Flood in Oregon, December 27 to 28, 2024

Area Forecast Discussion
National Weather Service Portland OR
319 PM PST Fri Dec 27 2024

.SYNOPSIS...The end of December will continue to be
exceptionally wet with periods of gusty winds, snow, and river
flooding. A series of warm fronts will move over the region;
tonight, Saturday night into Sunday, with showers lingering
through Monday. Windy conditions expected with each front. Snow
along the Cascades likely, though snow levels will be right near
pass level. Areas of flooding stemming from both tidal overflow
and rainfall accumulation expected. High sneakerwave threat
with higher surf conditions.

&&

.SHORT TERM...Now through Saturday Night...The forecast over
the next several days just keeps on giving with very few breaks.
We will see a little bit of everything when it comes to
sensible weather so let's start by setting the stage with the
overall synoptic pattern.

The overall pattern is fairly typical for the Pacific Northwest
in the winter with a rotating cycle of low pressure systems.
Starting at the uppermost levels, the jet stream is very active
with zonal flow producing a 150+ kt jet streak directly over the
forecast area. These winds are coupled at the mid-levels at 500
mb where winds are around 60 kt. These two components show the
overall flow of the atmosphere and how unstable it is. This
instability stems from a number of low pressure systems that are
moving throughout the northern Pacific. Each one of these storm
systems is energetic producing ample precipitation and breezy
winds. The key with the sensible weather is just where each of
these lows end up making landfall.

The first low is currently located at 47.4N, -135.4W and slowly
spinning eastward. Based on ensemble models, the low will push
inland over Vancouver Island late tonight. This has been a
fairly typical path and models are in great agreement with this
overall which is a welcomed break from the last several
systems. On the leading edge of the low is a warm front which
can easily be seen in the satellite imagery. This warm front is
taking on the appearance of a weak atmospheric river which will
bring ample rain to the area. Rainfall totals will be terrain
dependent with the coast/coast range seeing the highest amounts,
and the Willamette Valley to a lesser degree. Snowfall is
expected along the mountains with varying accumulations. Snow
levels are sitting right around pass elevation and the front is
on the warmer side and therefore snow ratios will be lower. This
means that it will be a wet snow and, as our cohorts at the WPC
said, would fall as "concrete" - a very heavy dense snow that
would compact quickly. Because of the variability in snow levels
and the potential impact of slick roads, have extended the
Winter Weather Advisory to encompass this upcoming system
through Saturday morning.

Winds will be southerly at the surface and westerly aloft. The
westerly flow along the mountains will cause those winds to
intensify, especially with the addition of geostrophic flow
around the low. In regards to wind there is some model
disagreement between the high-resolution models and the blend
however, based on current observations, the NBM is running on
the low side, while the higher resolution solutions are a bit
high. Have trended towards the NBM 90th percentile for the
higher terrain of the Cascades and along the coast. Based on the
probabilities, there is less than a 10% chance of meeting high
wind warning criteria for either locations. However, if you are
planning on being on the mountains, know that it will be windy
with nearly a 40% chance of exceeding 60 mph.

Another system arrives Saturday night into Sunday which is on
track to be slightly stronger in regards to precipitation. It is
once again a wet and warm system, but there is a track
consideration that must be made. Right now, there are two
different options for a track: a northerly one which would be a
repeat of tonight, and a southerly one that moves over northern
Washington. The northerly track would force more wind and colder
temperatures. Thus, snowfall could be greater. Could see another
winter hazard headline but at this point, conditions are
favoring the warmer scenario. Highest precipitation would be in
the southern Willamette Valley and the Oregon Cascades. The NBM
mean 24 hour precipitation for Eugene produces around 1.75
inches of rain, the lowest 10% (driest scenario) is around 1
inch, and the highest 10% (wettest scenario) is around 3 inches
of rain. These rainfall totals could increase flooding concerns.
-Muessle

.LONG TERM...Monday through Friday...Raise your hand if you're
ready for dry weather? Well, while it may not be dry per say,
it will be drier with showers expected Monday through Tuesday.
But what would the first day of 2025 be without even more rain!
Southwesterly flow will dominate with the potential for yet more
active weather. There is a lot of consistency issues in the
long term forecast which has been the trend this week. Looking
at ensemble low centers, they are all over the northeast Pacific
with no real consensus. Because of this, we could see anywhere
from 0.00 inch to 1.0 inch of rain on Wednesday and again
Thursday. -Muessle

