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Area forecast discussion...updated 
National Weather Service Pendleton or
410 PM PST Friday Dec 19 2014

Updated aviation discussion

Short term...tonight and Saturday...will see a brief break in
precipitation this evening, then a strong Pacific storm will move into the
region late tonight and Saturday. This system will cause significant
precipitation and increasing winds, especially during the day
Saturday. Snow levels across much of the region will be between 3500
and 4000 feet overnight. These snow levels will gradually rise
across the area Saturday as the precipitation and winds increase. Still
uncertain as to the amount of snow that will fall over the higher
elevations east of the Cascades. The main snow fall in these areas
will occur during the day Saturday, and will therefore hold off on
issuing advisories in these areas. Do have decent confidence that
significant snow will accumulate, especially above 2000 feet, in
western Kittitas County late tonight into Saturday. Thus will issue
a Winter Weather Advisory for snow in this area. Winds will increase
and become very windy over portions of the area Saturday afternoon
and will wind advisories for a number of our central Oregon and
Washington zones, as well as the Blue Mountain foothills. 90

Long term...Saturday night through Monday night...a very strong
upper level jet stream will bring above normal moisture and strong
dynamic lift to forecast area Saturday night through Sunday night.
The jet stream will usher in a warm front with rising snow levels
Saturday night and Sunday resulting in copious amounts of rain
falling on the snowpack in the mountains. Surface temperatures will
also remain above freezing in much of the mountains Saturday night
through Sunday night resulting in runoff from snowmelt and rainfall.
As such area rivers, streams and creeks will rise to or above
bankfull in some areas. At a minimum 1.00-1.50 inches of rain are
expected in the Blue Mountains and the Wallowa Mountains for the 36
hour period from Sundown on Saturday afternoon to sunup on Monday.
Lower elevations can expect to receive anywhere from 0.20 of an inch
to 1.00 inch of rain from Saturday night through Sunday night. Snow
levels rise to around 6000-8000 by daybreak on Sunday with the lower
snow levels in the Washington Cascades. Snow levels will remain high
on Sunday, around 6000 feet along the east slopes of the Washington
Cascades and rising to 7500-8500 feet in central and northeast
Oregon while the warm front remains overhead on Sunday. Snow levels
lower to 4500-5500 feet Sunday night along the east slopes of the
Washington Cascades and lower to 6000-7500 feet Sunday night in
central and northeast Oregon. An upper level ridge of high pressure
builds over the region Monday and Monday night for sinking air
yielding mainly dry conditions, although the higher mountains will
keep a low chance of precipitation Monday/Monday night. Polan

Tuesday through Friday night...weak high pressure will remain over
the area through the morning of Christmas evening. Models are struggling
with the amplitude of the high...as the European model (ecmwf) is more suppressed and
will provide a more moist westerly flow to the area. The GFS has the
ridge more amplified with the moisture overruning to the north.
Assuming minimal cloud cover this would present a fog scenario for
the lower elevations.

Wednesday evening a weather system is expected to impact the area
with more favorable chances of precipitation. But models have been
struggling to handle the snow levels and when the rain will
transition to snow. Latest GFS guidance indicates that the lower
elevations may not cool down enough for snow by Friday night. But
the European model (ecmwf) has snow at all levels late Christmas evening. Right now snow
amounts are too difficult to forecast with the run to run
inconsistency and lack of current model agreement. As the system
exits the area...cold dry northwest flow sets in for the remainder
of the extended. Weber

&&

Aviation...00z tafs...a fast moving cold front exited into Idaho
late this afternoon with improving conditions in its wake.

VFR conditions are expected for the remainder of this afternoon into
this evening, except at kykm where IFR will prevail due to low ceilings.
Krdm and kbdn will have MVFR ceilings tonight beginning around 05z/20th.
A warm front will bring rain at lower elevations and snow in the
mountains above 4000 feet tonight. Rain begins at kdls, krdm, and
kbdn around 05z/20th and at kykm at 07z/20th rain will spread east
to kpdt and kalw by 09z/20th an arrive at kpsc by 11z/20th. Polan

&&

Hydrology...the next Pacific system will bring significant
precipitation to the region this weekend. Initially the precipitation will
fall as snow at the highest elevations, but snow levels will rise
Saturday into Sunday. Thus there will be a combination of rain and
snow melt leading to significant rises on area rivers Sunday into
Monday. As such will issue flood watches for a large portion of the
area. 90

&&

Preliminary point temps/pops...
PDT 36 49 47 56 / 20 80 90 80
alw 39 50 45 56 / 20 80 100 90
psc 37 48 43 55 / 20 80 80 40
ykm 35 41 36 49 / 20 80 80 40
hri 36 49 44 58 / 20 80 80 50
eln 32 39 34 46 / 20 90 100 60
rdm 35 49 42 55 / 50 80 80 70
lgd 35 45 40 51 / 20 90 100 100
gcd 35 48 42 51 / 20 80 100 80
dls 36 45 44 54 / 40 90 100 90

&&

PDT watches/warnings/advisories...
or...Flood Watch from Saturday afternoon through Monday afternoon
orz041-509>511.

Wind Advisory from noon Saturday to noon PST Sunday orz510-511.

Flood Watch from Saturday afternoon through Monday afternoon
orz049-050-502-503-505>508.

Wind Advisory from noon Saturday to noon PST Sunday orz507.

Wind Advisory from noon Saturday to noon PST Sunday orz508.

Washington...Flood Watch from Saturday afternoon through Monday afternoon
waz024-026-027-520-521.

Flood Watch from Saturday afternoon through Monday afternoon
waz029-030.

Wind Advisory from noon Saturday to noon PST Sunday waz027-521.

Wind Advisory from noon Saturday to noon PST Sunday waz029.

Winter Weather Advisory from 10 PM this evening to 4 PM PST
Saturday for western Kittitas County in the waz520.

&&

For additional weather information, check our web site at...
www.Weather.Gov/Pendleton

$$

90/99/89/99

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