Ice Storm in Oregon, December 22 to 25, 2022

Area Forecast Discussion
National Weather Service Pendleton OR
836 PM PST Thu Dec 22 2022

.UPDATE...A very complex weather situation continues over the
forecast area. Light snow has been slowly moving northward from
central OR to the north and east. So far, observation data has
shown the snow to progress as far north as Madras, OR. However,
due to the lack of observations across north central OR, the snow
has likely progressed further than this. Temperatures across
central OR are still mostly in the single digits, so it will be a
while before the precipitation mixes and changes over to freezing
rain. The latest model 850 mb temperatures by the GFS brings above
freezing air to central OR between 12Z-18Z Friday. This is when
the snow will mix with and/or change over to freezing rain at the
surface where cold air will still be entrenched. Then slowly, this
mixture of snow and freezing rain will spread north and east
through the day on Friday and Friday night. By late Friday
evening, the precipitation over central OR will be mostly rain
with above freezing temperatures in the 40s Friday afternoon and
cooling to the mid and upper 30s in the evening at the surface.
Elsewhere, colder below freezing air will remain trapped,
resulting in a continuation of a wintry mixture of snow and
various intensities of freezing rain. This will also be the time
when the precipitation with this first weather disturbance begins
to move out from south to north, and the precipitation is expected
to end temporarily over southern and eastern areas of the
forecast area. However, there will still be a wintry mix over
northwest portions of the forecast area. A variety of winter
weather highlights from Winter Storm Warnings and Winter Weather
Advisories will be continued in effect through at least late
Friday night or Saturday morning. Winter Weather Advisories

The next weather system will then arrive Saturday morning, with
mainly rain or snow in southern and eastern areas, and a large
area of freezing rain over north central OR to the Lower Columbia
Basin of OR and WA. A mixture of freezing rain and snow is
expected to persist in the northwest most portions of the CWA.
Freezing rain may become moderate at times from north central OR
northward to the WA Cascades and the Yakima/Kittitas Valleys.
These areas are likely to receive a significant amount of ice
accumulation. Therefore, the Winter Storm Warnings will continue
there. The Winter Storm Warning for the OR Cascades, north
central OR, the Simcoe Highlands, and the Foothills of the
southern Blue Mountains will continue as well due to continued
chances of freezing rain. Elsewhere, there are Winter Weather
Advisories in effect for much of the CWA, with modest amounts of
snow and light accumulations of ice from freezing rain are
expected. However, most areas south and east of the Lower Columbia
Basin and north central OR will likely have changed over to all
rain by this time, except for the higher mountains where a
mixture of snow and freezing rain/rain is still expected. This
weather system will persist through Christmas day morning and then
there will be a short break again in the southern areas of the
CWA before the next weather system moves into the forecast area by
early Monday morning. There will still be a chance of freezing
rain in the Lower Columbia Basin and areas north and west, but
most other areas will be warm enough for mainly rain, except for
the higher mountains in the eastern areas. This active weather
pattern will persist through at least mid week. However, by then
temperatures should be warm enough in most areas for mainly just
rain as colder air mixes out of most areas.

Lastly, winds are expected to reach low end advisory criteria in
the southern Grande Ronde Valley by late this evening, and strong
winds are expected to continue there through Friday evening. A
Wind Advisory is in effect there for this time frame. 88

&&

.AVIATION...06Z TAFs...A mixture of snow and freezing rain will
impact all terminals, with low CIGs and/or low visibility in snow.
As a result conditions through the next 24 hours will be mostly
IFR to LIFR. As the snow changes over to freezing rain, the
visibility will increase, which may allow for short periods of
MVFR or possibly even VFR conditions. Winds will be light through
the period and mostly under 10 kts. 88

