Winter Storm and Heavy Snow in Washington, December 22 to 24, 2022

Area Forecast Discussion
National Weather Service Pendleton OR
953 PM PST Wed Dec 21 2022

.Updated Aviation Discussion.

.UPDATE...A very weak wave in a northwest flow over the region and
forecast area brought some flurries to the Blue Mountains and
Foothills this past afternoon. This has ended, and the mid and
high clouds are moving out. What is left is a widespread area of
low stratus over the valleys, canyons and the Lower Columbia Basin
and adjacent lower elevations. Temperatures have dropped to mostly
the 5 to 15 degree range, some some locations, such as Ellensburg
is now a couple degrees below zero. Tonight and Thursday will be
very cold, but not record cold. However, adding the wind to the
very cold temperatures, wind chill values will range from about
-10 to -25 across the forecast area. Currently, the wind chill at
Ellensburg, with only a 6 mph wind is -16 degrees, with a
temperature of -2 degrees.

The next weather system will begin spreading moisture over the top
of this dome of cold air, causing warm air advection snow to
begin from southwest to northeast. As the cold air begins to
slowly erode from southwest to northeast, the snow will transition
to sleet and/or freezing rain, first in central OR beginning late
at night between 09-12Z Thursday night. This transition will
slowly work its way north and east on Friday. Sleet and freezing
rain will reach north central and northeast OR by late morning or
early afternoon Friday, as well as along the OR/WA Cascade east
slopes. The amount of icing with this first disturbance will be
light due to the system weakening as it moves over the top of the
upper ridge that will be over the PacNW. Then a stronger and
wetter system will move into the region on Saturday, bringing
more significant icing from freezing rain, mainly from north
central OR across the Lower Columbia Basin, Blue Mountain
Foothills and along the WA Cascades and adjacent valleys to the
east. This second period of freezing rain has a good potential to
bring significant ice accumulation, especially along the Cascade
east slopes of WA, and the Lower Columbia Basin/North Central
Oregon and the eastern Gorge area. Winter Storm Watches have been
issued, beginning at 1 PM for the OR Cascade east slopes, and
then at 4 PM for north central OR northward to the Simcoe
Highlands and the WA Cascade east slopes. These watches are valid
through late Saturday night. Elsewhere across the forecast area,
additional winter weather highlights will be likely, as greater
confidence in the forecast increases.

There is very good consistency between the ensembles and
deterministic models in the short term, but that consistency
decreases by Friday night, so will hold off on issuing additional
winter weather highlights for now. The NBM 1D viewer temperature
forecasts are in very good agreement, with very low variance
through the short term period, as compared to last week. Tonight
and Thursday will be the coldest time period, and then significant
warming is expected to begin on Friday through Christmas Day with
strong warm advection expected. Temperatures will rise into the
mid 40s central OR and upper 30s to around 40 over northeast OR,
but likely remaining near or slightly above freezing over the
Lower Columbia Basin, Yakima and Kittitas Valleys by Christmas
Day afternoon. This will result in precipitation changing over to
all rain eventually on Christmas Day, especially areas outside of
the Lower Columbia Basin of WA and the Yakima/Kittitas Valleys
and the WA Cascades. 88

&&

.AVIATION...06Z TAFS...Variable conditions across all sites, with
VFR at DLS/YKM/PSC, MVFR at PDT/RDM/ALW, and IFR at BDN due to
reduced ceilings of OVC005 and visibilities of 3SM. Conditions are
expected to improve to VFR-MVFR for all sites after 08Z as a weak
ridge infiltrates into the region ahead of the next system.
Degrading conditions will be initially felt at BDN/RDM tomorrow
afternoon as light snow will be possible around 01Z before freezing
rain is anticipated toward the end of the period. This will reduce
visibilities to less than 1SM for BDN/RDM before reducing
visibilities and ceilings to MVFR conditions for all sites near the
end of the period. However, precipitation should hold off until
after the period for sites outside of Central Oregon. Winds will
stay below 10kts through the period 75

&&

.PREV DISCUSSION... /issued 232 PM PST Wed Dec 21 2022/

SHORT TERM...Tonight through Saturday...A ridge of high pressure
will move over the region overnight and early Thursday for a dry
but very cold period. Overnight lows will range from minus 10 below
zero up to 10 above zero. Not record cold but still well below
normal for this time of year. A weakening shortwave will begin
passing through the ridge late Thursday bringing some precipitation
into central Oregon then spread across the rest of the forecast area
overnight. Precipitation type will start off as snow but some warm
air aloft will begin spreading over the region transitioning precip
type over to sleet and freezing rain. This will begin in central
Oregon late Thursday night and then spread north across the forecast
area on Friday. The cold air near the surface will be slow to erode
away but central Oregon will change over to rain Friday morning to
midday. The cold air will be deeper and harder to erode away across
north central Oregon and areas northward so expect freezing rain to
persist into Friday night before decreasing. Main concern with this
system will be the snow followed by the ice accumulations due to
freezing rain. Snow and ice accumulations amounts are generally
expected to be near to low end advisory amounts. However, a second
weather system is expected to move into the region Saturday with
higher QPF amounts in what will generally be a freezing rain event
around the Columbia Basin due to the cold air remaining trapped. The
highest ice accumulations are expected along the east slopes of the
Cascades from about the Madras area northward and especially around
the Columbia Gorge region. As such, Winter Storm Watches have been
issued through Saturday night. The Saturday system will bring
widespread freezing rain to the Columbia Basin region too but only
thinking advisory amounts at this time or around a tenth to two
tenths of an inch of an inch but higher amounts across north central
Oregon and northward along the east slopes of the Cascades.

LONG TERM...Sunday through Wednesday...The overall ridge pattern
will shift the main track of precipitation northward Sunday but
continue to clip the forecast area will some precipitation. Most of
the cold air at the surface will have retreated north of the Horse
Heaven Hills meaning precipitation will be mainly rain except for a
possible wintry mix in the north. Ensemble models keep the overall
position of the ridge in place Monday into Tuesday before it moves
off to the east ahead of an incoming upper level trough. First of
the week will be warm with high snow levels and some possible
lingering freezing rain from the Tri-cities area northward Monday.
By Tuesday models are warming temperatures to above freezing in the
upper Basin region ending the freezing rain threat. The incoming
trough late Tuesday into Wednesday will lower snow levels bringing
snow to the mountains but rain to elevations below 3000 to 4000
feet.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT   0   7   4  20 /   0  10  70  80
ALW  -2   8   5  21 /   0   0  70  80
PSC  -2   9   7  16 /   0  10  70  80
YKM  -6   8   2  15 /   0  10  70  80
HRI   1   9   5  19 /   0  10  70  80
ELN -12   4   0  15 /   0  10  60  80
RDM   4  14   7  39 /   0  30  90  60
LGD   1  18  11  29 /   0   0  60  80
GCD   6  25  16  37 /   0  10  80  70
DLS   6  12   7  23 /   0  10  90  90

&&

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

     Winter Storm Watch from Thursday afternoon through late Saturday
     night for ORZ041-510.

     Winter Storm Watch from Thursday afternoon through late Saturday
     night for ORZ509.

WA...Wind Chill Advisory from 10 PM this evening to 10 AM PST Friday
     for WAZ024-026>029-521.

     Winter Storm Watch from Thursday afternoon through late Saturday
     night for WAZ024-520-521.

&&

UPDATE...88
SHORT TERM...91
LONG TERM....91
AVIATION...75