Area Forecast Discussion
Issued by NWS Pendleton, OR

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843
FXUS66 KPDT 051149 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
349 AM PST Mon Dec 5 2016

Updated Aviation Discussion

.SHORT TERM...Today through Tuesday night...A system approaching
Vancouver Island early this morning will swing southeast across the
Pacific Northwest late today through Tuesday morning. This system
will bring increasing chances of showers today into tonight. Snow
levels will be low, only rising to between 600 and 1200 feet MSL
this afternoon before falling to the Basin floor again this evening.
Thus most of the showers will produce snow. Showers will begin this
afternoon over the Cascade east slopes then spread east by mid
afternoon. The showery nature of the precip will make determining
the exact locations of heaviest snow difficult. At this time data
indicates that the Oregon Cascade east slopes could see 3 to 7
inches by Tuesday morning. Also Central and North-Central Oregon,
the Southern Blue Mountain Foothills and the John Day Basin could
see between 1 and 4 inches tonight into Tuesday morning. Other low
elevation locations look to see 1 to 2 inches and other mountain
locations look to see 1 to 5 inches. For now will issue winter
weather advisories for the Oregon Cascade east slopes, Central and
North-Central Oregon, the Southern Blue Mountain Foothills and the
John Day Basin zones. The snow showers will wind down Tuesday
afternoon, and be largely over by midnight Tuesday/Wednesday. 90

.LONG TERM...Wednesday through Sunday...An upper level ridge will
move east across WA and OR Wednesday. Subsidence from the ridge will
keep dry conditions over eastern WA and OR. Wednesday evening an
occluded front will move into SW WA and central OR. Cold air will
remain in place at the surface so the precipitation will be snow.
Light snow will slowly spread into the OR Cascades and central OR
Wednesday evening before rapidly spreading east early Thursday
morning. Light snow is likely Thursday. Some snow will accumulate,
even in the Columbia Basin, but how much is uncertain. Models
typically handle QPF with occlusions poorly. A wave will move along
the frontal boundary Thursday night with more snow likely. Showers
will persist Friday through Sunday, especially in the mountains, as
a series of disturbances move across the area in westerly flow.
The difficult part of the forecast is how fast the overrunning warm
air mixes to the surface Friday or Saturday. For example the GFS
ensemble MOS guidance for high temperature at Yakima ranges from 52
to 27. The higher elevations should mix out Friday and show the most
increase in temperatures. The Yakima and Kittitas Valleys usually
mix out last so the temperatures may not increase until Saturday. I
tried to keep the forecast highs slower to rise in the lower
elevations of the Columbia Basin and surrounding valleys.  Coonfield

&&

.AVIATION...12Z TAFS...VFR conditions will persist this morning.
A storm system will spread mid level clouds over the area from west
to east during the morning hours...with a mixture of light snow and
rain developing at KDLS and KYKM by midday. The mix of snow and rain
will spread east across the area during the afternoon and then change
to all snow this evening. This light snow or mixture of snow and
rain will bring MVFR CIGS/VIS to all terminals by late afternoon or
evening...with tempo IFR conditions also possible in heavier
showers. The shower activity is expected to taper off at KYKM and
KDLS in the late evening, while continuing elsewhere. Winds will
generally be light and of varying directions.  76

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  24  30  15 /  30  60  30  10
ALW  40  27  30  20 /  30  60  30  10
PSC  41  27  35  16 /  40  70  20   0
YKM  39  22  34   9 /  40  40  10   0
HRI  41  26  34  14 /  40  50  20  10
ELN  36  21  31  11 /  60  40  10   0
RDM  37  20  29   2 /  40  60  60  10
LGD  35  23  29  10 /  40  50  30  20
GCD  35  24  28   8 /  40  60  60  10
DLS  42  29  36  20 /  50  40  20   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Tuesday for ORZ505-508-510-511.

     Winter Weather Advisory from 1 PM this afternoon to 10 AM PST
     Tuesday for ORZ509.

WA...None.
&&

$$

90/76/76



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