817
FXUS66 KPQR 112252
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
352 PM PDT Wed Mar 11 2026

.SYNOPSIS...The bulk of our impacts the next several days center
around an atmospheric river event which is currently underway
across the region. Expect periods of moderate to heavy rain,
breezy conditions, and mountain snow (mainly WA Cascades).
While the probability for mainstem river flooding is low, rises
on rivers and streams across much of northwestern Oregon and
southwest Washington will be a feature to watch due to the
potential for anomalous rainfall rates - a Flood Watch is in
effect through Friday evening. From there, another weather
system clipping the area to the north late in the weekend may
bring additional precipitation in addition to ushering in much
warmer temperatures for early next week.

&&

.SHORT TERM...Now through Friday Night...Rainfall is currently
ongoing across the region this afternoon in addition to breezy
winds as a warm-frontal boundary pushes overhead and moisture
associated with a prolonged Atmospheric River continues to
stream overhead. Model integrated waver vapor transport values
near 500kg/m*s persist for another 48+hrs is associated with a
moderate strength atmospheric river, but it will be the wobbles
of this moisture longitudinally through Friday evening which
constitutes an area of uncertainty and will help to modulate
the axis of the heaviest rainfall. In this regard, the two time
periods to watch locally will be tonight into Thursday morning
and Friday into Friday evening during which rainfall rates
locally will be maximized. There is high confidence that
somewhere within the Pacific Northwest will see significant
rain, even potentially even in the valley but where exactly is
still under review - likely somewhere between Salem and Kelso
with the highest amounts in the Oregon Coast range. The long
period of rain will produce some hydrological concerns which
will be discussed in the "Hydrology" section below.

Rainfall projections remain highly sensitive to the atmospheric
river placement and supporting dynamics like the frontal
boundary placement/progression tonight and again Friday into
Friday evening. In Portland and across the north Willamette
Valley, NBM 72-hour totals ending early Saturday morning (close
to a storm-total) range roughly 2 inches on the low end (10th
percentile) to around 5.5 inches at the high end (90th
percentile). Along the north Oregon Coast Range, probabilities
span from roughly 4 inches at the low end to over 9 inches at
the high end within that same time frame. These values,
especially on the high-end, in combination with periods of
higher rainfall rates have the potential to drive small stream
and urban flooding beyond river-flooding concerns and promoted
the issuance of a Flood Watch.

As far as snowfall is concerned main threat is over the south
Washington Cascades where a Winter Storm Warning remains in
effect for the Gifford Pinchot Natl Forest and Mt Adams through
Thursday night to account for a brief surge in precipitation on
that day. Additional totals through this period range from 5-12
inches at 2500-3000 ft up to nearly 25 inches above 4500-5000
ft. The location of the atmospheric river will ultimately be
key. The area of concern sits right on the southern edge of the
cold airmass, and 850 mb temperatures are right around -4 deg C.
If the cold air dips further south, then the snow ratios will
increase and accumulation will rise. Several high-resolulation
models like the NAMNEST also present a interesting "dynamic
cooling" scenario across the lower elevations of southwest
Washington Friday morning when the cooler air on the north side
of the frontal boundary and high precipitation rates combine to
artificially push snow levels lower, potentially near 500ft.
There is significant uncertainty as whether this will actually
unfold and confidence is very low due to the complexity of the
set-up, but`s it`s something we`ll have to watch closely.
-99/27

.LONG TERM...Saturday through Tuesday night...Rainfall tapers
late Saturday as weak ridging develops. This ridge of high
pressure will come into play on Sunday and Monday though as yet
another atmospheric river is on deck. A broad low pressure
system forms in the Gulf of Alaska on Saturday night. Ensemble
500 mb heights show good consistency between clusters in the
ridge amplifying, a deep digging trough over the northern Plains
and the low pressure system in the Gulf of Alaska staying to
the north. About half of the clusters show a drier solution,
while the other half favor rain. Vapor transport again shows a
spike in values on Monday into Tuesday but uncertainty is quite
high. If the ridge remains dominate we will see high
temperatures well above climatology. For example in Eugene on
Monday, the high temperature spread is around 67-73 degrees F
(25th to 75th percentiles) with a 10% chance of seeing
temperatures as high as 75 degrees. Further north in Kelso, WA
the max temperature spread is closer to 55-65 degrees F with a
10% chance of highs of 67 degrees F.

