Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 142343 AAA

Area Forecast Discussion...Update
National Weather Service Pendleton OR
343 PM PST Thu Dec 14 2017

Updated Aviation Discussion

.SHORT TERM...Tonight through Saturday...An upper level ridge remains
centered along the coast today. The ridge will slowly move east
tonight, but stratus and freezing fog will persist through
tomorrow morning. This will keep the pool of cold temperatures in
the Columbia Basin and adjacent areas until Friday afternoon.

By tomorrow morning the ridge will be flattened and the axis will
be in Idaho. An upper level trough will be offshore with an
associated front approaching the Cascades. Precipitation will
begin in the Washington Cascade tomorrow morning...then spread
across eastern WA and OR in the afternoon. The Cascades will have
a chance of rain and snow and some pockets of freezing rain.
There will be significant rain shadowing east of the Cascades and
in the Yakima and Kittitas Valleys. The Columbia Basin west of the
Tri-cities and central and north central Oregon will have limited
precipitation with a mix of rain and freezing rain possible.
Further east, a wintry mix of rain, freezing rain, and mountain
snow is expected through early evening before switching over to
rain and mountain snow. The best chance of freezing rain will be
in the Blue Mountain Foothills. The front is not strong and
precipitation amounts will be generally under a tenth of an inch
with up to two tenths of an inch in the mountains. Ice
accumulations will be a few hundredths of an inch but will be
sufficient to create difficult driving conditions. The cold air
in the lower basins and valleys will mix out late in the afternoon
or early evening and temperatures will rise to the mid 30s while
further south temperatures will be in the lower 40s.

The trough will move into ID early Saturday morning. Upslope flow
will keep a slight chance of snow showers in the mountains
Saturday. Temperatures will be in the upper 30s to mid 40s.

.LONG TERM...Saturday night through Tuesday...West-northwest to
northwest flow aloft looks keep conditions mostly cloudy Saturday
night into Sunday. Some light snow may be possible along the Cascade
east slopes, especially in Washington, Saturday night. Increasing
moisture within the flow could result in some precip spreading into
the Kittitas valley, the Blue mountains and Wallowa county Sunday. A
possible disturbance within the west-northwest flow could produce
precip over a broader portion of the area Sunday night through
Monday night (higher terrain would see best chances). Snow levels
look to be higher, generally above 2500 feet Sunday night then
rising further Monday. A Pacific system looks to approach the region
Tuesday, with a surface low possibly moving into southwest British
Columbia by the end of the day. This system would increase the south
flow across the region, further raising snow levels. Meanwhile would
see an increase in precip chances through the day, especially over
the our south-central Washington zones. Temps through this period
look to be near to slightly above seasonal. 90

Tuesday night through Thursday...Models in general agreement with a
trough of low pressure diving from the northwest across the Interior
Pacific Tuesday night, with the trough axis moving east of the
region by Wednesday night. Given the trajectory of the system from
the northwest, will maximize PoPs Tuesday night/Wednesday to likely
in the eastern mountains and the immediate Cascade east slopes, with
much less precipitation coverage in the Columbia Basin to Central
Oregon. Precipitation will decrease Wednesday night as the trough
exits to the east, and drier northerly flow takes hold. Snow levels
at 2500-4000 feet Tuesday night will fall to 1500-3000 feet
Wednesday, and to 500-1500 feet Wednesday night, with some light
snow as low as the Blue Mountain foothills Wednesday night before
ending. Seasonable temperatures will prevail Tuesday night/Wednesday
with slightly below normal temperatures Wednesday night. Models
still diverge late week regarding position of the trough to the
east. The latest GFS digs the trough across the Four Corners region,
resulting in colder NE flow across the local area. ECMWF/Canadian
dig the trough further to the east and not as potent, with more of a
N/NW flow regime across the interior Pacific Northwest. Will split
the difference for now, with dry but colder conditions for the
region, with highs on Thursday only in the lower/mid 30s for the
lower elevations, with lows in the teens to mid 20s Thursday night.
With the upper level ridge axis well offshore per the model
consensus, do not expect low clouds/fog.


.AVIATION...00Z TAFs...Strong low level inversions with cloud tops
between 2500 and 4500 feet will persist across the Interior
Northwest through noon Thursday. An approaching system will begin to
force the inversions to lower after midnight. Widespread low clouds
and fog will continue under the inversions. Expect MVFR and IFR
cigs/vis to persist at KDLS, KYKM, KPSC, KALW and KPDT. KRDM and
KBDN will see VLIFR or LIFR conditions redevelop this evening. A
cold front associated with the next system will move across the
region Thursday afternoon. This front will bring precip to most TAF
sites. Expect to mainly see rain, though temps at KALW and KPDT will
be between 30 and 32 degrees with freezing rain possible. This front
will also begin to break up the inversions and increase mixing.
Winds will remain less than 10 kts. 90


PDT  27  31  29  39 /   0  50  40  10
ALW  29  33  31  41 /   0  60  60  20
PSC  28  33  30  42 /   0  30  20  10
YKM  28  32  27  41 /  10  20  10  10
HRI  28  33  29  41 /   0  40  20  10
ELN  28  33  28  37 /  10  10  10  10
RDM  16  46  22  38 /   0  30  20  10
LGD  27  35  29  35 /   0  40  50  20
GCD  28  43  28  37 /   0  30  50  20
DLS  31  38  36  45 /   0  20  10  20


OR...Air Stagnation Advisory until 10 AM PST Friday for ORZ041-044-

     Freezing Rain Advisory from 10 AM to 7 PM PST Friday for ORZ507.

WA...Air Stagnation Advisory until 10 AM PST Friday for WAZ024-

     Freezing Rain Advisory from 10 AM to 7 PM PST Friday for WAZ029.



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