&&

.AVIATION...Generally MVFR cigs throughout the area, with an
approaching front that is arriving within an hour or two (by 02z
Sat). Once the front fully moves in, 90%+ chance of MVFR or lower
ceilings. Likely a mix of MVFR and IFR ceilings, with high res
ensemble guidance showing 60% chance for all terminals to see IFR
cigs under heavier bands of rain (70-80% at coastal terminals).
The front finishes passing through around 08-10z Sat, after which
we enter a showery post-frontal pattern. Expect more variable
cigs, generally between VFR/MVFR.

Slightly stronger southerly gusts begin once the front moves in,
up to 25 kt for inland terminals and 30-35 kt at coastal
terminals. Elevated winds continue through essentially the end of
the TAF period, finally beginning to weaken around 22z Sat.

PDX AND APPROACHES...Generally MVFR cigs, with the trend remaining
MVFR while the next front moves in around 02Z Fri. Around a 50-60%
chance of intermittent IFR ceilings while the front is passing
through, with chance of IFR ending around 07-09z Sat. Afterwards,
showery and variable conditions. 70% chance that it remains VFR
through the end of the TAF period after 09z Sat. Winds southerly
with gusts around 25 kts throughout the period. /JLiu

&&

.MARINE...One more front bringing relatively impactful weather is
expected to arrive later this afternoon going into the evening.
Seas are currently around 15 ft at 15 seconds, and will continue
to rise as the front pushes a westerly swell into the area. Seas
peak Saturday morning with 20-22 ft seas at 15-16 seconds, after
which seas continue to fall through next week.

Winds continue to rise, with elevated winds expected through
Saturday morning. Latest ensemble guidance supports the Gale
Warning, with 95%+ confidence in 24 hr max wind gusts over 34 kts.
Storm force level winds look quite unlikely, with only around a
10-15% chance of max wind gusts reaching that high. Winds begin to
weaken Saturday morning, and we see calmer seas and winds through
early next week. /JLiu

&&

.BEACH HAZARDS...A Coastal Flood Advisory has been issued for the
northern Oregon coast; with many coastal rivers fairly full from
recent rainfall and high tides close to threshold, an advisory has
been issued through high tide tomorrow, at 11 am Sat. 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 Sat.

With regards to a possible High Surf Advisory through Saturday
morning, current model guidance indicates even peak swells
Saturday morning (18 ft at 15-16 seconds) may just barely remain
below criteria. Considering a Beach Hazard Statement for sneaker
waves has already been issued, have elected not to issue another
High Surf Advisory. 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/JLiu

&&

.HYDROLOGY...Continued wet weather will lead to rises on rivers
and creeks through at least December 30th. The Luckiamute River
has the highest chance of hitting minor flood stage at 95% so
there is currently a Flood Watch in effect for this river. For
details, please see the warning product. here is a 40-45%
chance that the Pudding River will reach minor flood stage, and
a 20-25% chance that the Wilson River, Siuslaw River, and Marys
River will reach minor flood stage tonight into Saturday.

On Sunday into Monday, there is a 10% chance the southern
Willamette Valley will receive 2-3" of rain and the adjacent
higher terrain of the Coast Range and Cascades will receive 3-5"
of rain. This represents the worst case scenario, which would
result in more widespread flooding issues. That said, the most
likely scenario is 1-2" 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, 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.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory until 2 PM PST Saturday for ORZ101-102.
     Winter Weather Advisory until 1 PM PST Saturday for ORZ126>128.
WA...Winter Weather Advisory until 1 PM PST Saturday for WAZ211.
PZ...Small Craft Advisory until 5 PM PST this afternoon for PZZ210.
     Gale Warning until 4 AM PST Saturday for PZZ210-251>253-
     271>273.
     Hazardous Seas Warning from 4 AM to 4 PM PST Saturday for
     PZZ210.
     Hazardous Seas Warning until 5 PM PST this afternoon for
     PZZ251>253-271>273.
     Hazardous Seas Warning from 4 AM Saturday to 4 AM PST Sunday
     for PZZ251>253-271>273.
&&

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
x.com/NWSPortland