Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 081646
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
845 AM PST Thu Dec 8 2016

.SHORT TERM....Oh the weather outside is frightful...or at least
it soon will be. A warm front has approached central and northeast
Oregon where widespread snow has developed. Snowfall amounts of
1-3 inches have been observed at Sisters, Prineville, and
Sunriver. John Day has also reported light snow over the past
hour. The front will soon makes its way across eastern Oregon
this morning then across south central and southeast Washington
this afternoon. Warm air aloft will cause some freezing
precipitation to fall this afternoon over portions of the Columbia
Deschutes Plateau and the John Day Basin then farther north near
Grass Valley this evening. Snow levels in Central Oregon will
rise to near 4000 feet by sunset this evening with precipitation
below 4000 feet changing to rain. Current Winter Storm Warnings
and Winter Weather Advisories are on track but forecasters will
need to keep a close eye on the Blue Mountains where prolonged
light snow from this afternoon through Saturday morning may bring
accumulations up to warning criteria. Wister



&&

.PREV DISCUSSION... /issued 227 AM PST Thu Dec 8 2016/

SHORT TERM...Today through Friday Night...A complex pattern will
set up over WA and OR through the short term period. All zones of
the CWA have a Winter Storm Warning or Advisory. Snow will begin to
develop along the East Slopes of the Oregon Cascades in Deschutes
County and the Bend/Redmond areas 5-8 AM this morning. The snow will
then spread over the remainder of eastern Oregon and eastern
Washington during  the day. Snow could begin to mix with sleet, and
eventually freezing rain during the late afternoon and evening hours
across Central Oregon, mainly south of hwy 26. Periods of snow will
continue across all of northern Oregon and Southern Washington into
Thursday evening. Blowing and drifting snow will be a concern in the
Grande Ronde Valley with south winds 20-35 MPH Thursday evening.
Snow and mixed precip will taper off and nearly end late Thursday
night and Friday morning for all lower elevations.

Snow amounts of 2-4 inches are expected for the John Day Basin and 3-
5 inches in the Ochoco Mountains today before warm air spreads and
increases snow levels to around 3500-4000 feet. In the Blue
Mtns...light snow will be continue from Thursday afternoon through
Friday night...and this will be a prolonged light to moderate snow
event. The snow amounts for any period probably will be below
warning criteria, but this may change. Snow amounts in the Grande
Ronde Valley and Wallowa County will be border line for advisory
amounts...but windy conditions in the Grande Ronde Valley late today
will cause blowing and drifting of snow.

The only change I intend to make in winter highlights is to extend
the Winter Storm Warning in the eastern Columbia River Gorge until 10
PM today since the snow will last longer than earlier anticipated.

Friday afternoon and evening another disturbance will move through
eastern WA and OR. Snow will start in the WA Cascades and the Yakima
and Kittitas Valleys Friday afternoon and spread across the rest of
the area Friday evening. More winter highlights are likely then.
Coonfield

LONG TERM...Saturday through Friday...an active weather pattern
will persist during the extended period. There will be several
weather systems as shown by the latest models. However there is
still some model differences so confidence is not real high at this
time yet due to run to run and model to model inconsistencies. The
first system will arrive on Saturday with snow in the mountains and
rain or a mixture of rain and snow in the lower levels. Cold
modified arctic air will try to spill southward into the central CWA
Saturday which will keep all precipitation frozen from the Basin
northward. It will be a close call for the Foothills of the Blue
Mountains as the 1300 1000-850mb thickness line will be right over
the Foothills. The surface pressure gradient will be weak so there
may be freezing rain with cold air at the surface and warmer air
aloft under a southwest flow between 850-700 mb. There will be a
break in the weather on Saturday night and then another system will
move in on Sunday. This system will be warmer so most of the lower
elevation precipitation will be rain with snow levels around around
1700-2500 from the Foothills southward. Precip may be frozen or
freezing in the normally colder Basin and the Yakima/Kittitas
Valleys where snow levels will only be around 500 feet. Another shot
of modified arctic air is then forecast to move south on Tuesday
turning any rain showers to snow showers for most of the CWA before
ending. Then dry conditions will prevail Tuesday and Wednesday
before another system moves into the region on Thursday and Friday.
Again precipitation type will be tricky as the rain snow line will
be somewhere between the Lower Columbia Basin and the Blue Mountain
Foothills. A shift either way of this line will result in the
difference between freezing/frozen precipitation or just rain. Cold
arctic air is then expected to push further southward after the
system departs for the weekend of the 17th of December. 88

AVIATION...12Z TAFs...VFR conditions will prevail this morning
except for at KRDM and KBDN where snow is expected to begin this
morning around daybreak and conditions will lower to MVFR or IFR
there at times. Conditions will slowly deteriorate as the day
progresses at the rest of the TAF sites as the weather system and
warm front pushes north across the area. Then late tonight
conditions will improve as the system moves out of the area. Winds
will be mostly light through the period...though it could become
locally breezy from the south at KRDM and KBDN. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  23  18  31  25 /  90  80  60  80
ALW  27  20  33  26 /  80  90  50  80
PSC  26  21  28  23 /  80  80  50  60
YKM  25  17  26  20 /  80  90  50  60
HRI  25  20  28  23 /  90  80  60  60
ELN  24  19  26  20 /  50  90  60  60
RDM  23  20  42  26 / 100  50  50  70
LGD  22  20  38  28 /  80  80  70  80
GCD  26  26  41  29 /  90  70  60  80
DLS  27  21  30  29 / 100  90  70  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 PM PST this evening for ORZ505-
     506-509-511.

     Winter Storm Warning until 10 PM PST this evening for ORZ510.

     Winter Weather Advisory until 4 AM PST Saturday for ORZ503.

     Winter Weather Advisory from 1 PM this afternoon to 4 AM PST
     Friday for ORZ050.

     Winter Weather Advisory from noon today to 4 AM PST Saturday for
     ORZ502.

     Winter Weather Advisory until 1 AM PST Friday for ORZ044-507.

     Winter Storm Warning until 10 PM PST this evening for ORZ041.

     Winter Weather Advisory until 4 AM PST Friday for ORZ049.

     Winter Storm Warning until 1 AM PST Friday for ORZ508.

WA...Winter Weather Advisory from noon today to 4 AM PST Saturday for
     WAZ030.

     Winter Weather Advisory until 1 AM PST Friday for WAZ026>029.

     Winter Storm Warning until 3 PM PST Saturday for WAZ521.

     Winter Storm Warning until 10 PM PST this evening for WAZ024.

     Winter Storm Warning from noon today to 3 PM PST Saturday for
     WAZ520.

&&

$$

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