Flooding in Oregon, November 11 to 12, 2021

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
947 PM PST Thu Nov 11 2021

Updated aviation and hydrology sections.

.SYNOPSIS...An atmospheric river will result in periods of
moderate to heavy rain through Friday, especially for locations to
the north of Salem. High pressure will bring a break from the rain
this weekend, except for southwest Washington zones which will most
likely see precipitation return by Sunday. Then, a cold front will
bring widespread precipitation to the region on Monday and set the
stage for a cool and showery weather pattern heading into Tuesday
and Wednesday.

&&

.SHORT TERM...Tonight through Sunday...Water vapor satellite imagery
reveals a broad shortwave trough over the northeast Pacific with an
elongated front and attendant atmospheric river taking aim at far
northwest Oregon and western Washington still this evening. Doppler
radar and surface observations show most of the rain remains across
the northern half of the CWA. Models are in good agreement in
suggesting that rain will continue to be focused primarily across
western Washington, including the Willapa Hills, before shifting
southeastward across the rest of the area early Friday morning.

HREF guidance suggests hourly rainfall rates will peak in the
0.25-0.50" across the higher terrain of the north Oregon Coast Range,
south Washington Cascades and Willapa Hills overnight. HREF guidance
also suggests that there is over a 90% chance of 1" of rain in 6
hours across the higher portions of the north Oregon Cascades and
Willapa Hills this evening and later tonight, with the odds of 2" of
rain falling in 6 hours at ~30-40%. This will result in rising
rivers overnight and potentially some flooding of low lying areas
across Pacific and Clatsop Counties. Will monitor rainfall trends
before issuing any additional products. Models are in good agreement
rainfall rates will gradually decrease as the front drops
southeastward across the area on Friday. This should result in the
threat of flooding diminishing as rainfall amounts decrease with
time. /Neuman

The cold front will continue to drop south Friday evening, allowing
rain to diminish from northwest to southeast, although hanging into
the central OR Cascades into Saturday morning time frame. Some
partial clearing skies Friday night will allow for the moisture rich
lower boundary layer to easily allow the formation of fog. Have
added areas of fog into the forecast, especially in the lower
valleys.

Upper ridging will stretch along the US west coast Saturday while a
surface high moves inland. This will promote dry weather for the
day. Lower stratus in the morning will give way to increasing high
level clouds from the next incoming atmospheric river system off the
Pacific. Temperatures over most of the forecast area should be
slightly cooler following Friday night's cold front passage, however
areas south of Salem may remain unchanged from Friday's highs due to
warm advection from the south under the ridging.

As for the next AR system, this one has been trending slower and
further north into Washington, so have decreased PoPs for Saturday
afternoon across the whole area, then slowly reintroduced them into
the northern third of our forecast area, mainly north of Salem,
Saturday night and into Sunday. This AR looks to be another weak to
moderate event, but again further north and a quicker mover. Snow
levels will once again rise to above 9000 feet. At this time, a
quarter to three-quarters of an inch of rain is forecast Saturday
night into Sunday afternoon, again, mainly north of Salem with the
highest amounts in the Willapa Hills and the southern WA Cascades.
/Kriederman

&&

.LONG TERM...Sunday night through Thursday...The momentary drier
period of the weekend looks to end by Sunday night/Monday as the
upper level ridge starts to break down and an upper level trough
drops down from western Canada as the typical work week starts.
Models are showing good agreement in the overall pattern as well as
timing. There are still some subtle differences within the models
themselves, but they all show the same general solution. The system
looks to bring precipitation and cool temperatures across the CWA.
While this will bring colder temperatures and lowering snow levels,
the bulk of the moisture will likely be east of the Cascades by the
time the colder -3C to -4C 850mb temperatures arrive. As a result of
the cooler air expected, daytime highs look to be in the mid 40s to
low 50s along the coast and within the Willamette Valley, with highs
in the low 30s to low 40s in the Cascades. In addition snow levels
look to drop from 8000 ft Sunday towards 2500 to 3500 through the
middle of the week.

