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Area forecast discussion...updated 
National Weather Service Pendleton or
925 am PST Monday Nov 30 2015

Updated aviation discussion

Short and tonight...high pressure remains over the
region this morning but it will begin to weaken as a low pressure
system off the coast moves towards the region. This low will weaken
rapidly and fall apart as it moves inland. We could see a little
light precipitation along the east slopes of the Cascades late this
afternoon and overnight. This means there could be a little wintry
mix of precipitation to deal with from central Oregon towards
Snoqualmie Pass. Precipitation amounts will range from a trace to a
hundredth or two but don't anticipate enough precipitation to
warrant any kind of advisory highlights. The stratus deck will
continue in place which means some issues with freezing fog and
reduced visibilities mainly in areas where the terrain reaches the
bottom of the stratus deck. The air stagnation advisory will
continue and is now expanded until Friday.


Aviation...18z tafs...MVFR/IFR conditions will be prevalent at
Columbia Basin taf sites under a low stratus deck at about 2000-2800
feet mean sea level. Visibility may be reduced to 4-6sm at times. Krdm and kbdn
are south of the stratus deck. A passing disturbance tonight will
lift the stratus and it may move north into krdm. The disturbance
may provide enough lift to create light snow flurries after 03z at
all taf sites but confidence was too low to add to tafs aside from
vcsh at kykm and kdls. Winds will remain below 10 kts for the next
24 hours. 76


Previous discussion... /issued 400 am PST Monday Nov 30 2015/

Short through Wednesday...the forecast area will be in
an area between an upper ridge that is centered over southwest
Canada and a low pressure system over the Rocky Mountains today.
Conditions will be dry and stable...except for some occasional snow
flurries under a stratus deck that stretches across the Columbia
Basin into the adjacent valleys of both Oregon and Washington. The
stratus layer extends southward to Jefferson County and includes the
city of Madras...and north to include the Yakima and Kittitas
valleys. It also stretches as far east as the grande ronde and
Wallowa valleys. There is some patchy fog under this stratus deck
where higher terrain meets the base of the stratus deck such as the
Horse Heaven Hills and the slopes of the mountains surrounding the
Columbia Basin and adjacent valleys. A low pressure system off the
coast will send a weak trough inland this afternoon and evening
while it weakens further as it encounters the upper ridge. The
result will mainly be just increasing middle and high clouds and it may
cause enough lift to squeeze out some snow flurries under the
stratus deck. Then there will be a short break with a weak upper
ridge on Tuesday morning before a stronger and wetter trough moves
into the region Tuesday night into Wednesday. This next system will
bring a mixed bag of precipitation with mainly snow in the mountains
and mainly rain with some freezing rain in the valleys and lower
Columbia Basin. Believe the winds will cause enough mixing to scour
out the cold air along the Blue Mountain foothills for mainly just
rain there. Farther north in the lower Columbia Basin there may be a
period of light freezing rain or snow before changing to rain on
Wednesday. Snow levels will increase to around 3500-4500 feet on
Wednesday in western and southern areas of the County Warning Area while it remains
fairly low over the eastern mountains for mainly snow there in the
mountains on Wednesday. Winds will increase mainly in the southern
Grande Ronde Valley around Ladd and pyles canyons and portions of
Wallowa County...otherwise winds will be light in the County Warning Area. Expect
warmer and wet weather in the extended forecast period. 88

Long term...Wednesday night through Monday...unsettled weather will
continue through the long term period. Wednesday night and Thursday
a front will move through the area with a chance of mainly rain
across the area with snow levels around 6000 feet. The rain will
taper off to a slight chance Thursday afternoon. Thursday night and
Friday the GFS shows another wave moving through the area while the
European model (ecmwf) is weaker with this wave. Current forecast leans towards the
wetter GFS so have kept a chance of mainly rain Thursday night and a
slight chance of rain Friday. This system will be colder with snow
levels around 3500 feet so the mountains will see snow with this
system. Friday night and Saturday will be a break between systems
and dry weather is expected though there will be a slight chance of
light snow in the mountains. Another system moves ashore either
Saturday or Saturday night and hangs up along the Cascades for 24
hours or so before moving into the area. Have leaned towards the
faster GFS solution and have a slight chance of rain and snow in and
near the Cascades Saturday then a chance of rain and snow across the
area Saturday night through Sunday as the system slowly moves
through the area. The European model (ecmwf) is slower so have kept a slight chance
of rain and snow for Sunday night and Monday in case the European model (ecmwf) is
right. Snow levels will vary from 4000 to 5500 feet with this system
so snow will be confined to the mountains. Temperatures will be in
the 40s in the lower elevations with middle 30s to lower 40s in the
mountains. Perry


Preliminary point temps/pops...
PDT 25 21 32 26 / 10 10 0 40
alw 29 27 33 31 / 10 10 0 30
psc 28 26 33 27 / 10 10 10 50
ykm 27 23 37 25 / 10 10 10 60
hri 28 27 33 27 / 10 10 10 50
eln 23 21 34 24 / 10 10 10 60
rdm 31 18 43 27 / 10 20 10 60
lgd 29 18 36 29 / 0 0 10 30
gcd 29 14 39 29 / 10 10 10 30
dls 32 30 41 33 / 10 20 10 60


PDT watches/warnings/advisories...
Washington...air stagnation advisory until noon PST Friday for waz026>029.


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