Area forecast discussion 
National Weather Service Pendleton or 
940 PM PST Thu Dec 8 2016 


Short term...warm front continues to slowly move northward across 
the region this evening. Radar indicates the front extending from 
the Washington Cascades east across the Columbia Basin with steady 
light to moderate snow. Expect this feature will move out of the 
forecast area by late evening. In the wake of this warm front 
across Oregon precipitation in mostly scattered snow showers. 
Expect this will continue overnight with mostly cloudy skies. 
00z models show another round of snow developing later Friday with 
an upper low approaching the coast and weak warm advection over 
the region. Warmer air aloft will likely result in mixed 
precipitation for some locations especially in Oregon. The active 
weather pattern continues into the weekend. 94 


Aviation...06z tafs. Steady snow moving off to the north at 06z. 
Scattered snow showers for the remainder of the night. Ceilings bkn- 
ovc 020-050. Some taf sites will have restrictions due to fog 
overnight with 3-6sm br. Another round of steady snow moves through 
the area later Friday with possible mixed precipitation at bdn rdm. 
94 


&& 


Previous discussion... /issued 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 Washington 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 southeast Washington 
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 southeast Washington and NE or...to 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 southeast Washington 
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 Washington 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 Washington 
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 County Warning Area 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 European model (ecmwf) were in fairly good agreement on this system. However the 
models have trended much further south with this system compared to 
yesterday's runs...so 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 European model (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 


&& 


Preliminary point temps/pops... 
PDT 17 28 25 36 / 100 70 80 30 
alw 18 32 29 37 / 100 60 80 40 
psc 21 29 24 36 / 100 70 60 20 
ykm 18 27 20 35 / 100 70 60 30 
hri 21 29 25 36 / 100 70 70 20 
eln 18 25 20 33 / 100 70 70 40 
rdm 17 35 24 37 / 80 60 80 30 
lgd 21 34 29 35 / 90 80 90 40 
gcd 22 35 30 37 / 80 80 80 30 
dls 24 33 29 42 / 100 80 80 50 


&& 


PDT watches/warnings/advisories... 
or...Winter Storm Warning until 10 PM PST this evening for orz041- 
509>511. 


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- 
506. 


Winter Storm Warning until 1 am PST Friday for orz508. 


Washington...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. 


&& 


$$ 


94 



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