Winter Storm in Washington, December 22 to 24, 2022

Area Forecast Discussion
National Weather Service Portland OR
259 PM PST Thu Dec 22 2022

.SYNOPSIS...An arctic air mass will remain over the region, bringing
the coldest temperatures the region has experienced since February
2014. A wintry mix of snow, sleet, and freezing rain will occur
tonight into Friday, resulting in significant travel impacts. While
most locations should warm above freezing Friday night into Saturday
morning, the Gorge will remain below freezing with freezing rain
lingering through at least Saturday night. Becoming warmer and wetter
next week.

&&

.SHORT TERM...THURSDAY THROUGH SUNDAY...

KEY POINTS...

*  Very cold temperatures and dangerous wind chills will continue
   through tonight into Friday morning.

*  A mix of wintry precipitation will spread across the area tonight
   and continue into Friday. Will see light snow transition to sleet
  and freezing rain this evening for much of the area.

*  Another weather system will move through on Saturday, with mainly
   rain expected and snow levels around 7000 to 8000 feet. The
   exception will be in the Columbia River Gorge, as persistent
   below freezing surface temperatures will likely result
   in another round of freezing rain and accumulating ice. Freezing
   temperatures in the Central Gorge could last into Sunday.

Be prepared for an impactful mix of wintry precipitation of freezing
rain and/or sleet along with very cold temperatures across much of
northwest Oregon and southwest Washington tonight through at least
Friday. As of 2 PM satellite and radar shows leading edge a warm
front moving into western OR & WA. Very light freezing rain reported
in the Eugene area so far. Pcpn will increase later this afternoon
and evening as freezing rain spreads from southwest to northeast.

The freezing air is deeper than earlier thought it would be and is
spilling across the gaps on the central coast in working into the
valleys of the Lane county Cascades. With the expected pcpn in those
areas have issued a Winter Weather Advisory for freezing rain through
tonight. Also issued a Winter Weather Advisory for freezing rain in
the Lane county Cascades as freezing air spills through the passes.

Now onto the most challenging part of the short-term forecast as a
warm front lift across the region tonigh and Friday. Given the below
freezing surface temperatures,

Forecast NAM BUFR soundings at Portland and Troutdale show  the
duration of sleet likely end up being longer than initially expected,
thus delaying the transition over to freezing rain. The HREF
continues support this idea and suggests the dominant
precipitation type will be sleet across the greater
Portland/Vancouver metro and east slopes of the Coast Range through
most of tonight, before finally switching over to freezing rain
between 2-4 AM PST Friday morning. Freezing rain is then expected to
continue through the day on Friday before temperatures slowly warm
above freezing Friday night, except in the Columbia River Gorge (this
zone is discussed below).  Freezing rain will prevail in southern
Willamette Valley, and central OR Coast Range upto 0.5 inch ice is
expected in these zone so will continue the Ice Storm Warning for
these zones tonight.

Unlike the rest of southwest WA and northwest OR, the main
precipitation type in the Gorge should be snow and sleet initially,
especially in the central Gorge near Hood River. Still expecting at
least 3-6 inches of snow and sleet in the central Gorge and Upper
Hood River Valley before precipitation finally switched over to
freezing rain Friday night. Below freezing surface temperatures
will most likely persist through Sunday across the Gorge as the cold
air east of the Cacades beging to retreat, but remain deep enough to
keep subfreezing air trapped there. So will see likely see
significant ice accumulations as a secondary weather system moves
into the region. Total ice accumulations will likely exceed one inch
through Saturday night through much of the Gorge.

Very little to no warming is expected through tonight as very strong
low-level offshore flow continually advects a fresh supply of arctic
air from east of the Cascade crest and strong winds maintain
dangerously cold wind chills. Therefore, Wind Chill Warnings and
Advisories remain in effect for the Cascades, the Gorge, the Cascade
foothills, the Portland/Vancouver metro, and the north OR Coast
Range. Make sure to keep your pets warm and take any precautions you
may need to take to prevent pipes from bursting. Anyone who is
exposed to the outdoors and is not dressed properly will be at risk
of developing frostbite and/or hypothermia.

East winds are gusting to 30-45 mph across the Portland
metro, but considerbly less in Clark County, as the brunt of the wind
turn towards the south once out of the Gorge. Not much change to the
winds as the TTD-DLS forecasted pressure gradient remains around -12
to -13 mb through the night, then gradually eases Friday. Winds are
around 50-60 mph in the western Gorge. But the windy spot, Crown
Point, seems to have losts it's wind sensor. The last report a gust
to 78 mph gust at 1245 pm, then silence.  The High Wind Warning
remains in effect for the western Gorge and a Wind Advisory remains
in effect for the Portland metro through early Friday.