&&

.PREV DISCUSSION... /issued 245 PM PST Thu Dec 22 2022/

SHORT TERM...Tonight through Saturday...A low amplitude ridge of
high pressure will be over the region tonight through Saturday. A
weakening weather system will move over the ridge tonight and
Friday. This system will bring some precipitation to the forecast
area with varying precipitation type. It will begin as snow and then
transitioning to sleet and freezing rain. This transition will begin
over central Oregon late tonight and then shift northward during the
day Friday through Friday night. Snow amounts are generally running
near low end advisory levels through Friday but combined with the
aspect of freezing rain have opted to issue winter weather
advisories for several zones. The system will exit the region Friday
night providing a brief break from the winter precipitation before
the next system arrives on Saturday. There will be enough warm air
aloft with the Saturday system that the primary precipitation type
will be either rain or freezing rain, except for the highest
elevations where it will be snow. The combination of these two
systems has led to the issuing of Winter Storm Warnings in several
zones along the east slopes of the Cascades where a quarter to half
an inch of ice accumulation will be possible. This system will also
bring a second round of freezing rain to the Columbia Basin and
surrounding Foothills Saturday. It is very likely that advisories
will eventually need to be issued or extended to cover the Saturday
event.

The very cold air air will remain over the region tonight with some
gradual warming on Friday. As the warmer air aloft moves into the
region the higher elevations will begin to warm up but the cold air
is expected to remain trapped in the Columbia Basin. By Saturday the
Foothills of the Blue Mountains in Oregon will begin to see some
eroding of the cold air but expect the cold air to remain trapped
along the Columbia River from The Dalles to Hermiston and all areas
northward.

LONG TERM...Sunday through Thursday...The overall pattern of a low
amplitude ridge of high pressure remains over the region on Sunday.
This will direct the main storm energy and precipitation across
Washington and just clipping northern Oregon. Still expect the cols
air to be trapped in the Columbia Basin Sunday mainly north of the
Oregon border. This will place the threat of freezing rain mainly
over the washington portion of the Columbia Basin.  A weather is
expected to move through the ridge on Monday bringing precipitation
back to the entire forecast area with snow levels at 5000 to 7000
feet. Even the Columbia Basin is expected to see temperatures
eventually climb above freezing by afternoon ending the threat of
freezing rain. An even wetter system is expected to arrive on
Tuesday with initially high snow levels that lower Tuesday night and
Wednesday to 3000 to 4000 feet with a transition to showers that
continues into Thursday. The Tuesday through Thursday period will
see much warmer temperatures return to the Lower Columbia Basin as
winds finally scour out the remaining cold air.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT   2  21  15  31 /  60 100  70  30
ALW   3  22  16  32 /  50 100  70  40
PSC   5  19  14  25 /  50 100  60  70
YKM   2  17  12  25 /  60  90  50  90
HRI   5  20  15  28 /  60 100  60  60
ELN  -2  13   9  25 /  60  90  50  90
RDM   5  40  33  44 / 100  50  20  50
LGD  10  28  26  38 /  50  90  70  20
GCD  14  37  34  43 /  60  80  40  20
DLS  10  25  20  32 /  90 100  60  90

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Chill Advisory until 10 AM PST Friday for ORZ041-044-049-
     050-505-507-508-510-511.

     Winter Storm Warning until 4 AM PST Sunday for ORZ041-508-510.

     Winter Weather Advisory until 10 AM PST Saturday for ORZ044-507.

     Wind Advisory until 10 PM PST Friday for ORZ049.

     Winter Weather Advisory from midnight tonight to noon PST
     Saturday for ORZ502.

     Winter Storm Warning until 11 PM PST Saturday for ORZ509.

     Winter Weather Advisory until 10 AM PST Saturday for ORZ511.

WA...Wind Chill Advisory until 10 AM PST Friday for WAZ024-026>029-
     521.

     Winter Storm Warning until 4 AM PST Sunday for WAZ024-521.

     Winter Weather Advisory until 10 AM PST Saturday for WAZ028-029.

     Winter Weather Advisory from midnight tonight to noon PST
     Saturday for WAZ030.

     Winter Weather Advisory from midnight tonight to 10 AM PST
     Saturday for WAZ026-027.

     Winter Storm Warning from midnight tonight to 11 PM PST Saturday
     for WAZ520.

&&

UPDATE...88
SHORT TERM...91
LONG TERM....91
AVIATION...88