Heavy Snow in Oregon, December 22 to 23, 2021

Area Forecast Discussion National Weather Service Pendleton OR 932
AM PST Wed Dec 22 2021

Updated aviation discussion

.MORNING UPDATE...Forecast is largely on track this morning as
snowfall begins in the WA Cascades. Temperatures have looked mostly
fair compared to forecast minimums from the morning, leading to good
confidence in today's warm up as southwesterly flow occurs due to
the approaching trough and the sun pokes through the clouds here at
the office in Pendleton. Will also continue to monitor trends in the
Blues for any additional messaging necessary in regards to the
expected advisory level amounts of snowfall in conjunction with our
product in the WA Blues. Goatley/87

AVIATION...18Z TAFS...Most TAFS are VFR this morning except YKM that
remains IFR to LIFR due to low clouds. Expect YKM cigs to lift to
IFR this afternoon. Another surge of moisture is expected across the
forecast area this evening and early night which will produce some
MVFR conditions as cigs lower. YKM remains the one location where
some initial light snow could also lower vsby but otherwise mainly
expecting rain at TAF sites. Conditions improve overnight and
Thursday morning as system departs but leaves some lingering
showers.

&&

.PREV DISCUSSION... /issued 341 AM PST Wed Dec 22 2021/

Updated for Aviation...

SHORT TERM...Wednesday through Friday...Forecast area remains under
an area of enhanced SW flow as a result of a low pressure system
dropping in from the Gulf of Alaska and absorbing a cutoff low
centered off the California coast. As a result, orographic snow
showers will be possible across our mountain zones through much of
the day Wednesday. The overall synoptic pattern favors persistent
chances for mountain snow through much of the short term and even
beyond, but looks like the period of most enhanced moisture
transport will be Wednesday afternoon through the overnight hours
into Thursday morning. Winter storm warnings remain in effect for
our Cascade zones, with a winter weather advisory issued for the
northern Blues of WA, as model QPFs have favored a period of
moderate to heavy snowfall more towards late Wednesday night into
early Thursday morning. Right now, looking like OR Cascades will
have a better shot at meeting warning criteria for snowfall compared
to the WA Cascades. Not expecting as much impactful snow for the
major mountain passes around the Blues, but those traveling for the
holidays should prepare for difficult driving conditions nonetheless.

By Thursday into Friday, another trough dropping in from the Gulf of
Alaska will work to amplify the synoptic pattern and create
substantial ridging over Alaska and broad troughing over the PacNW.
The net effect will be a trend towards cold temperatures heading
into Christmas weekend. Thursday looks to be a bit breezy to windy,
especially around the foothills of the Blues and Grande Ronde
Valley, as the front associated with Wednesday's system passes
through, before broad troughing works to cycle several weak systems
into our area. Precip will be limited mainly to our mountain zones,
while temperatures begin to slide towards unseasonably cold beyond
the short term. Snow levels will be below 1500-2000 ft across the
entire forecast area by the end of the week. While only a couple
stray showers are possible across the lower elevations and valleys,
travel may become challenging across mountain passes for the
holidays as the overall pattern supports persistent snow chances for
both the Cascades and Blues. 74

LONG TERM...Saturday through Wednesday...Several troughs will move
onshore during the extended period.  The first will be during the
day Saturday. The next one will follow on its heels on Sunday.  A
stronger, deeper trough will then move through on Monday.  All of
these systems will bring mainly light snow to the mountains, with
the one on Sunday and Monday possibly bringing some light snow to
the portions of the lower elevations as well. Monday may also be a
bit breezy as the trough moves through.

However, the big story will be the much colder temperatures, by the
end of the period.  Highs Christmas Day will mainly be in the 30s
and 40s across the Columbia Basin and Blue Mountain Foothills.
Temperatures will drop each day and by Monday Highs will struggle to
reach freezing in most areas and by Tuesday most locations will be
in the low to mid 20s, with central Oregon, possibly making it to 30
degrees.  It gets even colder through midweek.  Overnight lows start
in the 20s and 30s, and bottom out in the single digits.

The ECMWF Extreme Forecast Index (EFI) approaches 80-90% for both
high temperature mainly in central Oregon from on Monday, and is 70-
80%  Sunday through Wednesday.  All of the ensemble clusters are in
good agreement that there will be well below normal 500 mb heights
over the west from Saturday through Monday.  Beyond Monday, the
height patterns and anomalies begin to differ as there is some
disagreement in the exact location of the axis lowest heights.
However, all guidance does agree that the midweek period will be
very cold.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  45  36  45  31 /  10  40  50  20
ALW  43  38  45  33 /  10  50  60  30
PSC  44  37  46  35 /  20  30  10  10
YKM  40  30  43  26 /  30  60  40  30
HRI  44  36  47  33 /  10  40  30  10
ELN  36  28  37  23 /  30  60  40  40
RDM  47  33  42  27 /  20  60  60  30
LGD  38  33  39  26 /  10  60  70  50
GCD  42  36  41  26 /  10  60  60  40
DLS  42  36  43  33 /  70  90  90  70

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 4 PM PST Thursday for ORZ509.

WA...Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Thursday for WAZ030.

     Winter Storm Warning until 10 AM PST Thursday for WAZ520.

&&

SHORT TERM...87
LONG TERM....77
AVIATION...91