Heavy Snow in Oregon, December 24 to 27, 2021

Area Forecast Discussion
National Weather Service Pendleton OR
253 AM PST Fri Dec 24 2021

.SHORT TERM...Today through Sunday...Through the short term
period, the PacNW will be under a cyclonic flow aloft with a
series of embedded waves. Each passing wave will bring a chance of
showers. The challenge for this holiday weekend is pinpointing the
position of each wave and where the best potential for showers
will be located. The NAM12 was disregarded for the short term
period, as it appears too low on the QPF.

A closed upper low near the Olympic Peninsula will fill this
afternoon and will be north of Astoria this evening. An open
trough will sit over WA/OR tonight. An embedded wave along the
bottom of the trough will bring heavy snow to the Oregon Cascades
where a Winter Storm Warning is in effect through 6 AM on
Christmas morning. It's possible the warning may need to be
extended, but the timing looks good based on the high probabilities
of > 1 inch/hour snowfall rates tonight that decrease
significantly on Christmas Day, based on the HREF. The Winter
Storm Warning for the WA Cascade east slopes will be downgraded to
an advisory in effect until 3 PM this afternoon. As the low
tracks southward, the threat of heavy snow ends, especially north
of Mt. Adams.

On Christmas day and night, the forecast area will be under the
open trough and a west-southwest flow aloft. The WA/OR Cascades
will have numerous snow showers, and central OR and the eastern
mountains/valleys will have scattered light snow showers. With the
exception of the Cascade crest, snowfall amounts should only be a
trace to 2 inches on Christmas Day. On Christmas night, an
additional 1-3 inches are forecast but heavy snow is possible
along the Central OR Cascades including Mt. Bachelor. Although
snow levels will be around 500-1000 feet, the Columbia Basin and
surrounding valleys will see little to no accumulation on
Christmas.

Colder air will spread from the north on Sunday as the trough
deepens and a northerly surface gradient tightens over the
northern half of WA. Lapse rates will steepen and showers will
become numerous, mainly over the mountains and valleys as well as
the Columbia Deschutes Plateau. Isolated to scattered snow
showers are possible in the Lower Columbia Basin and surrounding
valleys with snowfall amounts less than 0.5 inch. For other
areas, 1-3 inches are forecast, and this includes the Bend and
Redmond areas. The east slopes of the OR Cascades will likely have
heavy snow on Sunday. To avoid confusion with the current Winter
Storm Warning, will hold off on winter highlights for the OR
Cascades but one may be needed in the next day or two. Wister/85

.LONG TERM...Sunday night through Thursday. Overall weather
pattern will be changing from the westerly flow/broad trough to the
strong north flow ushering in drier and very cold conditions. The
upper low will be dropping south over Washington Sunday night with
cyclonic flow and continued mountain snow for southeast Washington
and the eastern Oregon. On Monday, the trough shifts east with
northerly flow and cold air pushing south across Washington. The
coldest air stays to the north, but most of the area should see
temperatures 15 to 20 degrees below normal Tuesday through Thursday.
GFS/GEFS ensembles are the strongest with the cold air indicating
the Columbia Basin could see temperatures 20 to 30 degrees below
normal persisting through Thursday, while EC ensembles show the cold
air starting to moderate on Thursday.  It will be mostly dry for the
lower elevation during this cold period, but the mountains could see
some snow from time to time. Winds are also expected to be on the
lighter side during the coldest period, but will need to be
monitored for any wind chill concerns. 93

&&

.AVIATION...
12z Tafs...VFR conditions for most of the period as west to
southwest flow continues over area with mainly mountain snow
showers.  Look for shower activity to increase in the afternoon with
the best chance for rain/snow at RDM/BDN/DLS and showers nearby at
other sites. CIGS will be sct-bkn CIGS 030-060kft and some MVFR
conditions are possible the heaviest showers. Showers will decrease
with mainly dry conditions after 06z. BDN/RDM will see winds
west/southwest wind  10-20kts and gusty with winds under 10-15kts at
other locations. Winds will become under 10 kts after 04z.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  43  29  41  27 /  30  20  10  30
ALW  43  32  42  29 /  30  30  20  40
PSC  46  33  44  30 /  20   0  10  10
YKM  40  24  38  20 /  40  10  30  30
HRI  46  31  44  30 /  20  10  10  20
ELN  35  20  32  17 /  40  20  30  30
RDM  40  23  38  21 /  60  40  30  50
LGD  35  22  34  20 /  40  40  20  50
GCD  37  24  35  21 /  40  30  20  30
DLS  41  29  39  28 /  70  60  70  90

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 6 AM this morning to 6 AM PST Saturday
     for ORZ509.

WA...Winter Weather Advisory until 3 PM PST this afternoon for WAZ520.

     Winter Storm Warning until 4 AM PST early this morning for
     WAZ520.

&&

SHORT TERM...85
LONG TERM....93
AVIATION...93