While we know that people are wondering when/if the valley floor
will see snow, we look for 850 mb temperatures around -7C to -8C
when under onshore flow along with precipitation. On average the
first snowfall for the valley will happen around mid December. /42

&&

.HYDROLOGY...The Grays River near Rosburg, Washington has recently
surpassed the minor flood stage of 12 feet, and is still rising
quickly at this time. It is expected to peak just shy of 15 feet at
some point Friday morning before gradually subsiding to less than
minor flood stage Friday night or Saturday morning. Hence, a Flood
Warning has been issued for the area surrounding the Grays. The
chances it will reach major flood stage (i.e., 16') still appear
relatively low at around 30% at this time, though the effect of the
recent burst of intense rainfall rates on the river won't be fully
realized for a couple more hours. The Wilson River at Tillamook is
expected to surpass the minor flood stage of 12 feet shortly as well,
with river levels expected to peak at around 13.5 feet Friday
morning/early afternoon before subsiding quickly Friday night. A
Flood Warning has hence been issued for the Wilson as well.

The Nehalem at Foss will bear close watching over the next several
hours, with the most recent forecasts putting it right on the edge of
minor flood stage for Friday afternoon. Unless the Willapa
Hills get 1.75-2.50"+ of rain in two 6 hour periods tonight (which is
not out of the question), it appears the Naselle and Willapa should
escape reaching minor flood stage. Rainfall amounts will also be high
across the south Washington Cascades overnight so will need to
monitor the Cowlitz River in case upstream dam releases are larger
than expected.

It appears rainfall rates should diminish enough as the frontal rain
band shifts southeastward across the area on Friday that the threat
of debris flow and flash flooding appears to be diminishing across
the Oregon Cascade burn areas. Will keep the flood watch in effect
for now to make sure everything continues to evolve as expected, but
we may be able to drop our central Coast Range watches and our
Oregon Cascade Flood Watches early. /Neuman/Bumgardner

&&

.AVIATION...06Z TAFs: At 04Z widespread MVFR to IFR conditions
were noted across much of the area. The main exception was the
central and southern interior valleys. A band of moderate to
heavy rain extending from the south Washington Cascades to near
KONP has been nearly stationary since early afternoon. At 04Z NWS
doppler radar suggests weakening rainfall intensity within the
band. This is evident by the general improving visibility at
METAR sites. Model guidance, including the higher resolution
models, shows this frontal boundary slowly sagging southeast
overnight through Friday evening as a cold front. Thus, expect
general MVFR to IFR conditions to continue across much of the
area through the 06Z TAF period. Interior lowland areas south of
KSLE will likely see more VFR than MVFR through Fri morning,
becoming more MVFR after 18Z Fri as the frontal boundary slides
southeast.

For detailed  regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...Low MVFR to IFR at the terminal and
vicinity as of 04Z. Not anticipating much change in overall
conditions through 06Z Sat. Will likely see some pooling of
water on runways and decreased visibility due to increased rain
rates at times. Weishaar

&&

.MARINE...No changes to marine forecast. The warm front continues
to push inland today, and will shift slightly to the north
tonight. This shift will cause winds to begin to ease from 20 to
25 kt with gusts up to 30 kt to 15 to 20 kt with gusts up to 25
kt around Friday morning. Seas will remain around 7 to 9 ft at 9
seconds, increasing to 12 ft by Friday afternoon. Saturday likely
will be fairly settled with lower winds and seas around 7 to 9
ft. On Sunday though, another active weather pattern is expected
which will cause winds to increase once again to 20 to 25 kt with
gusts up to 30 ft.

-Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through late Friday night for Central Coast Range
     of Western Oregon-Central Columbia River Gorge-Central
     Oregon Coast-Central Willamette Valley-Coast Range of
     Northwest Oregon-Greater Portland Metro Area-Lower
     Columbia-North Oregon Coast-Northern Oregon Cascade
     Foothills-Northern Oregon Cascades-Upper Hood River Valley-
     Western Columbia River Gorge.

WA...Flood Watch through late Friday night for Central Columbia
     River Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-South Washington
     Cascades-South Washington Coast-Western Columbia River
     Gorge-Willapa Hills.

PZ...Small Craft Advisory until 10 AM PST Friday for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 nm.

     Small Craft Advisory until 4 PM PST Friday for coastal waters
     from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory until 1 AM PST Friday for Columbia River
     Bar.

     Small Craft Advisory from 8 AM to 2 PM PST Friday for Columbia
     River Bar.

&&

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