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Area forecast discussion 
National Weather Service Pendleton or
750 PM PST sun Nov 29 2015

Update...areas of freezing fog will persist across portions of the
lower Columbia Basin and the forecast was updated to reflect this
along with adding some occasional light snow. Little accumulation is
expected. Otherwise high pressure remains entrenched across the
region with dry conditions continuing. Temperatures remain in The
Ball park and the patchy freezing fog will persist across the entire
Columbia Basin into Monday morning.


Previous discussion... /issued 141 PM PST sun Nov 29 2015/

Short term...tonight through Wednesday...high pressure will persist
over eastern Washington and Oregon through Monday. Areas of low
clouds and patchy freezing fog will continue over the Columbia Basin
and adjacent areas below 3000 feet including the Blue Mountain
foothills...north central Oregon...Yakima Valley...Kittitas
Valley...and Grande Ronde Valley. The height of the surface
inversions is high enough that most of the fog will be areas where
the stratus layer intersects with the terrain.

Monday night a weak wave will shear apart as it moves into the
ridge. This wave may bring some slopover snow to areas near the
Cascade crest as it falls apart. Lift associated with the wave may
squeeze a few snow flurries out of the stratus layer as well Monday
night and Tuesday morning.

A stronger weather system will spread moisture across eastern
Washington and Oregon Tuesday night and Wednesday. There will be a
mix of wintry precipitation. The Kittitas Valley and upper Yakima
Valley will receive mainly snow due to evaporative and upslope
adiabatic cooling. Mountains will receive mainly snow as well.
Freezing rain is possible across the rest of the area until midday
Wednesday due to the existing surface cold pool and a warm layer
aloft. At this point confidence in the location of freezing rain is
low since models show warming temperatures. However models typically
scour out cold pools too quickly so I kept a chance over the area.

Long term...Wednesday night through Sunday. Unsettled conditions
will continue through the extended period as several systems move
through the region. A system will move through Wednesday night and
Thursday and is expected to bring mainly rain to the lower
elevations and snow to the mountains as it appears the colder air in
the lower basins will be cleared by then. This system exits the
region on Friday with a brief break on Friday night and may last
into Saturday before another system moves onshore and across the
area on Sunday. The models are in fairly good agreement until late
Saturday when the GFS is more aggressive and has more moisture than
the European model (ecmwf) with the system Saturday night. Temperatures should be
warming through the week with near to slight above normal readings.

Aviation...00z tafs. Taf sites should expect prevailing MVFR/IFR
conditions to occur...with tempo LIFR conditions possible by after
06z tonight. Low clouds with ceilings at or below 015 feet and visibility to around
2sm in fog overnight at sites except for krdm and kbdn. Krdm and
kbdn will see some higher cirrus clouds that may slow fog
development. Higher cirrus clouds will spread across the region
through the day on Monday with little change to low clouds and ceilings
in the lower basins. Ml


Preliminary point temps/pops...
PDT 19 29 22 31 / 0 0 10 0
alw 22 29 26 32 / 0 0 10 0
psc 21 29 21 32 / 0 10 10 10
ykm 15 33 16 36 / 0 10 10 10
hri 23 30 19 32 / 0 10 10 10
eln 11 30 14 33 / 0 0 10 10
rdm 2 37 12 42 / 0 10 10 10
lgd 12 32 19 35 / 0 0 0 10
gcd 6 34 10 38 / 0 10 10 10
dls 21 37 25 40 / 0 10 10 10


PDT watches/warnings/advisories...
Washington...air stagnation advisory until noon PST Tuesday for waz026>028.


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