Debris Flow and Flooding in Oregon, December 3 to 4, 2023

Area Forecast Discussion
National Weather Service Portland OR
427 AM PST Sun Dec 3 2023

.SYNOPSIS...Another atmospheric river will bring widespread
heavy rain to the region through today, resulting in river
flooding and some urban flooding. Yet another atmospheric river
is expected to impact portions of the area Monday night through
at least Tuesday night, with periods of moderate rain continuing
into Wednesday and Thursday, bringing a continued threat of
river flooding and urban flooding.

&&

.SHORT TERM...Today (Sunday) through Tuesday...The forecast
remains on track with two additional atmospheric rivers (AR)
slated to bring river and urban flooding impacts to NW Oregon
and SW Washington through the middle of this week. Specific
forecast rain amounts and flooding impacts for both ARs are
discussed in the hydrology section below. One atmospheric river
began pushing into the region from the south last night. The
GEFS/EPS IVT plume forecasts continue to show excellent
agreement regarding the strength of this AR as both suggest IVT
values will peak just under 750 kg/ms, which is in the moderate
category. Rain has slowly become more widespread across the
region as a surface warm front has pushed north. Through 3 AM
PST, rain rates have been pretty low, around 0.1-0.2 inch per
hour. However, heavier rain will occur this morning and
afternoon as the surface cold front pushes east through the
area, bringing the main surge of moisture associated with this
system. Rain rates will pick up after 5 AM PST, peaking ahead of
and along the cold frontal passage through the morning hours,
decreasing behind the front as it moves into the southern zones
by mid to late afternoon. Expect rain rates up to 0.25-0.5 inch
per hour along the Coast Range and Cascades and up to 0.1 to
locally 0.25 inch per hour for the interior lowlands. The
heaviest rainfall is still expected to be south of a line
extending from Tillamook to northern Marion County. Locations
north of that line, including southwest WA and the
Portland/Vancouver metro, will still see heavy rain this morning
along with a threat of urban and river flooding. However, heavy
rain will not last as long as locations further to the south.
Gusty winds are also expected as the cold front pushes inland.
Expect winds similar to yesterday morning with gusts up to 30-45
mph for most locations and occasionally up to 45-60 mph for
wind prone locations along the coast and high terrain. Winds
will begin picking up after 5 AM PST, peaking through around
noon PST.

Precipitation should become light and showery across all of
northwest OR and southwest WA by Sunday evening as the
atmospheric river weakens and shifts southward into southwest
Oregon. Scattered showers will continue into Monday ahead of
the next AR that will bring another round of widespread heavy
rain Monday night through Tuesday. This system is shaping up to
be the strongest of the storm systems as both GEFS & EPS suggest
IVT values will peak around 900-1000 kg/ms, which is in the
strong category. The period of heaviest rain still remains north
of a line from northern Lincoln county to Marion County, though
uncertainty remains in the exact southern track of this storm.
Within this area, NBM indicates 50-80% chance of 5+ inches of
rain along the Coast Range, Willapa Hills, and southern
Washington Cascades Monday through Tuesday night. Additionally,
NBM indicates a 50-80% probability of 2+ inches of rain over
the interior lowlands for Marion County and north and an
80-100% probability along the entire coast, Coast Range, and
Cascades for the same time period. This heavy rain will bring
additional river and urban flooding impacts. Please read the
hydrology section below for more details. -HEC

.LONG TERM...Wednesday through Saturday...Latest guidance
indicates the tail end of the AR will continue bringing periods
of moderate rain to the region Wednesday. There is uncertainty
on exactly how much rain and the location of heaviest rain will
continue on Wednesday, but the latest NBM forecast indicates a
50-70% chance of an additional 1+ inches along the coast, Coast
Range, and Cascades and a 20-30% chance elsewhere. Expect urban
and river flooding concerns to continue into Wednesday.

On Thursday, a cold front will begin to drop down from
the Gulf of Alaska. While initially this pattern appeared to be more
of an AR type scenario, as the days have passed, it is not
necessarily taking the same shape and rather that of an elongated
trough. This trajectory would bring in colder air from Canada,
which could bring another round of snow for the Cascades and
moderate rain for the rest of the region. NBM 10th percentile
snow levels indicate the potential for a rain/snow mix down to
around 1500-2000 feet Friday. Looking into the weekend, GEFS &
EPS guidance are indicating the potential for another warmer
weather system sometime late Friday into Saturday. Uncertainty
remains in exact timing and amounts, but ensemble members
indicate the potential for another round of moderate to heavy
rain. -HEC

&&

.HYDROLOGY...A series of atmospheric rivers will bring periods
of widespread heavy rain through mid week. Moderate to heavy
rain is expected through today with these additional rain
amounts expected through tonight: 0.75 to 2.0 inches along the
south Washington and north Oregon coast, 2.0 to 4.5 inches along
the central Oregon coast, 1.0 to 2.0 inches over the Willapa
Hills, 1 to 3.0 inches over the north Oregon Coast Range, 2 to
4.0 inches over the central Oregon Coast Range, 0.75 to 1.5
inches across the interior lowlands of southwest Washington and
northwest Oregon (highest in the Eugene- Springfield area), 1.5
to 4.0 inches across the Cascades and western Columbia River
Gorge, and 1.0 to 3 inches over the central Columbia River Gorge
and Upper Hood River Valley.

