Flood and High Wind in Washington, December 17 to 18, 2024

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1136 AM PST Tue Dec 17 2024

...Updated aviation and watch, warning and advisory section...

.SYNOPSIS...A frontal system will maintain active weather today
through Wednesday morning. Snow for the passes early this morning
before snow levels rise today above 7000 ft. Moderate to heavy rain
through this afternoon, mainly for the coast and higher terrain.
High winds at the coast later today as well with gusts up to 60 mph
along the beaches and headlands through this evening before a cold
front pushes inland tonight into Wednesday morning.  Drier
conditions likely later Wednesday through Friday, although weather
remains active over the Pacific and near the coast. Rain returns for
the weekend as the upper level ridge shifts east allowing for
frontal systems to push inland. Active weather likely to continue
into early next week.

&&

.SHORT TERM /Through Wednesday Night/...Radar imagery as of 3 AM
Tuesday morning shows showers being replaced by more stratiform
precipitation as a warm front lifts northward into NW Oregon. The
front will continue to slide beneath an upper level ridge that is
building over the western US and into British Columbia today. Later
this morning, a plume of subtropical moisture with PWAT values
around 1.0-1.2 inches will begin enhancing rainfall over the area,
especially along the coastal mountains north of Newport and into the
far north Oregon and southwest Washington Cascades. This will likely
bring a period of moderate to heavy rain through this afternoon. The
main plume of atmospheric river moisture appears that it will most
likely lift farther north into NW Washington by this evening. Model
QPF amounts have decreased slightly from 24-hours ago, though still
expect the coastal mountains to pick up around 3-4 inches of
rainfall in a 24-hour period. This lowers concerns for widespread
coastal river flooding, though will still maintain a 40-50% chance
for the Grays River to reach minor flood stage later Tuesday
evening. HEFS guidance suggests all other rivers maintain around a
10-25% chance of minor flooding, though those higher numbers are
likely skewed to chances into early next week. Rain amounts inland
through Wednesday morning expected to range from 0.50-1.25" with
amounts decreasing to the south.

As the warm front lifts north across the area, south to
southwesterly winds are also expected to become quite breezy,
especially at the coast. At 925 mb, models show these winds
increasing aloft later this morning through this evening up to
around 65 kt. If these winds are able to mix down to the surface,
would likely see winds gusting up to 55-65 mph, strongest along the
beaches and headlands. The 00z HREF mean supports these high winds
predominately between 4 PM and midnight, with the strongest winds
likely occurring after 8 PM tonight as an approaching cold front
compresses the winds into a coastal jet before pushing inland late
tonight into early Wednesday morning. With probabilities for winds
gusting over 58 MPH around 50% (NBM) to 80% (HREF), will be
upgrading the High Wind Watch to a Warning. Southerly winds inland
are also likely to become breezy, with gusts generally up to 30-40
mph, with isolated gusts up to 45 mph possible across the central
Willamette Valley. The stronger winds inland, though, are more
likely to remain elevated within the stable layer.

As the warm front lifts across the region, expect snow levels to
gradually rise from around 4000 ft early this morning to around 8000
ft by this afternoon. Webcams continue to show heavy snow falling at
Cascade passes while temperatures hover around 32 degrees. The
Winter Storm Warning remains in effect through 10 AM this morning
with a few more inches of snow likely before temps warm and snow
levels rise. A shortwave trough will push a cold front across the
area Wednesday morning. Expect dry conditions by late Wednesday
morning. May see some sun briefly, before the next offshore system
brushes by with light rain possible (60% chance) at the coast by
Wednesday evening. /DH

.LONG TERM...Thursday through Monday...Not much change in the
forecast for later this week. While there still remains some
uncertainty, there is good agreement among the models and their
ensembles in the overall synoptic pattern. Cluster analysis of 500
mb heights have been consistent in showing an amplified pattern with
upper level ridging over much of the western US for Thursday and
Friday. With upper level troughing over the Pacific, the weather
will remain active offshore. Frontal systems may brush by northwest
Oregon and southwest Washington at times, but at most light rain at
the coast, with inland locations trending mostly dry with offshore
low level flow. Generally less than a 30% chance of measurable rain
east of the Coast Range until Saturday. Best chances for dry weather
on Friday with increasing offshore flow and breezy east winds
through the Columbia River Gorge. The EFI is also highlighting high
chances for warmer than normal temperatures at the coast on Friday
with highs approaching 60 degrees.

Clusters are also in good agreement that the upper level ridge
shifts east by the weekend, allowing for frontal systems to push
back inland. There is high confidence that wet weather over the
weekend continues into next week. Snow levels appear to stay
relatively elevated above 5000-6000 ft into early next week. /DH

&&

.AVIATION...Active weather continues across the airspace through
at least 18Z Wed. A series of fronts will cross bringing widespread
rain. Currently, expect the general VFR conditions will slowly
degrade towards MVFR, with a 15-20% probability for intermittent
periods of IFR. These frontal systems will also bring the potential
for LLWS (in the form of speed shear) to most of the terminal at
various times through the TAF period. Southerly winds will persist
with gusts up to 45 kt along the coast through around 12Z Wed and
gusts up to 30 kt for inland locations.

PDX AND APPROACHES...Currently VFR conditions with rain. The VFR
conditions are expected to trend towards MVFR as the day progresses
with persistent rain as a series of frontal passages occur. Frontal
passages will result in increased LLWS (in the form of speed shear)
probabilities. Southerly winds will persist with gusts up to 25 kt.
/42

&&

.MARINE...Active weather will continue through the majority of
the week as a series of systems move through the region. Each system
will bring building winds and seas across all waters. The first
system currently continues to bring Gale Force/low-end Storm Force
gusts up to 50 kt through at least Wednesday morning. The next trio
of systems will bring three quick rounds of systems to our waters,
with round one on Thursday, round two on Friday, and round three on
Saturday. The Saturday round is expected to be the strongest system
of the three, as of this time. For Thursday and Friday, could see
gusts up to 35 kt beyond 20 NM, but will see mostly gusts up to 30
kt over most of the waters (50-70% probability). Saturday, could see
gusts up to 40 kt beyond 20 NM, but will mostly see wind gusts up to
35 kt over most of the waters (50-70% probability).

Given the minimal downtime between systems this week, expect seas to
remain elevated. Today, seas around 16-18 ft. Wednesday, 12-15 ft,
highest Wednesday morning. Thursday and Friday, 10-12 ft. Saturday,
seas around 13-16 ft in the morning will build to 17-19 ft by
Saturday night into Sunday morning. There is a 25% chance that the
outer coastal waters will exceed 20 ft for Saturday.
Chances will increase towards 50% into late Sunday and Monday.
~Hall/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Wind Warning until 4 AM PST Wednesday for ORZ101>103.

WA...High Wind Warning until 4 AM PST Wednesday for WAZ201.

PZ...Storm Warning until 1 AM PST Wednesday for PZZ210-251>253-
     271>273.

     Gale Warning from 1 AM to 7 AM PST Wednesday for PZZ210-251>253-
     271>273.
&&

www.weather.gov/portland

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