Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 060410
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
810 PM PST Mon Dec 5 2016

.UPDATE...A shortwave trough will move across the region tonight and
early Tuesday. This will deepen the cold air over the region and
also continue the snow showers into tomorrow. Several winter weather
advisories are currently in effect for mainly light snow showers. It
is possible that the snow will continue a bit longer than earlier
anticipated across the Blue Mountains Foothills and East Slopes of
the Washington Cascades where favorable upslope flow will develop on
Tuesday. Otherwise attention will focus to a winter storm for late
Wednesday night and Thursday as a warm front lifts north across the
region. QPF amounts dont look too high except near the Cascades but
the potential does exist for snow changing to a wintery mix which
could result in significant impacts Thursday into early Friday. 78

&&

.AVIATION...06Z TAFS...MVFR/IFR CIGS and VSBYS in -SN are likely
tonight and at least early Tuesday for KPDT...KALW...KRDM...KPSC and
KBDN. Other TAF sites may see MVFR cigs tonight before becoming VFR
Tuesday morning. Winds will be 10 kt or less.

&&

.PREV DISCUSSION... /issued 440 PM PST Mon Dec 5 2016/

SHORT TERM...Today through Wednesday Night...Multiple winter
weather advisories for snow are in effect through Tuesday morning.
This mainly includes the lower elevation zones that either will have
the first accumulative snow of the season or areas where 2-4 inches
meet advisory criteria.  The East Slopes of the WA Cascades and the
eastern mountains will also receive about 2-4 inches of snow...but
these are minor accumulations that do not meet advisor criteria.  A
shortwave trough with the center of the low over Vancouver Island is
dipping southward...and WA/OR will be under a cyclonic flow aloft
tonight through Tuesday morning. A weak unorganized cold front will
also be tracking southward tonight.  By sunrise Tuesday...the back
edge of the front will be over southern Oregon and a drier northerly
flow aloft will follow.  Precipitation will taper off as the day
progresses...and Tuesday night will be dry.  Tuesday night will be
COLD as the region will lie under a northerly flow aloft and partly
cloudy skies.  Lows Tuesday night will be in the single digits and
teens...and some of the colder valley spots such as Bear Valley will
fall below zero.  Wednesday will be cold and dry...and very few
locations will warm above freezing.  The deep cold air will remain
over the region so that the next system arriving from southwest
Oregon late Wednesday night will bring snow as it approaches the
forecast area.  The 12Z GFS is the fastest model at pushing moisture
northward...so I will not jump into this solution with likely PoPs
for anywhere other than the East Slopes of the Oregon Cascades and
Central Oregon. Snow may develop as far north as
Goldendale/Bickleton and as far east as Seneca/John Day late
Wednesday night. Wister

LONG TERM...Thursday through Friday...Strong low pressure will move
onshore coastal WA/BC by Thursday night. A warm frontal boundary
will move south to north across the local area Thursday. With good
agreement between the GFS/ECMWF with timing/locations of the deepest
moisture/forcing...have increases PoPs further on Thursday with
likely for all lower elevations...with categorical for the mtns.
Despite the warm frontal crossing...with arctic air entrenched
across the region...snow levels will be slow to rise...with precip
remaining snow for most if not all areas into Friday. Thermal
profiles hint at some possibility for a wintry mix Thursday night
into early Friday as warmer air aloft may ride over the still
chilled surface air across the Basin. With ensembles showing as much
as a 25% chance of this...will insert a slight chance of freezing
rain Thursday night for the Basin and Gorge. Will keep precip for
other areas in terms of rain or snow. Snowfall amounts will likely
be significant across the East Slopes of the Cascades with light
accumulating snows probable for the lower elevations. Precip will
linger on Friday...but with somewhat lower coverage as forcing will
not be quite as significant. Snow levels will rise to 1500-2500 feet
across the lower elevations of SE WA Friday...with snow levels 2500-
4500 feet across most of NE OR. temepratures mainly in the 20s to
lower 30s Thursday/Thursday night will warm to the 30s to lower 40s
on Friday.

Friday night through Monday...A very energetic and progressive flow
aloft will occur over the Pacific Northwest due to an upper level
120 mph jet stream around 30000 feet above MSL, oriented west-to-
east over the region. The jet stream will have an attendant frontal
boundary and a moisture plume, which remains over the region with
fast moving disturbances moving along the jet stream and the frontal
boundary. Snow levels will generally be 1000-2000 feet through the
period. As such the precipitation type in the Lower Columbia Basin
will be a mix of rain and snow during daytime periods with rain
changing to snow by late afternoon or early in the evening each day.
Since the flow aloft will be westerly, some pronounced precip
shadowing will be possible across the Lower Columbia Basin in
Washington. Saturday night looks to be dry in central Oregon, the
Kittitas and Yakima Valleys and the Lower Columbia Basin of Oregon
and Washington. The mountains will have a chance of snow throughout
the period.  Polan

AVIATION...00Z TAFS...VFR conditions will occur over the next 24
hours, except at KRDM, KBDN and KALW where MVFR conditions will
occur due to -SHSN reducing visibility. Late this afternoon, KPSC
will have -SHRA, which should change to -SHSN early this evening
with -SHSN ending by around 05Z at KPSC. A mid/upper level trough
will transport more clouds and moisture over the area from west to
east tonight through Tuesday morning resulting in -SHSN at KRDM,
KBDN, KPSC, KPDT and KALW. I kept VCSH until 02Z in the TAF for
KDLS. Precip is not expected at KYKM due to downslope winds
producing subsidence aloft in the lee of the Washington Cascades
resulting in dry conditions in the Yakima Valley.

Winds will be 5-10KT through tonight. Winds increase a tad Tuesday
morning to 5-12 KTs. However, KDLS will have winds either near CALM
over periods of several hours or variable winds to around 4KT.  Polan

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  26  31  15  26 /  60  60  10   0
ALW  28  33  20  27 /  60  60  10   0
PSC  28  34  17  28 /  70  50   0   0
YKM  22  35  11  30 /  40  20   0   0
HRI  28  33  15  27 /  60  40  10   0
ELN  21  30  12  25 /  40  20   0   0
RDM  21  30   3  28 /  60  70  10   0
LGD  22  29  11  26 /  70  50  20   0
GCD  17  29   8  28 /  60  60  10   0
DLS  29  38  20  31 /  40  30   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST Tuesday for ORZ044-505-
     507>511.

WA...Winter Weather Advisory until 10 AM PST Tuesday for WAZ028-029.

&&

$$

78



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