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FXUS66 KPQR 161814 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1014 AM PST Tue Dec 16 2025

...Updated aviation discussion and watches/warnings/advisories
section...

.SYNOPSIS...Very active weather through the week as a few
different frontal systems and atmospheric rivers move through
the area. Preparing for widespread heavy rain, gusty southwest
winds, river flooding and potentially high Cascade snow. Wind
strongest Tuesday night into Wednesday. Rain heaviest on
Thursday and Friday with the highest probability for river
flooding. Snow potential highest on Friday but uncertain.

&&

.SHORT TERM...Now through Wednesday Night...The rest of the week
can be described in one word...chaotic. As has been advertised,
a series of frontal systems and atmospheric rivers will move
over the region each bringing their own impacts. The first
frontal system from Monday is exiting the area this morning with
post frontal showers persisting. The environment is quite
convective so cannot rule out the potential for thunderstorms
along the coast Tuesday morning though confidence is low. The
front that will be the most impactful for Tuesday arrives this
afternoon along the coast but will spread further inland through
the evening into early Wednesday morning. Rain will ramp up with
the frontal passage with the heaviest rain and moisture
transport occurring very early Wednesday morning. Rainfall
totals have varied as they will greatly depend on the location
of the atmospheric river. As we have seen time and time again,
while the rain may be widespread there typically is one area
that receives the highest rain accumulation and rain rates. At
this point models still struggle with the NAM taking a more
widespread pattern while other higher resolution models trending
further north along the Coast Range and the south Washington
Cascades. Because of the southwesterly flow the Willamette
Valley may miss out on the heaviest rain rates.

Another concern within the next 24-36 hours is the potential
wind that will be coming. In order to describe it better, lets
look at the overall synoptic pattern. The jet stream both at 250
mb (35,000 ft) and 500 mb (around 10,000 ft) is nearly zonal and
directed perfectly centered over the Pacific Northwest. Through
the day on Tuesday though it does appear to be shifting slightly
northward which is putting northwest Oregon and southwest
Washington into the right exit region of the jet. This location
is not necessarily favorable for incredibly active weather.
However, at 500 mb the overall flow is disconnected from it`s
aloft partner of the jet and remains right over the area. This
means that we could see some stronger winds. Bumping down lower
to around 5000 ft we are seeing wind speeds around 55-60 kt
(63-69 mph). Those speeds intensify even further with some
models suggesting winds at 5000 ft being around 75 mph. Looking
at the surface a component to consider is the overall pressure
gradient. Based on local knowledge, the pattern is setting up to
bring strong winds to the coast late Tuesday, and stronger
winds inland Wednesday morning. The challenge with the wind
forecast is that these warning and advisory level speeds will
not last for a very long period of time as they sit within the
frontal boundary. For those in the inland valleys and the
Cascade foothills, that forecast is a bit more complicated as
the orientation and an inversion near the surface may keep the
winds from mixing down. At this point there is a 50% chance of
gusts greater than 45 mph for hi-resolution models and a 25%
chance for the NBM. The foothills have a higher probability.
Even if strong winds inland do occur for only a short duration,
the impacts will still be high. Due to very saturated soils
downed trees are very possible and thus cannot rule out power
outages.

The last component of the forecast for the next 24-hrs lies with
convective weather. Behind the front currently moving inland and
the second system Tuesday into Wednesday sits cooler air which
enhances instability, and modeled soundings show a well mixed
atmosphere with convective signatures. There will be increased
chances for thunderstorms and in fact the Storm Prediction
Center has put the area under a Marginal Threat of Thunderstorms
which is quite unique for this time of year. With any
thunderstorms there may be periods of lightning, heavy rain, and
gusty outflow winds. -27

&&

.LONG TERM...Thursday through Monday...Another moderate to
strong atmospheric river takes aim at the region Thursday into
Friday, bringing additional rain on top of elevated rivers and
saturated soils. Initially, heavy rain rates raise the risk for
urban or small creek and stream flooding on Thursday, while the
risk for river flooding increases late Thursday into Friday as
the abundant runoff makes its way downstream. As is typical with
ARs, the highest precipitation totals are likely to be over the
coastal terrain and across the Cascades, and the warm dynamics
will also favor increasing snow levels during the heaviest
precipitation. Rainfall forecasts have tended to shift slightly
between forecast updates, but guidance remains in good agreement
that the AR will impinge on the northern OR coast before
tracking southward toward the southern OR coast by Thursday
night. Most recent guidance is suggesting that the system will
linger over the region a bit longer which will increase rainfall
totals within the central Oregon Cascades and the southern
Valley. Now that`s not to say southwest Washington won`t see
rain. Current forecasts are still forecasting ample rain with
around 1.5 inch in the Willapa Hills and the lowlands of
southwest Washington. The highest rain totals are currently
focused across the southern half of the forecast area with
precipitation totals highest within Lane County, and the
Cascades of Linn and Lane Counties. 24 hour rainfall totals on
Thursday for 2 inches of rain or more have remained nearly
unchanged. There is a 30-45% chance of 2 inches of rain within
the interior lowlands north of Salem, near 60% chance from
Corvallis south, and around 60% along the Coast.

