between 00-
12z Monday before becoming widespread MVFR to IFR once again as
another, more potent frontal system brings widespread heavier
rain to the region.

Winds generally from the south inland and southwest along the coast.
Gusts up to 25 kts inland and 25-30 kts along the coast through 21z
Sunday, decreasing quickly to generally around or below 10 kts by
00z Monday. With the next frontal period, winds begin increasing
again along the coast around 12z Monday with gusts up to 25-30 kts
along the coast. Winds increase after 15z Monday inland with gusts
up to 25 kts.

KPDX AND APPROACHES...Rain showers continue through Sunday with
predominately MVFR conditions. Conditions could improve to VFR
around 00-03z Monday, becoming MVFR again after 12z Monday. South
wind gusts up to 20-25 kts through 21z Sunday, decreasing to around
10 kts by 00z Monday. South winds increase again after 15z Monday to
gust up to 25 kts. -03

&&

.MARINE...A frontal system moving over the coastal waters this
morning will bring the continuation of south to southwest winds
with wind gusts upwards of 20-35 kt, strongest over the central
and southern waters where a marginal Gale Warning remains in
effect. Winds will decrease substantially Sunday afternoon and
evening, resulting in a brief period of benign conditions with
seas under 8-9 ft and wind gusts falling below 20 kt.

However, this break will be short-lived. A relatively stronger
frontal system is set to arrive on Monday, bringing gale force
wind gusts up to 35-45 kt, strongest over the inner waters and
near the Columbia River Bar. Gale Watches have been upgraded to
Gale Warnings now that confidence levels are high and 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. Seas
decrease slightly Tuesday into Wednesday but remain above 10 ft,
with a period of breezy westerly winds up to 25-30 kt likely
Tuesday night (70-80% chance). Seas and winds subside late in the
week. -23

&&

.BEACH HAZARDS...Tidal overflow flooding is possible around high
tide on Monday and Tuesday, December 8-9. 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. With several coastal rivers likely to reach at
least 80% of flood flow and total tide forecasts peaking near or
slightly above 9.5 ft, Coastal Flood Advisories may need to be
issued along all of portions of the coast if confidence
increases. Since the high tide alone will not be enough to cause
tidal overflow flooding, the outcome will be highly dependent on
which watersheds receive the heaviest rainfall and thus which
rivers reach at least 80% of flood flow. The timing of river level
rises will matter too, as it will need to coincide with high
tide. On Monday, river levels will be rising quickly, but it
appears rivers may stay below 80% of flood flow during high
tide before climbing above 80% Monday evening/night. If the timing
is correct, this should mitigate flood concerns around high tide.
On Tuesday, confidence is higher coastal rivers will reach at
least 80% of flood flow during high tide, but that is when the
total tide generally dips below 9.5 ft. Will need to monitor both
days closely and see how the river forecasts evolve. -23


&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210-
     251-271.

     Gale Warning from 7 AM to 10 PM PST Monday for PZZ210-251-271.

     Gale Warning until noon PST today for PZZ252-253-272-273.

     Gale Warning from 7 AM Monday to 1 AM PST Tuesday for PZZ252-253-
     272-273.
&&

$$

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