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Area forecast discussion...updated 
National Weather Service Pendleton or
912 PM PST Wednesday Dec 17 2014

Updated aviation discussion

Update...a few disorganized showers are moving across the County Warning Area
this evening. Dense fog is over parts of the Blue Mountain
foothills. Patchy fog has developed over the Kittitas Valley. Web
cams and observations do not show very much fog in central and
north central Oregon yet. Forecast is on track but will need to
monitor development of fog with moist and stagnant air mass over
the area. Coonfield


Aviation...06z tafs...IFR/LIFR conditions will prevail at kpdt and
kalw. MVFR conditions will prevail at other taf sites with
occasional IFR ceilings until 18z. These conditions will continue into
the morning hours on Thursday with some improvement possible after
18z Thursday. Winds will be light. 76


Previous disc... /issued 242 PM PST Wednesday Dec 17 2014/

Short term...tonight through Thursday last weak
disturbance will move across the region tonight. This system will
cause isolated to scattered rain or snow showers. The stagnant air
mass across the basin will continue into Thursday, and as such
patchy to areas of fog will develop or expand in coverage. The
densest fog will generally occur between the elevations of 1500 and
3000 feet mean sea level. At this time have enough confidence to issue dense
fog advisories for the Blue Mountain foothills and the Kittitas
Valley. Portions of north-central and central Oregon may need to be
added depending on development in these areas. Generally expect
temperatures to remain above freezing in the foggy areas, though some
locations will see lows between 30 and 32 degrees. Thus some local
freezing fog will be possible. Other than a few lingering showers,
conditions will be generally dry Thursday due to a transitory upper
level high pressure ridge. Most foggy areas should see improvement
by late morning Thursday. A stronger and wetter Pacific system will
begin to move into the region Thursday night. Precipitation will gradually
spread from west to east through the night. Snow levels will range
from near 2000 feet in western Kittitas County to between 3500 and
4500 feet elsewhere. Snow amounts at this time are expected to be 3
inches or less over the higher elevations. This system will also
begin to increase southerly winds over a good portion of our Oregon
zones. Should see at least windy conditions over the southern Grande
Ronde Valley. The increasing cloud cover, precipitation and winds should
limit fog development in the basin. 90

Long term...Friday through weather pattern is
store through the period with several storm systems to impact the
region. The flow pattern will be fast zonal for much of the time but
indications are that it will transition to more northwesterly Tuesday into
Wednesday as Arctic air begins to filter into the region. For Friday
into Sunday expect periods of mostly rain heavy at times...
especially mountains. There could be in excess of 2 inches of quantitative precipitation forecast in
the mountains with rises likely on rivers and streams. It is
possible that there could be some snow along the I-90 corridor in northwest
Kittitas County if snow levels are just a bit lower than currently
forecast. There is also the potential for a high wind event Saturday
night into Sunday lower elevations with a strong westerly low level
jet and tight surface pressure gradient. For now have beefed up the
winds to advisory levels for that time period. There will continue
to be a chance of rain on Monday and Tuesday but the rain will
transition to snow by Wednesday (christmas eve) as snow levels lower
with the arrival of the Arctic air. 78


Preliminary point temps/pops...
PDT 30 38 33 42 / 30 10 50 70
alw 32 39 35 42 / 30 10 50 80
psc 34 41 34 43 / 20 10 60 70
ykm 33 39 33 40 / 30 10 70 40
hri 34 41 33 43 / 30 10 60 70
eln 33 38 33 36 / 30 10 70 50
rdm 29 43 32 43 / 30 10 60 60
lgd 32 45 35 40 / 40 20 40 80
gcd 31 42 32 39 / 30 20 40 80
dls 38 44 37 44 / 40 10 70 70


PDT watches/warnings/advisories...
or...dense fog advisory until 10 am PST Thursday orz507-508.

Washington...dense fog advisory until 10 am PST Thursday waz026-029.

Air stagnation advisory until 10 am PST Friday waz026>028.


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