Heavy Snow in Oregon, December 28 to 30, 2024

Area Forecast Discussion...Updated Aviation
National Weather Service Portland OR
953 AM PST Sat Dec 28 2024

.SYNOPSIS...The end of December will continue to be very wet with
periods of heavy rain, gusty winds, Cascade snow and isolated minor
river flooding. The coast will continue to see minor tidal overflow
flooding, high surf conditions and a high threat for sneaker waves. A
weak shortwave ridge will bring a brief break from the stormy weather
Monday night into Tuesday with areas of fog and low clouds likely
developing Monday night. A wet weather pattern returns Wednesday
through Friday, albeit with higher snow levels and minimal winter
travel impacts for the Cascade passes.

&&

.SHORT TERM...Now through Monday...The short term forecast is
highlighted by more wet weather with periods of heavy rain, gusty
winds, heavy Cascade snow, minor river flooding and minor tidal
overflow flooding at the coast. The strongest winds continue to be
observed in the high Cascades above 5000-5500 feet where winds gusts
are generally ranging between 45-75 mph. This will make for less than
ideal conditions for recreation in the high Cascades today,
especially given the continued snow showers and dangerously cold wind
chills in the single digits. The strong winds in the high Cascades
will begin to decrease after sunset, however accumulating snow will
continue, especially Saturday night through Sunday afternoon when the
next round of moisture lifts northward from southwest Oregon into
northwest Oregon and southwest Washington with a weak to moderate
atmospheric river (IVT values likely peaking somewhere around 500
kg/ms). This round of moisture will initially be accompanied by a
warm front which will briefly send snow levels up to around 6000 feet
in the Oregon Cascades Saturday night (above pass level), except only
to around 3500-4000 feet in the south WA Cascades and north OR
Cascades around Mt. Hood where the warmer air aloft will not be as
significant. The increase in snow levels will be very short-lived as
a trailing cold front sends snow levels down to 3000-4000 feet by
late Sunday morning and then to 2500-3000 feet Sunday evening. Snow
levels then look to hover within that range through Monday. Although
the main slug of moisture will be out of the area by then,
post-frontal showers aided by strong orographic lift associated with
a westerly jet around 40-55 kt at 850 mb and 55-65 kt at 700 mb will
continue to bring accumulating snow to the Cascades, including at
pass level. As such, a Winter Storm Watch is now in effect for the
Cascades from 4 AM Sunday through Monday morning. During that time,
another 10 to 16 inches of snow is likely to fall (>80% chance),
except up to 2 feet above 5500 feet. This means anyone planning on
travelling over the Cascade passes on Sunday should either reconsider
travel plans or be well prepared for dangerous winter travel
conditions, especially if your travel plans are during the afternoon,
evening or overnight hours when snow levels will be below pass level,
snowfall rates will be heavy and westerly winds will be strong. If
you must travel over the passes on Sunday, try to do so in the
morning before conditions worsen.

Enough about the Cascades. What about the coast and interior
lowlands? These areas will continue to see fairly mild temperatures
with another round of moderate to heavy rain and breezy winds at
times. Ongoing scattered rain showers Saturday morning will give way
to a steadier stratiform rain Saturday night, continuing into Sunday
morning as the aforementioned atmospheric river lifts northward over
the area. Precipitation will become showery again Sunday afternoon
with post-frontal showers then continuing Sunday night into Monday,
albeit less widespread and not as heavy when compared to Sunday
afternoon. Although there are minimal hydro concerns over southwest
Washington and far northwest Oregon with this system, that is not the
case to the south of Tillamook and Salem where chances for minor
river flooding and minor urban flooding remain in the forecast. If
you want details regarding forecast rain amounts and the potential
for flooding, please refer to the hydrology discussion below. The
threat for tidal overflow flooding at the coast is covered in the
beach hazards discussion, as well as the ongoing high surf conditions
and high sneaker wave threat.  -TK

.LONG TERM...Monday night through Friday night...If you want a break
from the ongoing storm weather, Monday night into Tuesday looks to be
your only chance. This is when models and their ensembles are showing
weak shortwave ridging with a very brief period of high pressure.
This ridge will bring a break from precipitation, however clearing
skies and light winds Monday night will set the stage for areas of
fog and low clouds to develop. Fog and/or low clouds will then linger
into Tuesday for inland valleys, resulting in chillier daytime
temperatures.

The weather pattern becomes wet again either Tuesday night or
Wednesday (there is some model disagreement regarding timing). NBM
PoPs peak around 75-80% on Wednesday morning and afternoon, which is
when rain is most likely to return to the area as a Pacific frontal
system pushes inland. Post-frontal showers then look to continue late
in the week. Currently there are no hydro concerns or wind concerns
with this system, and snow levels look to be above pass level
Thursday through Friday. Although snow levels should be below the
passes on Wednesday, snow amounts look fairly light with minimal
travel impacts.  -TK

&&

.HYDROLOGY...A series of storm systems will continue to bring a wet
weather pattern across northwest Oregon through the weekend. This
will lead to rises on rivers and creeks through at least December
30th. As of 2 AM PST December 28th, the Luckiamute River remained
just above minor flood stage and is forecast to remain right at or
just above minor flood stage through Saturday afternoon before
falling below flood stage Saturday evening, Dec. 28th. Other rivers
across northwest Oregon remained below minor flood stage, however
river levels will continue to rise. With the rising river levels,
there is a 40% chance that the Pudding River at Aurora will also
reach minor flood stage sometime on December 30th. In addition, there
is a 15-25% chance that the Wilson River, Marys River, Mohawk River
and Siletz River will reach minor flood stage. The Siuslaw River has
a 35% chance of reaching minor flood stage.