When will temperatures warm elsewhere? That will depend on if and
when winds are able to switch directions, from east/north to
southerly. And often, models bring that warmer air in too soon. If
colder temperatures are able to hang on (longer than what the NBM
suggests), could make for some more interesting winter weather later
in the week, including the Portland metro. There is a chance that
temps warm above freezing Friday night, but confidence in that is low
and the actual timing may wind up being around 12 hours later than
that. Although some uncertainty remains with exact timing and exact
snow/sleet/ice amounts, confidence remains high that significant
impacts to travel will occur with this winter storm. Anyone with
travel plans Thursday night and/or Friday should either reconsider
their plans or ensure they are well prepared for a messy mix of
snow/sleet/ice on roads. Still expecting warmer temperatures and
widespread rain on Christmas Eve with rain amounts around half an
inch or more across the interior lowlands and around 1 inch or more
at the coast. Main concern on Christmas Eve will be for the Gorge and
Upper Hood River Valley where freezing rain will likely continue and
bring additional ice accumulations. -TK

.LONG TERM.../SUNDAY NIGHT THROUGH THURSDAY/...Major contrast in
weather regimes during the long term period. Instead of a cold arctic
air mass, the region will be dominated by a mild and very wet
conditions. The 12Z deterministic model runs are in good agreement
showing another juicy frontal system moving into the forecast area
Sunday night through Monday. This system looks to be the first of
multiple atmospheric river (AR) events during the long term period.
The GEFS and EPS Integrated Vertical Transport (IVT) values for this
first AR fall into the moderate to low-end strong range. The GEFS
IVT mean values are forecast to be a shade under 750 units. However,
the EPS shows a much lower IVT with this system, closer to 500 units
or at the high end of weak category. The GEFS maintains offshore
low- level flow through the Gorge Sunday night with a KTTD- KDLS
pressure difference around -5 mb. Thus, would still expect freezing
rain potential to continue around Hood River, the Upper Hood River
Valley and the east slopes around Mt. Adams.

Operational models show a brief lull Monday afternoon through Monday
evening, followed another robust system late Monday night through
Tuesday. The EPS Extreme Forecast Index(EFI) for this system is
around 0.6 to 0.7. These values are on a 0 to 1.0 scale, indicating
how unusual the event is during a 5-week period centered on the week
in question during the 2001 to 2022 time period. Thus, an EFI or 0.6
to 0.7 is not all that extreme, but does suggest something a little
more than what typically occurs. The corresponding EPS IVT forecast
is in the moderate category, with the control run well into the
strong category. The GEFS is a little weaker, but still well into the
moderate range. Snow levels will be well above the Cascade passes,
increasing the potential for rain-on-snow. There is also the
potential
for high wind along the coast with this system. The EPS ensemble
members for KONP show gusts to 50 mph, with a handful of members in
the 55-65 mph range. The EPS 96-hr precipitation total ending 00Z
Thursday shows up to 5 inches for the coast and Coast Range and also
the south Washington and north Oregon Cascades. This will likely
raise hydrological concerns toward the end of the extended period.

The upper level low/trough move through Tuesday night through
Wednesday,
with snow levels finally lowering to below the Cascade passes. The
deterministic runs show a weaker system moving through Thursday, but
snow levels are expected to be at Cascade pass level and the expected
precipitation amounts look to be much less than the two
aforementioned systems. Weishaar

&&

.AVIATION...00z TAFs: Dry offshore flow is maintaining VFR across
the forecast area. East winds will continue to gust to around
30-40 kt across the PDX metro through Friday morning, with winds
up to 50-60 kt just above the surface. A warm front pushing across
the area tonight will result in precipitation spreading inland,
while temperatures at the surface remain below freezing. This will
lead to a mix of wintry precip and lowering cigs overnight into
Friday. Likely will start as a brief period of snow or sleet,
transitioning quickly to freezing rain. Most areas should see
light precip by 00Z. Conditions are likely to begin to deteriorate
around 04-06Z Friday to MVFR, although areas north may see VFR
persist through 08Z Friday. Snow or sleet should be light, but
MVFR or IFR will be possible in any snow.

As usual, confidence in the timing of precip type transitions is
low. Perhaps the more important thing to note is that confidence
is high that there will be widespread impacts due to lowland snow,
ice, and blustery E-NE winds in the lowlands tonight into Fri
morning. It may be well into the day Friday that even KEUG warms
above freezing.

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

KPDX AND APPROACHES...VFR with cold east winds and increasing mid
level clouds this afternoon. Precip will likely start off as
flurries, with only minimal accumulations. Snow and sleet will
increase this evening, eventually transitioning to FZRA by around
06z. Occasional -FZRA will persist through Fri, along with MVFR
and perhaps IFR cigs/vsbys. East winds will continue to gust 30-40
kt through Fri morning. /DH

&&

.MARINE...Very strong and cold high pressure over the Columbia
Basin continues to drive strong offshore flow across the Pac NW
and adjacent waters. Solid Small Craft Advisory level winds are
occurring through most of our waters, along with localized gusts
to Gale Force abeam gaps in the coastal terrain. This will persist
through Friday morning, with winds slowly veering and decreasing
thereafter. S-SW winds likely increase again Fri night/Sat as low
pressure moves into Vancouver Island from the Pacific and its
associated front move across the Pac NW waters. The National
Blend of Models (NBM) v4.0 indicates a greater than 90 percent
chance of gale force gusts Saturday morning. Issued a Gale watch
for all coastal waters on Saturday. Yet another possibility of
gale force conditions looks possible Sunday.

