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fxus66 kpdt 180523 
afdpdt

Area forecast discussion
National Weather Service Pendleton or
923 PM PST Wed Jan 17 2018

Updated aviation discussion

Short term...tonight and Thursday...an upper level trough in the
eastern Pacific with a closed low off the coast of Canada will
slowly move eastward over the next 24 hours. This will push a cold
front across the forecast area. Currently the front stretches from
northeast Washington back to western Oregon and will be exiting the
forecast area late Thursday. The front will slowly move a band of
precipitation eastward with initial snow levels at 7000 feet. Snow
levels will lower behind the front late Thursday to around 2000 feet
but. However, limited precipitation behind the front in the colder
air means no significant snow accumulations. Main impact of the
front will be the rainfall and the windy conditions ahead of the
front. Wind advisories have been issued for some zones through
tonight.

Aviation...06z tafs...a cold front will move through the
region overnight and Thursday with a slow moving band of rain.
Cigs will lower to around 040-060 during the rain band with vsby
remaining VFR. Some windy conditions with gusts 30 to 50 kts will
precede the front mainly at kpdt and kalw and then decrease behind
the front early Thursday.

&&

Previous discussion... /issued 306 PM PST Wed Jan 17 2018/

Short term...tonight through Saturday. A deep upper low centered
near 48n/135w will track east-northeast toward northern b.C.
Tonight. The low will force a strong cold front east of the Cascades
tomorrow. Surface winds ahead of the front are gusting to 45-50 mph
in the southern Grande Ronde Valley, and a Wind Advisory was issued
earlier for this area in effect until 8 am thurs morning. Gusts to
35-45 mph will continue in the northern Grande Ronde Valley and just
along the base of the or Blue Mountain foothills east of Milton-
Freewater and Pendleton tonight, and these winds are just shy of
advisory criteria.

The cold front is well organized, and quantitative precipitation forecast amounts of 0.25-0.6 inch
are forecast for most of the area ahead of the front. Snow levels
will also lower significantly behind the front. Snow levels will
range from 4500-5500 feet early Thursday morning lowering to 2500-
3500 feet behind the front Thursday afternoon and evening. The
front will approach the eastern slopes of the Washington/or Cascades around
sunrise, across the basin and the Columbia Deschutes plateau during
the morning, then across the remainder of the forecast area during
the afternoon. Despite the cold air and lowering snow levels,
precipitation will taper off behind the front. From Thursday night
through Friday morning, precipitation will mainly be orographic
mountain snow showers with around 1-3 inches of accumulation in the
populated mountain areas and around 5-10 inches over the highest
peaks of the elkhorns, Wallowa Mountains, and the Strawberry
wilderness.

From Friday through Saturday, there will be two weak shortwave
troughs bringing scattered snow showers over the mountains and
isolated snow showers elsewhere. Elevations below 2500 feet will see
light rain or light rain/snow mix. Precipitation amounts will be
light with only 1-3 inches of new snow possible in the mountains
with each shortwave. Afternoon temperatures during this time will be
in the 30s over the mountains and 40s in the lower elevations which
is near average for this time of the year. Wister

Long term...Saturday night through Thursday morning...an active
weather pattern continues as a series of waves around the low in the
Gulf of Alaska continue to move through the region. At the outset of
the period weak ridging in the trough-ridge-trough pattern will be
over the region. This will quickly move off to the east ahead of the
first impulse associated with the aforementioned Gulf of Alaska low
entering the area during the day on Sunday. Precipitation will taper
off during the day on Monday as transient ridging builds back in for
Monday night/Tuesday with the next significant shot of energy
entering the region late Tuesday night/Wednesday morning. Snow
levels during the periods of heaviest precipitation associated with
each warm front will be 3000-4000 feet, falling to around 2500 feet
with passage of the cold front with each system. Both systems should
result in some helpful additions to the mountain snowpack, but will
also include impacts to those transiting the passes. Temperatures
will be near to slightly above period through the extended. Breezy
conditions will be possible late Sunday night/Monday morning as
trough crosses local maximum of 850mb winds along the Columbia
River, but given timing with convective mixing at a minimum may be
more of a localized threat for downslope favored areas and
ridgetops. 74



&&

Preliminary point temps/pops...
PDT 46 49 35 46 / 40 90 30 10
alw 48 50 36 47 / 40 90 30 10
psc 39 51 35 47 / 90 60 10 10
ykm 38 49 31 44 / 90 20 10 10
hri 43 51 35 48 / 80 80 10 10
eln 34 45 28 40 / 100 20 10 20
rdm 45 46 27 42 / 80 60 30 20
lgd 44 46 32 42 / 20 90 60 40
gcd 44 46 29 40 / 10 90 60 30
dls 41 51 35 47 / 100 40 30 30

&&

PDT watches/warnings/advisories...
or...Wind Advisory until 4 am PST Thursday for orz507.

Wind Advisory until 8 am PST Thursday for orz049.

Washington...Wind Advisory until 4 am PST Thursday for waz029.

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