Heavy rainfall may also lead to ponding of water, general lowland
flooding and localized urban flooding, especially in low lying areas
or other areas with poor drainage. Heavy rainfall will also increase
the potential for landslides in areas of steep terrain and debris
flows over areas that have been burned by wildfires over the past
few years.

There is still some uncertainty regarding the exact track of the
Saturday night/Sunday system and associated rain amounts. There is
currently a 10% chance the southern Willamette Valley will receive
2-2.5" of rain and the adjacent higher terrain of the Coast Range
and Cascades will receive 3-4" of rain Saturday evening through
Sunday morning. This represents the worst case scenario, which would
result in more widespread flooding issues. That said, the most
likely scenario is 1-1.5" of rain in the southern Willamette Valley
and 2-3" in the adjacent higher terrain of the Coast Range and
Cascades. This would result in minor and less widespread flooding.
Shifts in the forecast storm track can change the current outlook
and probabilities for river flooding, so be sure to monitor
weather.gov/portland for the latest weather updates, as well as any
watches, warnings, or advisories that may be issued. To view the
latest river forecasts, visit water.noaa.gov/wfo/pqr.  -TK

&&

.BEACH HAZARDS...A Coastal Flood Advisory has been issued for the
northern Oregon and south Washington coast; with many coastal
rivers fairly full from recent rainfall and high tides close to
threshold, an advisory has been issued through high tide, around
midday today. In addition will see large west swell reaching the
coast, elevating waters near shore. Concern should generally be
limited to the few hours at and around 11 am tomorrow, with some
minor tidal overflow possible between around 8am-2pm.

High Surf Advisory issued with with peak wave occurring around the
high tide this morning. Westerly swell forecast around 19 ft at
16-17 seconds, on the low end of advisory criteria. Along with the
rough conditions in the surf zone, with the longer period of the
swell sneaker wave potential are enhanced as well.

That said, beachgoers should stay far away from incoming waves
while remaining observant and vigilant. Sneaker waves will be a
threat and can catch beachgoers off guard, potentially knocking
you off your feet and pulling you out to sea.  /mh

&&

.AVIATION...Showers continue across the region, with mixed
VFR/MVFR conditions throughout the region. Breezy southwesterly
winds at all terminals, up to 25 kt. 70-80% confidence that the
Willamette Valley remains VFR while this showery pattern is
ongoing through 8-12z Sun, when the next front arrives. Once this
moves in, confidence for MVFR cigs at all terminals is 85%. Can't
rule out brief drops to IFR with heavier bands of rain (30%
chance). Winds look more mild with this front, with southeasterly
gusts up to 10-15 kt possible after the front moves in.

PDX AND APPROACHES...Mostly VFR conditions through 12z Sun,
though passing showers may briefly drop ceilings to MVFR. Some
localized low-level wind shear remains possible through 22z Sat,
with some speed shear in the first 2000 ft AGL. After 12z Sun, the
next front moves in, bringing MVFR ceilings (90% confidence).
Winds turn southeasterly and weaken, with gusts up to 10-15 kt
possible. /JLiu

&&

.MARINE...Low pressure near southwest B.C. early this morning
west to southwest wind gusting 25 to 35 kt. There is a Gale
Warning for the waters north of Cape Foulweather this morning.
The aforementioned low pressure weakens today and winds will back
ahead of the next low pressure. There is potential for gales to
return to the waters late Sunday morning into the afternoon.
Models have been varying on the solution so forecast confidence is
low. But at this time, appears the waters south of Cape Falcon
stand to see the gales so have issued a Gale Watch for those areas
for Sunday. More benign conditions develop later Sunday into
Monday, but active pattern resumes by next mid-week.

Seas peak later this morning around 20-22 ft seas at 16-17
seconds, after which seas continue to fall into next week.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Coastal Flood Advisory until 2 PM PST this afternoon for ORZ101-
     102.

     High Surf Advisory until 10 PM PST this evening for ORZ101>103.

     Winter Weather Advisory until 1 PM PST this afternoon for
     ORZ126>128.

     Winter Storm Watch from late tonight through Monday morning for
     ORZ126>128.

WA...Coastal Flood Advisory until 2 PM PST this afternoon for WAZ201.

     High Surf Advisory until 10 PM PST this evening for WAZ201.

     Winter Weather Advisory until 1 PM PST this afternoon for
WAZ211.

     Winter Storm Watch from late tonight through Monday morning for
     WAZ211.

PZ...Gale Warning until noon PST today for PZZ210-251-252-271-272.

     Hazardous Seas Warning until 4 AM PST Sunday for PZZ210-251-252-
     271-272.

     Gale Watch from Sunday morning through Sunday afternoon for
     PZZ252-253-272-273.

     Hazardous Seas Warning until 4 AM PST Sunday for PZZ253-273.
&&

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