Seas remain well below 10 ft through tonight, then are forecast
to exceed 10 ft by Friday morning. Seas hover close to 10 ft
through Friday night. Longer range guidance shows seas building
into the mid to upper teens Sunday night and possibly exceeding 20
ft early Monday.

Several forecast models show an even stronger system Tue or Wed,
with many showing winds to Storm Force and seas well above 20 ft.
We held our seas fairly close to the output from the NBM wave
guidance, if not just a little below it. The rationale behind
this is mostly uncertainty due to strength and timing of any
strong low approaching the Pac NW - however it should be noted
that the 06z run of the GFS global wave model shows a large slug
of seas 30 ft and higher reaching the Pac NW coast during the
middle of next week.  DH/Weagle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Storm Warning until 7 AM PST Friday for North Oregon
     Coast.

     Winter Weather Advisory until 4 AM PST Friday for Central
     Oregon Coast.

     Wind Chill Advisory until 6 AM PST Friday for Central Columbia
     River Gorge-Coast Range of Northwest Oregon-Greater
     Portland Metro Area-Northern Oregon Cascade Foothills-Upper
     Hood River Valley-Western Columbia River Gorge.

     Winter Storm Warning until 10 PM PST Friday for Coast Range of
     Northwest Oregon.

     Ice Storm Warning until 7 AM PST Friday for Central Coast Range
     of Western Oregon-South Willamette Valley.

     Winter Storm Warning until 1 PM PST Friday for Central
     Willamette Valley-Lower Columbia-Northern Oregon Cascade
     Foothills.

     Wind Advisory until 7 AM PST Friday for Greater Portland Metro
     Area.

     Winter Storm Warning until 10 PM PST Friday for Greater
     Portland Metro Area.

     Wind Chill Warning until 6 AM PST Friday for Northern Oregon
     Cascades.

     Winter Storm Warning until 6 PM PST Friday for Northern Oregon
     Cascades.

     Winter Weather Advisory until 4 AM PST Friday for Cascade
     Foothills in Lane County-Cascades in Lane County.

     Winter Storm Warning until 10 PM PST Saturday for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

     High Wind Warning until 1 PM PST Friday for Western Columbia
     River Gorge.

WA...Winter Storm Warning until 7 AM PST Friday for South Washington
     Coast.

     Wind Chill Advisory until 6 AM PST Friday for Central Columbia
     River Gorge-Greater Vancouver Area-South Washington Cascade
     Foothills-Western Columbia River Gorge.

     Winter Storm Warning until 1 PM PST Friday for I-5 Corridor in
     Cowlitz County-South Washington Cascade Foothills-Willapa
     Hills.

     Wind Advisory until 7 AM PST Friday for Greater Vancouver Area.

     Winter Storm Warning until 10 PM PST Friday for Greater
     Vancouver Area.

     Wind Chill Warning until 6 AM PST Friday for South Washington
     Cascades.

     Winter Storm Warning until 6 PM PST Friday for South Washington
     Cascades.

     Winter Storm Warning until 10 PM PST Saturday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     High Wind Warning until 1 PM PST Friday for Western Columbia
     River Gorge.

PZ...Gale Warning until 8 AM PST Friday for Columbia River Bar.

     Small Craft Advisory until 6 AM PST Saturday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 nm.

     Gale Watch from Saturday morning through Saturday afternoon for
     coastal waters from Cape Shoalwater WA to Florence OR out
     60 nm.
&&

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PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE PORTLAND OR
430 PM PST THU DEC 22 2022

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

1153 AM     NON-TSTM WND DMG SUNSET HILLS            45.51N 122.76W
12/22/2022                   WASHINGTON         OR   PUBLIC

            TREE FALLEN ON POWERLINE.

1221 PM     NON-TSTM WND DMG GARDEN HOME             45.47N 122.75W
12/22/2022                   WASHINGTON         OR   PUBLIC

            TREE FELL ON POWERLINE.

0223 PM     NON-TSTM WND DMG CENTRAL BEAVERTON       45.49N 122.78W
12/22/2022                   WASHINGTON         OR   PUBLIC

            TREE FALLEN ON POWERLINE.

0530 AM     NON-TSTM WND DMG BEAVER LAKE             45.32N 122.51W
12/22/2022                   CLACKAMAS          OR   PUBLIC

            TREE FALLEN ON POWERLINE.

&&