There is high uncertainty overall in the long-term forecast so we
will be continuing to evaluate as time passes. -27

&&

.AVIATION...MVFR conditions (primarily CIGs) continue at inland
terminals while LIFR/IFR conditions persist at coastal terminals.
Expect these conditions to last through the TAF period, with the
exception of the possibility of MVFR conditions at KAST after 06Z
Thu, and VFR conditions at KEUG between 00Z and 06Z Thu. Steady rain
will continue throughout the airspace through the TAF period. Low
end MVFR conditions will be possible if heavier rain bands pass
over terminals. Winds will also be a concern through this TAF
period, with southerly to southwesterly wind gusts increasing
through all north- south aligned terrain, especially for for
coastal and Willamette Valley terminals. Expect wind gusts up to
35-40 kt at coastal terminals until around 06Z Thu (highest gust
magnitudes at KONP). As for the Willamette Valley, expect wind
gusts up to 30 to 35 kt from now until before 12Z Thu. LLWS is a
threat for many inland sites, especially for airports that are not
susceptible to southerly surface winds like KTTD.

KPDX AND APPROACHES...Steady rain continues through the TAF
period, with MVFR CIGs and occasional MVFR VIS expected to persist
through the TAF period. Expect southerly to southwesterly wind
gusts up to 30 kt from now until around 06Z Thu. Southerly to
southwesterly winds up to 50 kt at 3000 ft through around 06Z Thu.
With surface wind gusts above 20 kt, the LLWS threat is not as
significant. However, east approaches will likely be affected due
to strong winds aloft, with speeds in the 50-60 kt range between
3000-5000 ft. ~12

&&

.MARINE...A strong frontal boundary moves over the waters,
bringing varying conditions over the next 24 to 48 hours.
Southwest winds will continue to increase through this afternoon,
bringing Gales Force winds (34+ kt) over all of the waters
through this evening. The strongest gusts will be located within
10 NM of the coast due to nearshore enhancement, allowing Gale
Force winds to persist a bit longer compared to the waters beyond
10 NM. Gale Warnings are in effect until 10 PM PDT this evening
for the outer waters (beyond 10 NM) and until 2 AM PDT Thursday
for the inner waters (within 10 NM), including the Columbia River
Bar. During this time, seas will also become steep and hazardous,
continuing beyond the Gale Warning time. With seas expected to
build to around 13 to 15 ft at 10 to 11 seconds, a Hazardous Seas
Warning has been issued for the outer waters from 10 PM this
evening to 5 AM PDT Thursday. Small Craft Advisories will follow
as seas remain elevated, with a Small Craft Advisory in effect
for the inner waters by 2 AM Thursday and outer waters by 5 AM
Thursday. The Small Craft Advisories will continue until late
Thursday.

As the front finally exits southward on Friday, winds and seas
will finally ease to below hazard criteria. As high pressure
builds over the Northeast Pacific, winds will shift northerly
Friday into the weekend while seas continue to ease below 5-6 ft
through this weekend.
~12


&&

.HYDROLOGY...Hydrologic concerns increase today through Friday
as a atmospheric river brings a prolonged period of moderate to
heavy rainfall to portions of western Oregon and southwest
Washington. While confidence is high that a moisture plume will
impact the region, confidence remains moderate regarding where
the heaviest rainfall will set up in conjunction with the two
primary impulses of rainfall - tonight into Thursday morning and
again on Friday. While most main-stem rivers have less than a
5% chance of minor flooding, a few of our "flashier" rivers have
greater chances. The Pudding River at Aurora, Johnson Creek
near Sycamore, and the Wilson/Trask near Tillamook are expected
to reach minor Flood Stage with a 5-20% to reach moderate.
River responses will likely vary in time due to rainfall
duration and location, with some likely occurring late tonight
through Saturday, and others occurring late Thursday through
Saturday. The most likely outcome at this time is elevated river
levels and within-bank rises, though trends will need to be
monitored.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 5 PM PDT this afternoon for ORZ126-
     127.

WA...Winter Storm Warning until 5 AM PDT Friday for WAZ211.

PZ...Small Craft Advisory until 2 PM PDT this afternoon for PZZ210.

     Small Craft Advisory from 2 AM to 5 PM PDT Thursday for PZZ210-
     251>253.

     Gale Warning until 2 AM PDT Thursday for PZZ210.

     Gale Warning until 2 AM PDT Thursday for PZZ251>253.

     Gale Warning until 10 PM PDT this evening for PZZ271>273.

     Hazardous Seas Warning from 10 PM this evening to 5 AM PDT
     Thursday for PZZ271>273.

     Small Craft Advisory from 5 AM to 5 PM PDT Thursday for
     PZZ271>273.
&&

$$

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