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FXUS66 KPDT 082254 CCA

Area Forecast Discussion
National Weather Service Pendleton OR
245 PM PST Thu Dec 8 2016

.NEAR TERM...This afternoon through Friday...Bands of moderate
to locally heavy snow continue to move north across Northern and
Central Oregon early this afternoon. The most organized band extends
from extreme SW coastal WA east across the Columbia Gorge and Basin.
Several reports of 3-5 inches of accumulation have been reported
since sunrise. This band...along a warm frontal boundary...will
continue to migrate north across the remainder of NE OR and SE WA
late this afternoon/evening. Additional activity can be expected to
the south of the band across Central OR. Several inches of
additional accumulation will be likely...thus all advisories and
warnings will remain in effect through most of this evening...with
warnings for Central/North-central Oregon ending late this evening
as the bulk of the heaviest precip moves to the north. By later
tonight...warm air advection will begin to work in from the south.
Snow levels will rise to 3000-5000 feet across most of Central
Oregon. With temperatures still near/below freezing...expect the
precip to change over from snow to a wintry mix of sleet and
freezing rain overnight.
With the precip coverage decreasing somewhat...will not extend the
Winter Storm Warning. Lows tngt will range from the teens and 20s
across SE WA and NE just below freezing across portions of
Central Oregon.

Friday...A slight break from widespread precip can be expected
Friday morning in between systems...with residual upslope precip
across the Cascade East Slopes and the John Day Highlands. As
warming continues aloft...some of the precip across Central Oregon
will be sleet and/or freezing rain. The next system will enter the
region from the west on Friday afternoon. Given the trajectory of
this system...deep layer moisture will not be quite as significant
as the previous system but still expect likely/categorical PoPs in
the afternoon. With enough warming at the surface occurring across
Central Oregon...the precip there will change over to mainly rain
for the valleys. Other areas will retain the colder air with another
round of snow. With upslope flow developing across the Blue
Mountains...will have to monitor the current advisory for a possible
upgrade to warning...but for now will keep the advisory as amounts
look to remain under warning criteria.
High temperatures will range from the mid 20s to lower 30s for SE WA
and NE OR...with mid 30s to lower 40s further south.

.SHORT TERM...Friday Night through Sunday Night. Although there are
no significant storm systems approaching this weekend...a continuous
westerly flow and embedded shortwaves will bring periods of snow and
low elevation rain/snow mix.  The Winter Storm Warning for heavy
snow for the East Slopes of the WA Cascades through Saturday
afternoon looks reasonable based on the slop over and prolonged
period of snow.  The Winter Weather Advisory for snow in the
northern Blue Mountains through Saturday morning also looks on
track.  Rain shadowing will take place for most of the Columbia
Basin and surrounding valleys down to the Columbia Deschutes Plateau
Friday night through Saturday evening where any showers will be
isolated to scattered.  Winds aloft and a relatively tight pressure
gradient should provide breezy conditions and mixing to help scour
out some of the cold air in the Columbia Basin and bring
temperatures in the mid to upper 30s this weekend...but cold air
damming will likely continue along the East Slopes of the WA
Cascades. No wind highlights are expected...but there could be gusts
to 30-35 mph in the Simcoe Highlands Saturday night. A lee-side
trough east of the Oregon Cascades could also bring breezy/windy
conditions in Central Oregon on Saturday.  A cold front Sunday night
will lower snow levels down to the Basin floor with several inches
of snow in the eastern mountains.  Wister

.LONG TERM...Monday through Thursday night...A weak clipper like
system is expected to move through the area on Monday. This will
bring a high chance of snow showers to the mountains with a slight
chance or low end chance for snow and rain mixed showers in the
lower elevations. Snow levels will be below 1000 feet for northern
Oregon and southern Washington...but closer to 2000 feet in Central
Oregon. There are some model differences with this system as the
latest 12z GFS would also bring an arctic front through the CWA from
north to south Monday afternoon, while the Euro tries to filter
colder air into the region more gradually through the week. Behind
this system Tuesday and Tuesday night looks mainly dry and
colder...except for a chance of lingering mountain snow showers.
Another wave of low pressure is expected to track south of the
forecast area Wednesday afternoon through Thursday...both the GFS
and ECMWF were in fairly good agreement on this system. However the
models have trended much further south with this system compared to
yesterday's the highest PoPs are now over the southern
third to half of the forecast area...mainly south of the Washington
border. In this area there are high chance to low end likely PoPs
for snow Wednesday night and Thursday. In Washington the forecast
was updated to have mainly only slight chance or lower PoPs during
this time. Will need to watch the trend of the models in the coming
days to see if this system ends up any further south. If the system
holds its course it could bring any round of accumulating snow to
parts of the area. As mentioned above the GFS and ECMWF are much
different with temperatures though the week (and therefore snow
levels too). Decided to lean toward the colder GFS solution after
collaboration with WPC and surrounding offices. Also...the 12z GFS
Ensemble Mean is in excellent agreement that very cold air will
filter into our area by Tuesday/Wednesday...the ensemble mean was
actually showing surface temperatures of 20-30 degrees below average
over our area during this time. Did not go that extreme for the
forecast...but certainly trended colder based on this. 77


.AVIATION...00z TAFS...Snow is overspreading much of the area this
afternoon and evening. This snow will continue through the evening
hours and could mix with sleet in Central Oregon...including KBDN
and KRDM. This is bringing IFR/LIFR CIGS and VIS as the snow moves
through. The steady snow should taper off at most sites after 09/06-
09z. Through the overnight period there should be light showers of
mixed precipitation...with IFR or MVFR conditions expected. Another
round of light snow is expected across northern OR and southern WA
Friday afternoon...including KDLS...KPDT...KPSC...KALW and KYKM.
This could bring more MVFR/IFR conditions. Winds will be light under
15 kts through the period. 77


PDT  20  30  25  36 /  80  70  80  30 
ALW  24  34  29  37 /  80  60  80  40 
PSC  23  30  24  36 /  80  70  60  20 
YKM  19  29  20  35 /  80  70  60  30 
HRI  22  30  25  36 /  80  70  70  20 
ELN  20  27  20  33 /  90  70  70  40 
RDM  20  41  24  37 /  80  60  80  30 
LGD  22  38  29  35 /  90  80  90  40 
GCD  29  41  30  37 /  70  80  80  30 
DLS  25  34  29  42 / 100  80  80  50 


OR...Winter Storm Warning until 10 PM PST this evening for ORZ041-

     Winter Weather Advisory until 4 AM PST Saturday for ORZ502-503.

     Winter Weather Advisory until 4 AM PST Friday for ORZ049-050.

     Winter Weather Advisory until 1 AM PST Friday for ORZ044-507.

     Winter Weather Advisory until 10 PM PST this evening for ORZ505-

     Winter Storm Warning until 1 AM PST Friday for ORZ508.

WA...Winter Storm Warning until 10 PM PST this evening for WAZ024.

     Winter Weather Advisory until 4 AM PST Saturday for WAZ030.

     Winter Weather Advisory until 1 AM PST Friday for WAZ026>029.

     Winter Storm Warning until 3 PM PST Saturday for WAZ520-521.


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