This amount of precipitation over only 24 hours would support
river flooding as saturated soils will allow most of the rain to
make its way into area rivers as runoff. Chances for Minor to
Moderate river flooding continue to increase, with the highest
confidence in flooding for rivers draining the central OR Coast
Range, both toward the coast and into the Willamette River.
Those who live near areas prone to river flooding should keep a
close eye on the forecast moving forward, and additional
information can be found in the Hydrology discussion below.

The thoughts of snow in the Cascades has been on people`s minds
so let`s dive into that a bit further. The challenge with
receiving snow in this kind of set up is the shear structure of
an atmospheric river. By definition, these events typically are
associated with warm air with the cooler air to the north. The
warm air is what harnesses most of the precipitation. Therefore
we are entering into a period of time where we have been warm,
will continue to be warm, and thus getting significant snowfall
will be a challenge. Not saying it`s not impossible, but the
probability of 6" of snow or more for the duration of the event
(Thursday and Friday) is around 30% for the foothills, and
around 70% near the passes. Bumping up amount`s to 12" or more,
those probabilities drop significantly with around a 20% chance
within the passes. Something to remember about models and snow
is that it is accounting for all of that precipitation to fall
as accumulating snow and with this type of event, we will
struggle to see that come to fruition. Given the level of
uncertainty though, if planning to travel across the passes
this weekend you should prepare for snow in the forecast.

The veritable parade of weather systems does not look to let up
this weekend and into next week. While confidence in forecast
details is low at this lead time, the active pattern and
persistent chances for rain will most likely continue.
-27/36

&&

.AVIATION...Radar imagery and terminal observations as of late
Tuesday morning show decreasing showers with low-end VFR CIGs across
the Willamette Valley and a mix of LIFR/IFR/MVFR CIGs along the the
coast. The next frontal system will increase rainfall across the
region after 00z Wednesday. This frontal system is more robust than
the rain showers from this morning. Tonight, there is high
confidence (>80% chance) for IFR CIGs or lower and moderate
confidence (40-60% chance) for MVFR CIGs across Valley terminals.
The environment will be convective, so there is also a 15-20% chance
of thunderstorms over the coast, Coast Range and some of the
interior terminals. These chances generally increase after 00Z Wed
and persist through the remainder of the TAF period. Heavy rain from
thunderstorms may briefly drop CIGs/VIS to IFR thresholds or lower
at any terminal.

Wind is also becoming more of a concern over the next 24 hours.
Around 00Z Wednesday, along with the passage of the front, south to
southwesterly winds will begin to ramp up not only at the surface
but aloft. Could see surface gusts up to 40-45 kt along the coast
and 25-30 kt across the Valley overnight. Around 2000 ft, guidance
suggests strong southwesterly winds up to 50 kt. This will lead
potentially strong cross winds for E-W aligned runways.
Added LLWS for inland terminals between 05-09z Wed, but shear should
be unidirectional.

KPDX AND APPROACHES...Predominately VFR through this afternoon.
After 00z Wed, there is a 30-50% chance for MVFR CIGs and a 5%
chance for southwesterly wind gusts exceeding 45 kt. Southwesterly
winds likely gust around 25-30 kt from 00z Wed through the end of
the TAF period with the next front. CIGs improve to predominately
VFR after 12z Wed.       -10

&&

.MARINE...Currently observing Small Craft Advisory seas around
10-13 ft at 12 seconds. These conditions will be maintained
until the atmospheric river moves over the waters in the late
afternoon. At that point the pressure gradient will increase
considerably due to a disturbance in the flow. Will quickly see
winds ramp up to gale force speeds over all of the waters
including the Columbia River Bar. The probability of maximum
wind gusts meeting Storm Force speeds (gusts greater than 48 kt)
is around 40-50% within the inner waters with a coastal jet that
forms, and around 25% in the outer waters. For portions of the
coast like moorages that are susceptible to southwesterly winds
there will be a trend towards higher end Gale Force Wind speeds.

