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Area forecast discussion 
National Weather Service Pendleton or
910 PM PST Monday Nov 30 2015

Update...a weak disturbance will track across the two state area
tonight producing some snow showers and little accumulation is
expected. Otherwise low clouds and fog will persist over the
Columbia Basin tonight. Overnight temperatures were lowered some
across the entire area and now the present short term forecast
appears on track.


Aviation...a weak disturbance may also produce some snow showers at
all taf sites until 15z. Meanwhile low clouds and fog creating areas
of MVFR and IFR conditions will persist across most taf sites except
for krdm and kbdn through tomorrow.


Previous discussion... /issued 305 PM PST Monday Nov 30 2015/

Short term...tonight through Wednesday night...high pressure
over the region is in the process of breaking down. This will allow
a transition to a more wet pattern with a variety of precipitation
types over the next couple of days. The first system will arrive
this evening and overnight with mainly some increasing clouds. The
system is weakening rapidly as it pushes against the departing high
pressure. Could still see a few light showers mainly over the
Cascades and perhaps along the east slopes. With cold air in place
there could be some brief freezing rain mixed in with the snow but
not expecting enough accumulation to warrant any advisories. The
next system will arrive Tuesday night and persist through Wednesday
night. This system will arrive as a weak cold front and then
transition into a warm front. The cold front will arrive Tuesday
night and weaken as it arrives but it will bring a good chance of
accumulating precipitation. Then on Wednesday it will begin to lift
northward as a warm front. The cold air trapped in the Columbia
Basin is expected to remain in place which will make this event
tricky as to what type of precipitation will occur and when. Above
the cold air this will be a snow event turning to rain with rising
snow levels as the warmer air aloft arrives. The main concern will
be the accumulation of snow and ice in the lower elevations starting
Tuesday night and continuing through Wednesday night. Some areas
could see around a quarter of in inch of ice or more from this event
as well as some snow accumulations. As such will be issuing a
Winter Storm Watch for this event.

Long term...Thursday through Monday...unsettled weather will
continue through the long term period. Thursday a warm front will
move northeast across the area with a chance of mainly rain across
the area. Snow levels will be 5000-6000 feet except in the northern
Cascades where they will be around 4000 feet. The rain will taper
off Thursday afternoon. Another system will move across eastern
Washington and Oregon early Friday with valley rain and mountain
snow. A transitory ridge will move over the area Friday night with
drier conditions. 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. Model spread becomes significant Sunday
and Monday so have leaned toward slight chance of valley rain and
chance of mountain snow. High temperatures will be in the 40s in the
lower elevations with middle 30s to lower 40s in the mountains.


Preliminary point temps/pops...
PDT 16 27 25 30 / 10 10 30 50
alw 19 31 30 33 / 10 10 20 50
psc 20 30 26 31 / 10 10 60 60
ykm 17 29 23 31 / 10 10 60 80
hri 21 30 27 31 / 10 10 60 60
eln 16 25 22 31 / 10 10 60 80
rdm 12 33 28 39 / 20 10 70 70
lgd 10 32 27 36 / 10 10 10 50
gcd 12 36 26 37 / 10 10 30 70
dls 25 33 29 32 / 20 10 70 80


PDT watches/warnings/advisories...
or...air stagnation advisory until noon PST Friday for orz041-044-507-

Winter Storm Watch from Tuesday evening through late Wednesday
night for orz041-044-508-510-511.

Air stagnation advisory until noon PST Wednesday for orz049-050-

Washington...air stagnation advisory until noon PST Friday for waz024-026>029-

Winter Storm Watch from Tuesday evening through late Wednesday
night for waz024-026-027-520-521.


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