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Area forecast discussion 
National Weather Service Pendleton or
744 PM PST Thursday Dec 18 2014

Short term...frontal boundary moving into the Pacific northwest
this evening. Some light rain and higher elevation snow has been
moving through the region over the past several hours. Satellite
enhancement along the coast and radar suggest more widespread
precipitation will occur later tonight into Friday morning. Low
clouds and fog likely in the Columbia Basin tonight as well. As the
front moves east of the forecast area Friday the precipitation will
quickly diminish with skies remaining mostly cloudy. Next system to
impact the Pacific northwest will be a warm front Friday night into Saturday.
This front has fairly significant moisture associated with it and a
period of steady rain, possibly heavy at times is likely. Southerly
surface gradients and winds aloft will be increasing Saturday
resulting in breezy to windy conditions by afternoon and evening. 94


Previous disc... /issued 454 PM PST Thursday Dec 18 2014/

Short term...tonight through Friday night...patchy fog lingers at a
few locations within the Columbia Basin. Do not expect the fog to
expand this evening due to approach and passage of the next Pacific
system. This next system will bring a good shot of precipitation to most of
the region tonight into Friday morning. Generally snow levels will
run between 4000 and 5000 feet, but snow levels will run around 2500
feet along the Washington Cascade east slopes in Yakima and Kittitas
counties. Snow fall overnight at and above 2500 feet in this area
will run between 2 and 6 inches, with the highest amounts near the
crest. As such will issue a Snow Advisory. Elsewhere snow amounts
will mainly run 1-3 inches above the snow levels. Will see breezy
conditions increase to the 20 to 30 miles per hour range in the southern Grande
Ronde Valley overnight. Precipitation will gradually decrease across the
area from late Friday morning into Friday evening. The next in the
series of Pacific storms will begin to push precipitation into the area
late Friday night. Snow levels will start out a bit lower with this
system and snow highlights may be needed until snow levels rise. 90

Long term...Saturday through Monday night...a very strong upper
level jet stream will bring above normal moisture and strong dynamic
lift to forecast area Saturday through Monday. The jet stream will
usher in a warm front with dramatically rising snow levels during
the day on Saturday resulting in copious amounts of rain falling on
the snowpack in the mountains. As such area rivers, streams and
creeks will rise to or above bankfull in some areas. At a minimum
2.00 inches of rain are expected in the Blue Mountains for the 48
hour period from sunup on Saturday through sunup on Monday. Lower
elevations can expect to receive anywhere from 0.40 to 1.00 inch of
rain. Snow levels start early Saturday morning around 3500-4000 feet
and then rise dramatically to around 4500-5000 Saturday afternoon.
Snow levels continue to rise in response to the warm front to around
6000 feet along the east slopes if the Washington Cascades by
daybreak on Sunday, and rise to around 7000-8500 feet in the
mountains of central and northeast Oregon. Snow levels remain high
persisting at the aforementioned levels through Sunday night, and
then lower to 4500-6500 feet during the day on Monday. Polan

Tuesday through Thursday...a ridge of high pressure over the area
will begin to break down on Tuesday. Daytime highs will be in the
40s to lower 50s as a weak disturbance brings a chance of rain and
High Mountain snow. A stronger shortwave will then swing through the
area on Wednesday bringing more precipitation along with a cold
front. Snow levels subsequently will lower to the basin floor on
Wednesday night. However, by the time the colder air arrives,
precipitation will be diminishing and only light snow showers are
expected on Christmas morning. On Thursday, northwesterly flow aloft
will keep conditions cool with daytime temperatures in the 30s, and
clearing skies Thursday night will allow temperatures to drop into
the teens and low 20s. Cobb

Aviation...00z tafs...VFR conditions initially at taf sites this
evening, except MVFR conditions at kykm, kdls and kalw expected this
evening, mainly due to low stratus. At 442 PM, kpdt radar showed
scattered rain showers moving east across khri and kpdt and toward
kalw. These showers should dissipate by 02z-04z. Conditions will
deteriorate to IFR/MVFR overnight as an approaching cold front
brings rain starting around 02z/19th at kykm and kdls, then -ra
starts at krdm and kbdn by 03z/19th. Rain spreads to other taf sites
by 06z/19th. Some periods of local LIFR possible under moderate to
heavy rain reducing visibility. Improving conditions will occur Friday
morning as rain ends at taf sites from west to east, beginning
around 11z-12z/19th at kdls and kykm, then ending at kpsc and kpdt
by around 16z-19z/19th. Winds will be at or below 10 kts. Polan


Preliminary point temps/pops...
PDT 35 46 35 47 / 70 70 20 80
alw 37 46 37 47 / 70 70 20 80
psc 36 49 36 47 / 70 50 10 80
ykm 35 43 32 43 / 70 30 20 80
hri 35 48 35 48 / 70 50 20 80
eln 33 42 31 43 / 70 40 20 80
rdm 34 46 33 47 / 70 50 30 80
lgd 37 44 34 46 / 70 80 20 90
gcd 34 45 33 46 / 80 70 20 80
dls 37 48 36 48 / 80 60 30 90


PDT watches/warnings/advisories...
Washington...air stagnation advisory until 10 am PST Friday waz026>028.

Winter Weather Advisory until 7 am PST Friday above 2500 feet
for the waz520.


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