During this period of time seas will ramp up due to a strong
westerly swell and enhanced wind wave. Overnight will see
widespread 16-18 ft seas at 10-11 seconds. There will be pockets
of seas up to 20 ft in the inner waters north of Cape
Foulweather. Some models have backed off of these heights, but
given the overall pattern they tend to underdo these types of
scenarios.

Behind this weather system seas will fall to around 12-15 ft at
11 seconds. There will be another  river that comes in on
Thursday into Friday but at this point it remains more of a rain
impact vs a wind and seas threat. -27

&&

.HYDROLOGY...Potential for river flooding is highest on Thursday
and Friday due to the abundant rainfall from an atmospheric
river. Heaviest rainfall occurs for a relatively short period
of time from Thursday through Thursday night. The already
saturated soils and high rivers combined with the incoming well
above normal rainfall has resulted in an elevated risk for
flooding late this week. As is typical in atmospheric rivers,
the highest rainfall totals are likely to be over areas of
terrain including the Coast Range and Cascades, while lesser
totals can be expected across inland valleys. Nonetheless,
rainfall is likely to be measured in inches across most of the
region. One difference between the last series of ARs and the
one arriving on Thursday is the placement. The late week system
favors a more southerly route and thus areas that previously
missed the heaviest rain will see the heavier accumulations this
time around. Due to the lack of previous rain basins may be
more capable of holding the incoming precipitation without
experiencing flooding. Other areas to the north within the Coast
Range in Lincoln, Tillamook, Benton, Polk, Yamhill, and
Washington Counties may see more overlap between recently
observed heavy rainfall and additional heavy rainfall with this
upcoming system. Similarly, river basins covering the central OR
Cascades in Lane and Linn Counties have a slightly more
optimistic outlook than north into Marion and Clackamas
Counties, but that does not mean flooding cannot happen in all
of the aforementioned areas. Luckily, where rainfall has so far
been the heaviest across southwestern Washington and far
northwestern Oregon may see the lowest totals from this system,
although even 1-2 inches of rain over 24 hours may be enough to
cause additional flooding in these very saturated areas.

At this point, the rivers with the highest confidence of
reaching flood stage are those which drain the central Oregon
Coast Range, including the Wilson, Trask, Siletz, and Siuslaw
draining toward the coast, and the Marys, Luckiamute, Yamhill,
and Tualatin draining toward the Willamette. The chances for
these rivers to reach at least Minor flood stage are 25-55% at
this time. Rivers draining the Cascades, including the Mohawk,
Santiam, Pudding, Molalla, Clackamas, and Sandy, have slightly
lower but still significant chances of 15-45%. These rivers will
begin to rise later on Thursday as heavy rain continues, not
cresting until Friday in most cases. The Willamette River main
stem itself also has a 5-10% chance of reaching flood stage
above Willamette Falls, and slightly higher chances in tidally-
influenced portions downstream of Oregon City, however runoff
will take longer to reach the Willamette and flooding would be
delayed later than in its tributaries. A Flood Watch has been
issues across all of northwestern Oregon and southwestern
Washington from 4 AM Thursday through 4 AM Saturday to highlight
these risks.

During the time of heaviest rain on Thursday we will also see
some of our highest tides of the week. Due to the rivers near
the coast forecast to sit below the 80th percentile CFS, there
is a low probability of tidal overflow flooding. Will continue
to monitor though as precipitation totals become more well
resolved.

These river forecasts remain very sensitive to rainfall
forecasts, and interested parties should expect changes as the
event nears. The latest river forecasts across the region can be
found at www.water.noaa.gov/wfo/pqr.-27/36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Wind Warning from 4 PM this afternoon to 6 AM PST
     Wednesday for ORZ101>103.

     Wind Advisory from 4 PM this afternoon to 6 AM PST Wednesday
     for ORZ104>107-123>125.

WA...High Wind Warning from 4 PM this afternoon to 6 AM PST
     Wednesday for WAZ201.

     Wind Advisory from 4 PM this afternoon to 6 AM PST Wednesday
     for WAZ202-203-208.

     Winter Weather Advisory from 10 PM this evening to 10 PM PST
     Wednesday for WAZ211.

PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210-
     251>253-271>273.

     Gale Warning from 4 PM this afternoon to 10 AM PST Wednesday
     for PZZ210-251>253-271>273.

&&

$$

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