Winter Weather in Oregon, December 20, 2022

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
838 PM PST Mon Dec 19 2022

Updated Aviation and Marine Sections

.SYNOPSIS...Still cool, with spotty light rain or mountain snow.
Cold front over northern Washington will slide south across the
region later Tuesday night and Wednesday. Dry and cold for Wed into
Thu. But, precipitation returns late Thu, with snow in the evening,
then transition to freezing rain and sleet overnight into Fri. Worst
will be in the Columbia Gorge, with frigid east winds and snow.
Moderating temperatures over the weekend, with next week looking
rather wet but mild.
&&

.SHORT TERM (TONIGHT THROUGH THURSDAY)...

KEY POINTS...

*  Rain later Tuesday, then colder air arrives.

*  Windy, cold and dry for Wednesday into Thursday.

*  Precipitation returns as snow later Thursday. Then, change
   to messy mix of freezing rain, sleet and snow Thu night and Fri.

*  Blizzard conditions in the Gorge possible Thu night and Fri.

Overall, some things coming into place, as far as clarity in the
forecast this week. As always, periodically check for latest
forecasts, as they will likely change over next few days.

First, weak westerly flow aloft continues, with variable moisture
aloft. This enough to keep some chance of showers across the region
tonight into early Tue. But, snow levels stay off the lowest
valleys, running 1000 to 1500 feet. Temperatures will generally stay
in the 30s tonight, with cooler temperatures for the mountains.

An arctic front remains rather stationary to our north, draped from
Vancouver Island to Washington Cascades then across eastern
Washington. A weather system well offshore will push into western
Washington on Tuesday. Now, it will be unable to push much to the
east, and instead, will drop southeast from south side of the
Olympics to the south Washington Cascades during the day on Tuesday,
then into north central Oregon Tuesday evening. This will bring a
band of rain across the region, moving from north to south. Still
enough onshore flow that will maintain snow levels at 1500 to 2000
feet in the north, and higher to the south. As such, no low
elevation snow issues expected. But, different story for the higher
terrain. Generally, 10 to 18 inches of snow likely for the south
Washington Cascades, and 4 to 12 inches across the Oregon Cascades.
So, will have Winter Storm Warning for south Washington Cascades,
and put up Winter Weather Advisory for Cascades of Oregon. Still bit
cool in the Hood River Valley, and with the track of the low, not
likely to see much boost in the snow level. Likely that it will hold
somewhere at 500 to 1000 feet. Will put up Winter Weather Advisory
for the Hood River Valley, where could see 2 to 5 inches Tue
afternoon into Tue evening. Precipitation will decrease later Tue
night from north to south.

As the low passes to the south, it will tug and pull the arctic
front southward. This will bring drier and cooler weather into the
region. But, the main burst of cold air will be arriving from the
east on Wed, as the arctic front slides south of the Columbia River.
Plenty of cold air east of the Cascades will filter westward on
increasing easterly winds Wed and Wed night, with temperatures
mostly falling during that time. Likely to see some of the coldest
air since Feb 2014, with lows in the teens for inland valleys, and 0
to 5 above for the Columbia Gorge. With east winds increasing,
gradients spiking near 10 to 13 mb offshore. Local studies project
wind gusts of 45 to 55 mph in east Portland/Vancouver metro, with 60
to 80 mph in the Columbia Gorge, and perhaps to 100 mph on exposed
spots such as Crown Point. With temperatures so cold, and winds
strong, expect wind chills to be in the single digits and colder for
some areas.

At risk populations include those houseless, and/or living in
vehicles/camps. This will be a time to ensure they are given
opportunity to seek safe and warm shelters. In addition, animals
such as pets will need to be looked after, ensuring adequate shelter
and water.

Not much change on Thursday, as remains cold with strong offshore
flow. Highs likely only back to the lower 20s for inland valleys,
with highs near 10 in the Gorge. Even along the coast, highs will be
much below normal, with Astoria and Tillamook only reaching 30 deg.
Again, very windy in/near the Gorge.

While Thursday starts dry, will see increasing clouds, with some
spotty precipitation arriving later in the day. Air mass is cold,
and will support snow with the onset. But, could be more as
occasional flurries, with any sticking snow holding off until
Thursday evening as moisture aloft increases.	      /Rockey

.LONG TERM (THURSDAY THROUGH MONDAY)...
Generally, not much in way of change. Still, confidence increasing
for timing and precipitation type, but still low confidence in
actual snow/ice accumulations.

Concerns for high impact weather impacting holiday travel have
increased for the long term forecast, especially late Thursday
through Friday when impacts are expected from a combination of
increasing winds, dangerously cold wind chills, and low elevation
snow/sleet/freezing rain. Will first discussion the wind potential,
as the latest iteration of the deterministic GFS/EURO have shown a
notable uptick in the strength of low-level offshore flow. In fact,
both models are now suggesting the surface pressure gradient from
Troutdale to the Dalles will peak around -13 mb Thursday morning and
remain strong through Friday before weakening Friday night into
Saturday. This is very strong and would suggest wind gusts over 100-
120 mph at Crown Point if the east winds were purely gap driven.
That said, this offshore flow event looks more like a downslope wind
event as the cold pool looks to deepen enough to spill over the
Cascade crest. Both the GFS and EURO show winds becoming due
easterly up to ~850 mb. This means wind gusts over 100 mph would not
be likely in the Gorge, however it would still be quite windy and
the wind impacts would be more widespread. For now, have nudged
heavily towards the NBM 90th percentile for both speeds and gusts.

