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Area forecast discussion...updated aviation discussion 
National Weather Service Pendleton or
930 am PST sun Nov 29 2015

Update...high pressure will persist over eastern Washington and
Oregon through Monday. Areas of low clouds and 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. Coonfield


Previous discussion... /issued 325 am PST sun Nov 29 2015/

Short through Tuesday...high pressure aloft with an
easterly flow will continue for one more day. Main forecast concern
in the short term will be the expanding fog and low clouds that will
eventually spread over all of the basin and foothills. Low cloud
cloud bases are high enough for most valley locations to have just
low clouds while the higher terrain against the mountains meets the
low cloud layer surrounding the basin and adjacent valleys. These
areas will see the most fog. The fog and low clouds will persist
through Tuesday morning and then decrease as a weather system moving
into the region Tuesday night and Wednesday brings enough mixing to
lift the stratus layer or at least the surface fog into a stratus
layer. The other concern will be the onset of precipitation which
may begin as freezing rain in the colder valleys or snow in the
mountains. Temperatures this morning are significantly warmer than
the past few days. The forecast soundings are showing air that
will be too warm for freezing rain along the foothills...but some
basin locations and areas against the Cascade east slopes may
see some snow or freezing rain to start. 88

Long term...Tuesday night through Sunday...a weather system will
continue to move inland Tuesday evening...starting with a winter mix
of precipitation. Snow will likely be the form of precipitation
over the east slopes of the Washington Cascades...the Kittitas and
Yakima valleys...and the mountains of northeast Oregon. All other
areas will likely see a rain/freezing rain mix. This is due to a
strong warm layer aloft...and cold air that has remained trapped at
the surface over the past several days. Lately models have been
mixing out the colder air fairly quick. Freezing rain looks to
start after sunset Tuesday...and may not change over to rain until
middle to late Wednesday morning. Areas that will be prone to cold air
trapped for longer periods of time include...the Washington
Basin...Yakima and Kittitas Valley.

This weather system is expected to linger over the area through the
end of the work week...providing a continued chance of rain and high
elevation snow. By Friday morning a cold front should push
through...lowering snow levels again. Keep in mind the lower
elevations will still be mild and may not notice the change as much
as some of the mountain areas. Another warm front is expected late
Saturday into Sunday. This will likely be a rain/high elevation
snow cloud cover will likely keep areas from getting
too cool at night to develop fog. Overall expect a fairly wet
extended...but temperatures turning more mild...once the cold air is
mixed out. Weber

Aviation...18z tafs...various flight categories will continue over
the next 24 hours. All taf sites should expect prevailing MVFR/IFR
conditions to occur...with tempo LIFR conditions possible. With
ridge axis over the region and loss of northerly flow would expect
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. Ml


Preliminary point temps/pops...
PDT 27 23 30 23 / 0 0 0 10
alw 28 27 30 27 / 0 0 0 10
psc 29 22 31 22 / 0 0 10 10
ykm 33 16 33 16 / 0 0 10 10
hri 29 20 31 20 / 0 0 10 10
eln 32 14 31 14 / 0 0 0 10
rdm 25 8 36 13 / 0 0 10 10
lgd 33 16 32 20 / 0 0 0 0
gcd 29 7 33 11 / 0 0 10 10
dls 34 25 38 25 / 0 0 10 10


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


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