Heavy Snow in Oregon, December 30 to 31, 2021

Area Forecast Discussion
National Weather Service Pendleton OR
928 PM PST Wed Dec 29 2021

.UPDATE...
Latest radar imagery is clear of any echoes across our area at
this time. However it will be short lived as the next system
approaches from the northwest and snow begins to fall by morning,
mainly over the Cascades and Blues. However, all of the guidance
suggests there will be a swath of moisture over the Blue Mountain
Foothills and a portion of the nearby Columbia Basin by morning.

However, the guidance, especially the high resolution guidance
also indicates a fairly large dry area covering most of the Basin
and even the Yakima Valley through mid to late afternoon. There is
good agreement here. After this time, the guidance begins to
diverge on what happens during the late afternoon and into the
evening hours. This is when there could also be the steadiest
snow.

In the real near term, adjusted some temps as in some areas they
are not as low as originally forecast and are expected to remain
steady and gradually rise overnight.

&&

.AVIATION...
06Z TAFS...With the exception of BDN and RDM who have had some IFR
CIGS, everyone has been VFR through the evening hours. Expect VFR
to continue until early morning when conditions will drop to MVFR
as -SN begins. Not confident on timing at all locations. Also,
especially at ALW, could see IFR if there is moderate or heavy
snow. If any other locations see moderate or heavy snow, could see
local lower conditions as well.

Once conditions decrease, expect them to stay down for the balance
of the day, though there could be improvements if snow stops then
decreases as it starts again.

Winds could gust 20 to 25 kts from the west at BDN, RDM, YKM and
ALW in the afternoon.

&&

.PREV DISCUSSION... /issued 254 PM PST Wed Dec 29 2021/

SHORT TERM...Tonight through Friday night...The last of a few
flurries over the Blue Mountain Foothills of OR are now
dissipating. It has been snowing non-stop at the Pendleton WFO for
over 30 hours until now.

Focus now changes to the next weather system, which will be a
stronger storm system with more moisture from the Pacific. It is
expected to arrive to the Cascades and the Blue Mountains at about
the same time (09Z - 12Z tonight) and increase in coverage and
intensity through the day Thursday, into the evening. This storm
has a good potential to produce heavy snow accumulations for the
northern Blue Mountains and all of the Foothills of the Blue
Mountains. Latest HREF indicates heavy snow totals by Friday
morning to be heavy in the warning/advisory areas. Have issued
Winter Storm Warnings for these areas beginning Thursday morning
and continuing through Thursday night. The snow will be
accompanied by another surge of very cold modified arctic air to
reinforce the current very cold temperatures already over the
forecast area. Night time lows are expected to be in the single
digits to the mid teens tonight across the CWA. However, they are
expected to rebound to the mid 20s to around 30 in the lower
elevations and the teens and 20s in the mountains on Thursday.
Temperatures will also not be as cold Thursday night due to cloud
coverage and still areas of snow. Winter Weather Advisories have
also been issued for both the OR and WA Cascade east slopes, with
the greatest amounts of snow near the crest. The Cascades will not
see Winter Storm Criteria from this system due to the track and
origin of the storm system that is expected to move into the
region. However, conditions could change and the Cascades will
need to be monitored in case an upgrade to a warning is needed.

There will also be breezy winds, mainly over exposed terrain and
ridge tops, which will create poor visibilities for travelers.
Also, the snow will be very dry and powdery, which will blow
around very easily due to passing vehicles. Travelers should also
be prepared for this as well. However, overall winds will be light
through the short term period.

This next storm will move out of the forecast area by mid to late
morning Friday, with just some lingering snow showers during the
day on Friday. Friday night, a deep northerly flow will be over
the region. This combined with a decent snow cover on the ground,
and possible clearing, temperatures will be very cold for Friday
night's lows. Expect Friday night's lows to be mostly in the
single digits to lower teens, even in the Lower Columbia Basin.
There could be local temperatures below zero, mainly in high
mountain valleys/meadows. 88

LONG TERM...Saturday through Wednesday...The area will be under
an upper-level ridge with northwesterly flow aloft to begin the
period on Saturday. This will bring a slight chance of snow to the
mountains, though lower elevations will remain dry. A deep upper
low is forecast to drop down from the Gulf of Alaska along the
coast of British Columbia by Sunday morning. This will turn the
flow aloft to southwesterly and increase precipitation chances for
the Cascades. Some disagreement between the EPS and GEFS
regarding the timing and amplitude of the longwave pattern does
show up in clusters.

Monday looks to be potentially impactful regarding winds as
surface pressure gradients strengthen due to very cold air across
far eastern Oregon and comparably mild conditions for the Basin.
Southerly winds are expected to develop in response across central
Oregon, the Grande Ronde Valley, and the Foothills of the Blues.
Currently, deterministic and ensemble guidance are both
advertising the potential for strong winds and wind gusts, as is
the ECMWF EFI, though the exact timing and strength will be
dependent on the evolution of the synoptic pattern.

Substantial uncertainty in the longwave pattern develops by
Tuesday and Wednesday. Currently, the EPS is favoring a more zonal
upper-level flow while the GEFS is favoring slightly lower
heights and more southwesterly flow aloft. Either way, the multi-
model ensemble and the NBM both signal precipitation chances will
continue Tuesday through Wednesday for the majority of our
forecast area with the majority of it falling as snow for low
elevations and the mountains. Plunkett/86

AVIATION...00Z TAFS...A mix of IFR, MVFR, and VFR conditions are
currently being observed at TAF sites this afternoon due to some
lingering low ceilings, low visibilities, and light snow across
central Oregon and the foothills of the Blues. Winds of 5 to 10
kts are expected to continue to weaken through the evening as
surface pressure gradients slacken. Another weather system will
arrive tomorrow morning and afternoon bringing more snow, breezy
winds, and widespread MVFR to IFR conditions. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT   5  29  13  20 /  40  80  60  10
ALW   9  31  15  22 /  40  90  50  10
PSC  15  28  17  24 /  30  50  20   0
YKM  15  24  13  25 /   0  50  20   0
HRI   8  30  16  23 /  40  60  40  10
ELN   8  21  11  23 /  10  60  20   0
RDM   7  34  19  32 /  10  30  30  20
LGD   5  27  16  23 /  30  90  70  20
GCD   8  30  19  28 /  10  60  60  30
DLS  17  35  22  30 /   0  80  70  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 7 AM Thursday to 4 AM PST Friday
     for ORZ509.

     Winter Storm Warning from 7 AM Thursday to 4 AM PST Friday for
     ORZ502-503-507-508.

WA...Winter Storm Warning from 7 AM Thursday to 4 AM PST Friday for
     WAZ029-030.

     Winter Weather Advisory from 7 AM to 10 PM PST Thursday for
     WAZ520.

&&

SHORT TERM...77
LONG TERM....86
AVIATION...77