Winter Storm Watch 5, NWS Pendleton, OR

Area Forecast Discussion
National Weather Service Pendleton OR
244 AM PST Sat Dec 18 2021

.SHORT TERM...Today through Monday...A complex pattern the next
several days poses forecast challenges, mainly fine tuning the
amount of precipitation and snow levels over individual forecast
zones. Several winter highlights will be added to the morning
package based on the high QPF. For now, the focus is on a warm
front over eastern WA and northeast OR that has brought
widespread light snow. Snow levels remain low, and almost all
automated weather stations are in the 20s to lower 30s. Current
forecast has a slight chance of sleet and freezing rain mixed with
snow and will keep but most ice accumulations will be a trace. The
eastern slopes of the WA Cascades may have ice accumulations up to
0.1 inch in some areas. The warm front will lift northward this
morning and will be along the WA and northern OR Cascades and
north of the Lower Columbia Basin. Therefore, precipitation will
end for most of the forecast area later this morning. The Kittitas
and Yakima Valleys and the northern Blue Mountains will have a
chance of precipitation. The northern Blue Mountains north of
Meacham may also observe a wintry mix of snow and freezing rain
later today.

Wind Advisories are in effect through Sunday morning for the
Grande Ronde Valley and the foothills of the Blues Mountains east
of Walla Walla, Milton-Freewater, and Pendleton. It is breezy now
but will increase as the day progresses with gusts 45-55 mph.
Main concern will be near Ladd and Pyles Canyons and near Cabbage
Hill. With increasing snow levels and more compact snow, the
potential for blizzard conditions from Cabbage Hill to La Grande
is low but drifting snow is likely and will include in the
forecast.

The front will sag southward tonight and remain stationary just
along the WA/OR border through Monday morning. There will be a
dramatic difference in snow levels in our northern zones to our
southern zones. While south central WA will observe lowering snow
levels down to around 1500-2500 feet, and the Blue Mountains east
to the ID border will have snow levels around 3500 feet, but snow
levels will be as high as 6000 feet in central OR. The position of
the front plays a major role on snow levels and precipitation
type. The front will tap in Pacific moisture and set up an
atmospheric river along the WA/OR border tonight through Sunday.
Boundary layer easterly winds will provide significant upslope
along the eastern WA Cascades south of the Yakima County Border as
well as the Simcoe Highlands. There will also be plentiful
moisture over the northern Blues, especially in the Ski Bluewood
area. Snowfall amounts in the forecast tonight through Monday
meet warning criteria in these areas. Will issue a Winter Storm
Watch as confidence is high but there is still some uncertainty on
snowfall accumulations. Forecasters will also be focusing on the
eastern Columbia River Gorge for elevations in the higher hills
west of The Dalles where current forecast accumulations Sunday
night are around 3 inches, meeting advisory criteria. Will hold
off as confidence for heavy snowfall in the higher elevations of
the Eastern Columbia River Gorge is about 50 percent.

The front will lift northward Monday as the upper flow increases
from the south with precipitation gradually tapering off from the
south.  Wister/85

.LONG TERM...Monday night through Saturday...What's left of a
frontal boundary on Monday evening will begin to weaken with
decreasing chances for rain and snow. For Tuesday through Wednesday
an upper low will be off the ORE/NRN CA coast with SW flow aloft
over the area. It does not appear that there will be any major
shortwave troughs impacting the region though there will be a chance
for mountain snow and a rain/snow mix in the valleys each day. For
Thursday the ensembles differ on the longwave pattern with the ECMWF
swinging the upper low inland while the GFS ensemble mean keeps the
low offshore. By Friday and especially Saturday the ensembles are in
better agreement on having an upper trough over the area though the
details in the deterministic models are a bit sketchy still. It
appears that there could be a decent wintertime precipitation event
during this period though confidence in the details is low. The
Extreme Forecast index (EFI) is not suggesting the potential for an
extreme precipitation event compared to climatology through the
extended period. It does appear that at least initially southerly
winds will develop over the area Friday into Saturday which should
help boost snow levels enough to keep the precipitation rain at the
lowest elevations. 78

&&

.AVIATION...12Z TAFS...VFR conditions are expected at KRDM and KBDN.
Elsewhere mostly VFR conditions are forecast today though periods of
MVFR/IFR CIGS and VSBYS could occur in -RASN mainly after 00Z. Winds
will increase along the Blue Mountain Foothills through the day with
gusts to 45 kt. Not expecting those strength of winds at KALW and
KPDT...but brief bursts of strong winds will be possible at the two
terminal sites. Winds could also gust to 15-20 kt at KBDN and RDM
otherwise winds will be 10 kt or less. 78

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  50  36  40  30 /  30  70  90  80
ALW  50  37  41  32 /  50  80  90  80
PSC  46  36  39  31 /  60  70  70  80
YKM  41  30  37  27 /  60  50  40  60
HRI  49  36  41  30 /  30  70  80  80
ELN  38  27  36  25 /  60  50  30  40
RDM  51  37  49  32 /  10  40  50  50
LGD  36  32  38  32 /  20  50  80  70
GCD  43  34  44  32 /  10  30  40  40
DLS  48  37  42  34 /  50  80  90 100

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 7 AM PST Sunday for ORZ049-507.

     Winter Storm Watch from this evening through Monday afternoon
     for ORZ502.

WA...Wind Advisory until 7 AM PST Sunday for WAZ029.

     Winter Storm Watch from this evening through Monday afternoon
     for WAZ030.

     Winter Storm Warning until 7 PM PST Monday for WAZ520.

     Winter Storm Watch from late tonight through Monday evening for
     WAZ521.

&&

SHORT TERM...85
LONG TERM....78
AVIATION...78