336
FXUS66 KPQR 070551 AAB
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
951 PM PST Sat Dec 6 2025

Updated aviation discussion.

.SYNOPSIS...A prolonged wet pattern continues through early
next week as multiple frontal systems push inland. The most
notable period continues to focus on Monday through midweek,
when a prolonged atmospheric river may bring significant
rainfall and rising rivers across southwest Washington and
northwest Oregon. Today and Sunday bring lighter rainfall, while
Monday is expected to produce the highest totals across
southwest Washington and northwest Oregon.

&&

.Discussion...Now through Friday...Weather will remain active
through at least the middle of next week. Upper level zonal flow
will keep conditions dreary through Sunday with another
shortwave disturbance passing this evening and overnight. This
will bring a decent round of rain to the area as IVT values
increase to between 250-500 kg/ms. Rainfall with this system is
expected to be around 0.5-0.75 inches across the interior
lowlands, 0.75-1.5 inches along the coast, 1.5-3.0 inches along
the coast range and Cascades with the exception of the Lane
County Cascades which will be lower at 0.75-1.5 inches.
Widespread impacts are not anticipated through Sunday; however,
if rainfall rates sustain around 0.2 to 0.3 inches per hour for
several hours, a few faster-reponding basins, such as the Grays
at Rosburg and the Willapa River, could rise quickly. Breezy
southerly to southwesterly winds are expected at times with
gusts typically 30-40 mph, strongest near exposed coastal and
elevated terrain. This system will be a precursor to an active
few days where several inches of rain could fall across NW OR
and SW WA, increasing the potential for river and urban
flooding.


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 on Monday with IVT values along the coast peaking
above 750 kg/ms and values inland peaking between 500-750 kg/ms. The
heaviest rainfall with this first round is expected to fall between
Monday and Tuesday morning. 45% of NCEP Ensemble members show
rainfall totals between 4 pm Monday and 4 am Tuesday reaching or
exceeding the 10 year Average Return Interval for the Portland area.
This indicates that the expected rainfall totals during a 12 hour
period has a 10% chance of occurring in any given year. This is
notable given the expected widespread heavy precipitation and
potential urban flooding impacts. This round of rain will be
falling on already saturated soil which has prompted WPC to add
a Day 3 Slight/Marginal ERO risk over the CWA. The slight risk
covers all of SW WA, down the coast rang in OR and along the
Cascades down to around Santiam pass. rainfall totals from 4 am
Monday through 10 am Tuesday are expected to be 1.5-2.5 inches
across the interior lowlands, 3.0-5.0 inches along the coast and
coast range, and 3.0- 6.0 inches along the Cascades and Cascade
Foothills. Amounts may be slightly lower farther south, mainly
across Lane county. These rainfall amounts will lead to
significant rises on area rivers, especially in the coast range.

The second round of high IVT values is expected to impact the region
Tuesday night into Wednesday. IVT values are expected to be
slightly lower than the first wave, peaking around 500 kg/ms
both inland and along the coast. However, the duration of these
higher values is expected to be slightly longer. Rainfall
totals from late Tuesday through Wednesday night are expected to
be slightly lower across much of the area except for the
Cascades where totals are forecast to be slightly higher. As it
sits now, rainfall totals from early Monday through Wednesday
night will significant and impactful. While the chances for
widespread impacts are rather low, it is almost certain that
localized impacts will occur somewhere within the CWA.

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. There
is a 10-20% chance for max wind gusts over 40 mph for inland areas,
and a 20-40% chance for gusts over 50 mph along beaches and
headlands. If wind gusts of this magnitude do materialize, expect
scattered downed trees and power outages.

Showery conditions will linger into Thursday and Friday as the flow
gradually weakens and dries. Snow levels are expected to remain high
(above 6000 ft) during this warm weather event, keeping most
precipitation as rain in the Cascades. Forecast confidence in exact
timing and magnitudes is still moderate given ensemble spread.
Overall, the Monday through Wednesday time period remains the
primary focus for potential high-impact hydrologic (see hydrology
section below for more details) and wind concerns. -19/12


&&

.AVIATION...A showery warm front will move over the area over the
next 24 hours. Showers continue within the vicinity of the area
with a mixture of flight categories. These showers will intensify
after 10Z Sunday causing CIGs to become widespread MVFR, and winds
to increase. The combination of wind and rain will cause
visibility to decrease, though how limited is unknown - ultimately
it will come down to rain rates. Given the south to southwesterly
winds amplifying, CIGs will hover right around the MVFR/VFR
threshold before decreasing to solid MVFR through the day. Wind
will ease towards the end of the forecast period.

Do want to note for those flight planning, a strong atmospheric
river system is going to move into the area on Monday. This system
will be accompanied by strong wind and very heavy rain.

KPDX AND APPROACHES...Exact timing is difficult for PDX as there
are a variety of factors that must come into play in order to get
any one cat level. Overall will see a trend towards MVFR after 10Z
Sun through at least 20Z. A diurnally driven weak easterly wind
will aid in clearing conditions temporarily. If they do clear, the
dewpoint depression will shrink and stratus may lower to IFR
levels. There is around a 20-30% chance of that occurring at some
point between 12-19Z Sun. With 30 kts of SSW wind at 2000 ft and
5-10 kt winds out of the SE to E at the surface, there will be a
period of modest low level wind shear values as we approach 18z
Sunday at KTTD and KPDX. Have included LLWS in the TAF at this
point but if the easterly winds at the surface shift to the south
then the risk will be far less. -27

&&

.MARINE...Seas will continue to slowly subside over the next
12-24 hours to less than 10 ft. A warm front will lift
northeastward across the area late tonight into Sunday morning
and result in southerly winds increasing across the waters. HREF
and NBM guidance suggest there is at least an 80% chance that
low end Gale Force wind gusts will occur across the waters south
of Cape Falcon. Seas will temporarily rise and become more wind
dominated due to these winds. Winds will then subside and
result in decreasing seas late Sunday before the next warm front
lifts northeastward across the waters. This suggests there is a
70% chance for stronger Gale Force winds to materialize across
the waters Monday. This will push seas into the mid teens Monday
afternoon with a 10-20% chance of seas climbing to at least
17-18 by 4pm Monday. Additional rounds of gusty southerly winds
are in store across the waters next week, but there is around
40-50% chance for brief Gale Force wind gusts across the waters
Tuesday and Wednesday. Southerly winds continue through the end
of next week, but there is 10% chance or less of Gale Force
winds materializing across the waters next Thursday, Friday and
Saturday.

&&

.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. Total rainfall amounts from
4am Monday to 4am Thursday (72 hour total) will most likely be
around 4 to 6 inches at the coast and interior lowlands, and 5.5
to 9.0 inches in the Coast Range and Cascades. Lane County
Cascades will be an exception, with 4-6.5 inches expected. There
is a 10% chance for rain amounts up to one foot in the Cascades
and Coast Range, up to 8-9 inches at the coast, and up to 7
inches in the Willamette Valley, Portland 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
5%). 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.

Considering the probabilities of flooding for rivers and urban
areas, a Flood Watch has been issued for all of southwest
Washington and northwest Oregon from 4 AM Monday to 4 AM Friday.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST Sunday for PZZ210-251-271.
     Gale Watch from Monday morning through Monday evening for
     PZZ210-251>253-271>273.
     Small Craft Advisory until 3 AM PST Sunday for PZZ252-253-272-
     273.
     Gale Warning from 3 AM to noon PST Sunday for PZZ252-253-272-
     273.
&&

$$

www.weather.gov/portland

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