Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 181759 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
959 AM PST Sun Feb 18 2018

Updated AVIATION discussion

.SHORT TERM...Today and tonight...A large upper level trough is over
the region today with a center circulation noted over western
Washington on satellite. This is placing most of the forecast area
under a west to southwest flow this morning with some showers. The
center of the low will move over eastern Washington late today and
overnight while the surface low associated with it tracks through
central Oregon tonight and and into southwest Idaho by Monday
morning. The track of this surface low means that it will pull
colder air southward creating upslope conditions as cold moist air
in the Columbia Basin is lifted orographically into the higher
terrain of eastern Oregon. The result will be some enhanced snowfall
along the Foothills of the Blue Mountains and central Oregon. As
such, winter weather advisories have been issued from late this
afternoon through early Monday morning. The winter storm warning for
the Blue Mountains continues as an additional 3 to 6 inches of snow
will be possible by this evening. This warning may need to be
extended or possibly reduced to an advisory for the overnight
period but will decide that this afternoon.

.AVIATION...18Z TAFs...A mid/upper low will which is over the
Puget Sound this morning will move across the interior Pacific
Northwest today through this evening and then move into Idaho and
Montana overnight. Expect broken to overcast skies today becoming
scattered to broken late tonight. Ceilings will be 3500 feet and
above today with rain showers expected at all TAF sites. As colder
air moves into the area late this afternoon and evening rain showers
will turn to snow though amounts will be light. Snow showers will
taper off after 03Z. With the heavier snow showers, MVFR ceilings
and visibilities will be possible, otherwise low end VFR conditions
are expected tonight. West to southwest winds of 10 to 20 kts with
gusts to 25-30 kts today will turn more north to northwest late this
afternoon and tonight and gradually decrease.

&&

.PREVIOUS DISCUSSION... /issued 359 AM PST Sun Feb 18 2018/

SHORT TERM...Today through Tuesday. Cold upper level low just
off the coast early this morning. This system will move inland
across the forecast area today. Onshore flow will continue to
produce snow over the Cascades through the day possibly heavy at
times. Winter storm warnings will remain in effect. East of the
Cascades expect periods of snow through the day. The snow could
mix with rain this morning in the Columbia basin. Otherwise cold
advection associated with the low will drop freezing levels and
would expect all snow by afternoon. Temperatures will be nearly
steady or falling through the day with highs only in the 30s and
breezy westerly winds of 10-25 mph. Cold northerly flow tonight
will maintain the chance of snow especially over the Blues into
central Oregon. An upper ridge building offshore Monday will
maintain the northerly flow. The airmass will be drying and
mountain snow showers will end. Skies will be partly to mostly
cloudy with highs in the 20s to lower 30s. Clear to partly cloudy
Mon night with lows zero to 15. A weak upper trough approaching
the coast Tuesday will spread mid and high level clouds across the
region by afternoon. 94

LONG TERM...Tuesday Night through Sunday...Cool and unsettled
weather is expected for much of the long term period. Models have
significant differences as to the timing and strength of short waves
crossing the area so overall confidence is below normal. Tuesday
night and Wednesday models have a trough over the area with a north
to northwest flow. The GFS has a small low that passes off the coast
and does little to our side of the Cascades. The ECMWF brings a wave
through our area with light snow showers mainly over the mountains.
Have leaned towards the ECMWF and have a slight chance of snow with
amounts generally less than an inch for the 24 hour period. The
Columbia Basin and adjacent valleys will be dry. Wednesday night and
Thursday both the GFS and ECMWF bring a low into the area and deepen
the trough. The GFS brings the light snow this time while the ECMWF
is mostly dry. Kept a slight chance of light snow over the area
including the Columbia Basin on Thursday. Thursday night the trough
moves off to the east and is replaced by a northwest flow over the
area from the eastern fringe of a ridge in the Pacific. Models show
a series of systems moving over the ridge and through the area
Friday through Sunday though they differ as to the timing and
details. Have kept a slight chance of snow in the lower elevations
through this period and a chance in the mountains. Have the Columbia
Basin dry Saturday night and Sunday. Temperatures will be in the 30s
and lower 40s with mid 20s to mid 30s Wednesday and Thursday then
warm to the 40s with 30s in the mountains Friday through Sunday. 83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  17  26   8 /  90  60  20   0
ALW  38  16  26   9 /  80  60  20   0
PSC  38  18  32  11 /  50  30   0   0
YKM  36  18  31  10 /  30  20   0  10
HRI  39  18  31  10 /  70  40  10   0
ELN  32  15  29   8 /  30  20   0   0
RDM  35  12  26   1 /  70  60  20  10
LGD  36  14  20   1 /  70  60  30  10
GCD  35  14  21   1 /  70  70  20   0
DLS  40  20  34  13 /  70  60  10  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 3 PM this afternoon to 9 AM PST
     Monday for ORZ505-507-508-510-511.

     Winter Storm Warning until 10 PM PST this evening for ORZ502.

     Winter Weather Advisory until 9 AM PST Monday for ORZ509.

WA...Winter Storm Warning until 10 PM PST this evening for WAZ030.

&&

$$

91/99/99



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