More rain is expected Monday through Wednesday with another
atmospheric river. Although some uncertainties remain on exact
numbers, latest and previous forecast guidance have suggested
these additional rainfall totals from Monday through Wednesday
with the heaviest rain Monday night through Tuesday: 2 to 6
inches for the Coast Range and Willapa Hills, 2 to 4 inches
along the coast, 2 to 5 inches for the north and central Oregon
Cascades, 2 to 8 inches for the southern Washington Cascades,
and 1 to 3 inches across the interior lowlands of southwest
Washington and northwest Oregon.

Given the forecast rainfall amounts expected this weekend into
mid week, river levels will continue to rise through Wednesday.
Forecasts for most rivers indicate a smaller peak late tonight
into Monday morning with another higher peak Tuesday afternoon
into Wednesday morning. Most coastal rivers show a 40-60%
chance of reaching minor flood stage and a 10-30% chance of
reaching major flood stage in the next 10 days. This includes
the Wilson River and Trask River near Tillamook, the Naselle
River near Naselle, the Nehalem River near Foss, the Nestucca
River near Beaver, the Grays River near Rosburg, and the Siletz
River at Siletz.

Additionally, many other rivers across northwest OR and
southwest WA still show anywhere from a 30-50% chance of
reaching at least minor flood stage in the next 10 days, except
for the mainstem Columbia and Willamette Rivers where the chance
of reaching minor flood stage remains lower, around 5-10%. To
view current and forecast river stages for any river gage
location across southwest WA and northwest OR, visit
https://water.weather.gov/ahps2/index.php?wfo=pqr

Lastly, urban flooding is also possible through Wednesday,
especially in low-lying areas with poor drainage. If you have
gutters and/or storm drains that still need to be cleared out,
now is the time. Significant ponding of water and localized
roadway flooding is expected on roads. Any motorists with travel
plans should also allow extra time for their commute as
hydroplaning will be a risk. Never drive through flooded roads.
-TK/HEC

&&

.AVIATION...Widespread rain and predominantly MVFR conditions
throughout the coast and Willamette Valley as the warm front
stalls, with moments of IFR possible through 20z Sunday. Southerly
winds are expected to increase beginning 14z Sunday, gusting
20-30 kts in the Willamette Valley. Coastal areas may see 30-40
kt gusts, with strongest wind south of KTMK. Winds will gradually
ease around 22z Sunday. Rain will taper to showers 21z-00z Monday
as a trailing cold front sweeps across the area, with conditions
improving to VFR across the north and high end MVFR as rain
lingers south of KSLE.

** The new Aviation Weather Center website is live. The new
 website can be found at aviationweather.gov **

KPDX AND APPROACHES...Predominantly MVFR cigs and vis throughout
Sunday morning. Wind will gust out of the south to 20-25 kt at the
terminal around 17z Sun into the afternoon. Rain tapers to
showers 21-00z Mon with cigs improving back towards VFR around
FL040. /JH

&&

.MARINE...Warm front continues to increase southerly winds,
lasting through Sunday morning. Expect to see gales across all
zones except 271 and 251 (waters north of Cape Falcon) until
late Sunday morning, then transitioning into hazardous seas.
A small portion of 271 & 251 may briefly see gales during this
time.

Another series of fronts will bring gales again to the waters
late Sunday night through Monday night. May even see a brief
period of storm force winds Monday evening. Current guidance has
seas just under 20 ft during this time, but considering this is
prolonged period of gales will probably see seas exceeding 20 ft
in the Monday/Monday night time frame.

Westerly swell around 13 to 15 feet late Sunday morning through
Monday morning, so a Hazardous Seas Warning has been issued 21z
Sunday to 12z Monday. Also with the elevated seas continuing
through mid-week, will maintain the threat of sneaker waves along
our coast. /JH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Flood Watch through Wednesday afternoon for Cascade Foothills
     in Lane County-Central Coast Range of Western Oregon-Central
     Oregon Coast-Central Willamette Valley-Coast Range of
     Northwest Oregon-Greater Portland Metro Area-Lower Columbia-
     North Oregon Coast-Northern Oregon Cascade Foothills-South
     Willamette Valley.

WA...Flood Watch through Wednesday afternoon for Greater Vancouver
     Area-I-5 Corridor in Cowlitz County-South Washington
     Cascade Foothills-South Washington Coast-Willapa Hills.

PZ...Gale Warning until 10 AM PST this morning for Coastal waters
     from Cape Falcon OR to Cape Foulweather OR out 60 NM.

     Gale Warning from 4 AM to 4 PM PST Monday for Coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Hazardous Seas Warning from 1 PM this afternoon to 4 AM PST
     Monday for Coastal waters from Cape Foulweather to Florence
     OR out 10 NM-Waters from Cape Foulweather to Florence OR
     from 10 to 60 NM.

     Small Craft Advisory until 4 PM PST Monday for Columbia River
     Bar.

     Small Craft Advisory until 4 AM PST Monday for Coastal waters
     from Cape Shoalwater WA to Cape Falcon OR out 60 NM.

     Hazardous Seas Warning from 10 AM this morning to 4 AM PST
     Monday for Coastal waters from Cape Falcon OR to Cape
     Foulweather OR out 60 NM.

     Gale Warning until 1 PM PST this afternoon for Coastal waters
     from Cape Foulweather to Florence OR out 10 NM-Waters from
     Cape Foulweather to Florence OR from 10 to 60 NM.
&&

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