Flash Flood Watch 1, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
243 AM PST Fri Nov 12 2021

.SYNOPSIS...An atmospheric river will result in periods of
moderate to heavy rain today, 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...Today through Sunday...Satellite loop shows an
atmospheric river embedded in the southwesterly flow associated with
a broad trough over the northeast Pacific,  aimed over western WA and
northwest Oregon this morning. Pcpn rates are around 0.5" to 0.8" per
3hrs over the south part of the Olympic Mountains, while the south WA
and north OR coast/coast range has pcpn rate around 0.3" to 0.6" per
3hrs, but locally near 1" per 3hrs.  Satellite also shows shortwave
approaching 130W off the Central OR coast. This is forecast to arrive
near 5 am and is expected to increase pcpn rates especially for the
north OR coast/coast range through the morning.

HREF guidance indicating 1hr pcpn rates of 0.3" to 0.5" with spots of
0.5" to 0.75" possible for the north OR coast/coast range & south WA
Cascades through 6 am. Rates then decrease to the 0.1" to 0.25"
range. As the mid and lower level flow weakens and buckles this
afternoon and tonight, rain rates further decrease and the focus of
the pcpn shifts south and east tonight.

Thunderstorm chances remain low but will continue with a slight
chance for the coast and coast range this morning. Model sounding are
marginal, but convergence at the coast may provide needed lift.  Snow
levels will be above all but the highest peaks today, then lower to
near 4000' north to around 6500' over the south part of the CWA by
Sat AM. By that time it should be dry. Some partial clearing skies
tonight will allow for the moisture rich lower boundary layer to
easily allow the formation of fog.

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.  The next AR system has been trending slower and
further north into Washington/southern B.C., so kept PoPs low 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.
/mh /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...Flood warning are out for the Grays River and Wilson
River near Tillamook where minor flooding is expected. Current
forecast has the Grays river cresting near 14.5 ft this afternoon and
the Wilson river near 14 ft later this morning or early afternoon.
The Nehalem River at Foss will probably have flooding as well, just
over flood stage.

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.

As rainfall rates diminish as the rain band shifts southeastward
across the area today and tonight, 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.

&&

.AVIATION...Moist southwest flow aloft will continue to maintain
a band of moderate to heavy rainfall across much of the region.
Expect periods of reduced cigs and visibilities through this
morning. At 10Z widespread MVFR to IFR conditions were observed
along the coast, while a mix of MVFR to lower VFR conditions were
noted through the interior. Model guidance suggests conditions
will deteriorate after 12Z Friday with more widespread MVFR
cigs expected across the interior and persisting through at least
00Z Saturday. A rather stationary frontal boundary is forecast
to slowly sag southeast later this morning through this evening.
Northern terminals may improve slightly after 06Z Saturday, but
most model guidance suggests MVFR conditions will still persist
into Friday night.

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

KPDX AND APPROACHES...Mix of MVFR and VFR at the terminal and
vicinity as of 10Z. Anticipate more widespread MVFR through at
least 06Z Saturday. IFR conditions will be possible later this
morning through the afternoon. Will likely see some pooling of
water on runways and decreased visibility due to increased rain
rates at times. /DDH

&&

.MARINE...A surface front will slowly slide southeast across the
coastal waters today. South-southwest winds will gradually
diminish through the day from north to south. Winds will shift to
more westerly later this afternoon across the northern zones and
then across the central coastal waters by this evening. Will
continue the Small Craft Advisory for all waters through this
morning as winds of 15 to 20 kt with gusts to 25 kt are expected
to persist, especially across the central waters. Seas will
remain around 7 to 9 feet at around 9 seconds as well through
this morning. A westerly swell around 9 ft at 12 seconds then
moves through this afternoon and into early Saturday morning.

High pressure will briefly build in behind the front and move
across the waters through Saturday morning. Winds will ease to 10
kt or less for nearly a 24 hour period shifting northerly tonight
and more offshore Sat morning. Seas will generally remain around
7 to 9 feet through Sat evening. The next frontal system will
approach the waters Sat night with southerly winds again
increasing to around 20 to 25 kt with gusts up to 30 kt by Sunday
through at least Sunday night. The cold front will likely move
across the waters on Monday. Seas are expected to build to around
10 to 12 feet on Sunday. /DDH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through late tonight 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 tonight 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 this morning for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     nm.

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

     Small Craft Advisory from 8 AM this morning to 2 PM PST this
     afternoon for Columbia River Bar.

&&

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
628 AM PST FRI NOV 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 AM     HEAVY RAIN       2 SSE FARGHER LAKE      45.85N 122.51W
11/12/2021  M1.00 INCH       CLARK              WA   TRAINED SPOTTER

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
705 AM PST FRI NOV 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0739 PM     HEAVY RAIN       5 WNW LEXINGTON         46.21N 123.00W
11/11/2021  M3.01 INCH       COWLITZ            WA   TRAINED SPOTTER

            3.01 INCHES SINCE MIDNIGHT.

0533 PM     HEAVY RAIN       2 ENE CAMAS             45.60N 122.38W
11/11/2021  M1.58 INCH       CLARK              WA   TRAINED SPOTTER

            1.58 INCHES PRECIP SINCE 6 PM NOV 11.

0731 PM     HEAVY RAIN       1 S AMBOY               45.89N 122.45W
11/11/2021  M2.65 INCH       CLARK              WA   TRAINED SPOTTER

            2.65 INCHES SINCE MIDNIGHT. 1.61 INCHES
            SINCE 9 AM.

1100 AM     HEAVY RAIN       1 E WELCHES             45.33N 121.95W
11/11/2021  M1.50 INCH       CLACKAMAS          OR   TRAINED SPOTTER

1159 PM     HEAVY RAIN       1 W CANBY               45.27N 122.70W
11/11/2021  M1.35 INCH       CLACKAMAS          OR   TRAINED SPOTTER

1159 PM     HEAVY RAIN       2 NE DURHAM             45.41N 122.74W
11/11/2021  M1.44 INCH       CLACKAMAS          OR   TRAINED SPOTTER

&&