Heavy Snow in Washington, December 22 to 23, 2018

Area Forecast Discussion
National Weather Service Pendleton OR
100 PM PST Sat Dec 22 2018

.SHORT TERM...Tonight through Monday night...A fairly active weather
pattern is expected with a number of winter weather concerns. First
a Pacific weather system will move across the region and forecast
area tonight and Sunday. This will bring mountain snow and lower
valley rain to the forecast area tonight and Sunday. There are a
number of winter weather advisories in effect during this period
with varying elevation based advisories and beginning/ending times
of the snow. First a winter weather advisory is in effect for the
Washington Cascades and adjacent Yakima and Kittitas Valleys
beginning at 4 PM this afternoon continuing until 4 PM Sunday for
the Cascade east slopes...and until 10 AM Sunday for the
Yakima/Kittitas Valleys (when snow levels rise too much to continue
the advisory in the Yakima/Kittitas Valleys). Elsewhere a winter
weather advisory is in effect for the Blue Mountains and the John
Day Highlands beginning at 7 PM this evening until 4 PM Sunday.
Finally a winter weather advisory beginning at 10 PM tonight until 4
PM Sunday for Wallowa County and the Grande Ronde Valley. It will be
windy in the Grande Ronde Valley, but winds will not be strong
enough to warrant a wind advisory (or Blizzard warning) for the GR
Valley, but enough for blowing and drifting snow...especially
tonight. Have added areas of blowing and drifting snow in the GR
Valley. This weather system will move out Sunday evening. Snow
levels are expected to rise to quickly for the Oregon Cascade east
slopes so have not issued any advisories for that zone (ORZ509).
However, there could still be several inches of snow in ORZ509
before snow levels rise there tonight.

Then another weather system will move into the Pacific Northwest by
Monday and continuing Monday night. This system will mainly impact
mainly Oregon as it will take a more southern track, leaving
Washington zones mostly dry. Will need to consider highlights for
central Oregon and the John Day highlands and possibly the southern
Blue Mountain zone for Monday and Monday night. Will wait for the
current advisories to either expire or be canceled (or near
expiration/cancellation) before issuing any kind of highlights for
these zones. The models seem to be having a little difficulty with
agreement with each other for the Monday and Monday night system
which is another reason that have decided to wait on issuing any
highlights for the second system. Temperatures will be near normal,
but the models have been trending a little colder than before.
Unsettled conditions with frequent weather systems will continue
into the extended forecast period. 88

.LONG TERM...Christmas Day through Saturday...Models in general
agreement in a cooler, more seasonable pattern with systems moving
through every 24 to 36 hours. While the pattern remains active the
best fetch of Pacific appears to miss us either to the north or
south with each passing system which should serve to keep any snow
accumulations in the manageable category. A departing system
Christmas morning will yield some residual snow showers mostly in
the Blues/Wallowas and foothills but with rather limited QPF would
not expect much more than a few inches in the mountains and less
than an inch (mostly on grassy surfaces) in the foothills with
surface temperatures at or just above freezing. The next system
arrives Wednesday and with snow levels initially near the surface
for all locations and slightly better moisture support would expect
accumulations in the 3 to 8 inch range in the mountains with a
dusting up to perhaps 3 inches in the lower elevations for locations
least shadowed under westerly flow. Models begin to differ
substantially in timing of subsequent systems into the weekend and
have nudged the model blend toward active pattern climatology with
mostly dry conditions for the lower elevations and chance PoPs in
the mountains. Peck

&&

.AVIATION...00Z TAFs...Precipitation associated with an approaching
upper level low will spread over the region this evening with
significant precipitation shadowing/downsloping limiting
accumulations expected at most of the terminals. Snow levels will
initially support snow at YKM and possibly PSC with a quick
switchover to rain at PSC. Could potentially see a brief switchover
to freezing rain at YKM before precipitation tapers off Sunday
morning as warm air moves in aloft. Winds remain light except for
locally enhanced downslope winds off W-E oriented terrain near
BDN/PDT/ALW. Peck

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  36  46  36  44 /  80  60  40  40
ALW  36  46  38  44 /  80  70  50  40
PSC  35  45  36  43 /  60  40  30  30
YKM  30  38  31  38 /  80  70  40  40
HRI  34  45  36  44 /  70  50  30  40
ELN  29  36  31  35 /  90  70  40  40
RDM  34  44  33  42 /  60  60  40  70
LGD  31  36  32  38 /  90  80  60  50
GCD  33  38  33  38 /  80  70  60  70
DLS  34  42  36  42 /  90  70  60  60

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 7 PM this evening to 4 PM PST
     Sunday for ORZ502-503-506.

     Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Sunday for ORZ049-050.

WA...Winter Weather Advisory from 7 PM this evening to 4 PM PST
     Sunday for WAZ030.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Sunday for WAZ520.

     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Sunday for WAZ026-027-521.

&&

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