Given forecast temperatures in the teens and 20s across the lowlands
Thursday/Thursday night, the strong east winds will result in
dangerously low wind chills. The current forecast suggests wind
chills in the lower single digits across the Portland/Vancouver
metro and below zero in the Gorge. The high Cascades could see wind
chills as low as 10 to 30 degrees below zero on Thursday. Even the
NBM 90th percentile keeps high temperatures below freezing on
Thursday, suggesting confidence is high that much colder than normal
temperatures will materialize even if the warmest model solutions
verify. This event will be a major concern for the houseless
community, people recreating outdoors who are not properly dressed
for the dangerously cold wind chills, and anyone without adequate
heating. Frostbite and hypothermia can occur in a matter of minutes
on exposed skin with wind chills as cold as 30 below zero, so
recreation in the Cascades is not advised late in the week. Bursting
pipes will also be a concern if precautions are not taken.

In addition to the wind and cold, a threat of snow and freezing rain
remains in the forecast (as well as a period of sleet between the
transition). In fact, GFS/EURO soundings match well and depict a
textbook-worthy example of snow transitioning to sleet and then
transitioning to freezing rain as a warm layer aloft deepens and the
sub-freezing layer near the surface becomes more shallow. Given the
fact that even the warmest model solutions keep temperatures below
freezing through the day, confidence is increasing that sleet and/or
freezing rain will occur at some point late Thursday into Friday
across most of southwest WA and northwest OR (including most of the
coast). However, confidence is very low in regards to exact
precipitation amounts. In fact, the NBM v4.1 shows QPF amounts near
zero for the 10th percentile and over an inch for the 90th
percentile. This would make the difference between a high impact ice
storm and a light glaze of ice. Given the high degree of
uncertainty, the current forecast closely matches the NBM 50th
percentile for QPF/snow/ice amounts. Regardless of the outcome,
anyone with travel plans on Thursday and/or Friday should pay close
attention to the forecast over the coming days as guidance comes
into better agreement and the details become more clear. Warmer
temperatures are still likely to move into the region over the
weekend.	   /TK

&&

.AVIATION...06Z TAFs: Predominately VFR conditions over the area
Monday evening. Localized IFR to MVFR was noted over the interior
lowlands generally around KSLE and also in the Gorge between KCZK
and K4S2. Little change in conditions through around early
Tuesday morning. A west-to-east oriented boundary slowly slips
south over southwest Washington and far northwest Oregon Tuesday
morning. Expect increasing MVFR in this area generally after 12Z
and persisting well into Tuesday afternoon. There may be
improvement to MVFR or even low-end VFR over the south Washington
and north Oregon coast and coastal valleys between 00Z and 06Z
Wednesday. Elsewhere, VFR to prevail into Tuesday morning, with
areas of MVFR mainly over the coastal areas. Increasing MVFR after
18Z, with areas of IFR, likely through 06Z Wednesday.

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

KPDX AND APPROACHES...VFR conditions at the terminal and vicinity
through 12Z Tuesday. MVFR becomes more likely after 12Z as a
west-to-east boundary slides across the area. There is the
possibility of IFR after 18Z, especially in the Tualatin River
Valley. There is a 60 percent chance of sub-VFR conditions Tuesday
afternoon through 06Z Wednesday. Weishaar

&&

.MARINE...A west-to-east oriented boundary resides over the
Washington waters Monday evening. As of 04Z wind speeds were
generally 15 kt or less, except to 20 kt at buoy 46089. Models are in
good agreement showing the wind veering to south to southwest Monday
night. The High-resolution guidance suggests wind speeds will
increase to 20 kt with gusts to 25 kt over the north zones overnight.
A couple models show boundary layer wind speeds close to 30 kt over
the far north part of PZZ271 and PZZ251 late Monday night through
early Tuesday morning. Believe gusts to 30 kt are a little overdone
and will stick closer to 25 kt for this forecast. A small craft
advisory will remain in effect for the north zones. Wind shifts to
northerly over the north zones by Tuesday afternoon and then the far
south zones by late afternoon.

Offshore flow develops over the north zones Wednesday afternoon
and then spreads to the remainder of the waters by Wednesday
evening. The offshore gradient strengthens Wednesday night and
Thursday with gusts to 30 kt likely, especially downwind of
coastal gaps. A couple models suggest possible low-end gale force
gusts late Thursday. The wind finally shifts to southwest Thursday
night. Solid small craft advisory level wind speeds can be
expected Friday through the weekend.

Seas will hover around 5 ft through Thursday. Seas get close to 10
ft Friday or Friday night then build into the lower teens over the
weekend. There is the potential for wave heights in the mid-teens
early next week. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory from noon Tuesday to 4 AM PST Wednesday
     for Cascades in Lane County-Northern Oregon Cascades-Upper
     Hood River Valley.

WA...Winter Storm Warning from noon Tuesday to midnight PST Tuesday
     night for South Washington Cascades.

PZ...Small Craft Advisory until 9 AM PST Tuesday for coastal waters
     from Cape Shoalwater WA to Cape Falcon OR out 60 nm.
&&

www.weather.gov/portland

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