Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 131725 AAA

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
924 AM PST Wed Dec 13 2017

Updated Aviation Discussion

.UPDATE...A weak shortwave riding along the eastern periphery of
upper level ridging has resulted in light freezing rain and snow
from the Columbia Basin eastward overnight, with as much as an
inch of snow across portions of Wallowa County. The shortwave
is exiting to the east this morning, and will continue to push
east through the remainder of the day, with upper ridging
amplifying just offshore WA/OR. Residual light precipitation
across far eastern OR will end by late morning, with dry
conditions from this afternoon into Friday areawide. Areas of
dense freezing fog persist across north central OR, and will
monitor for extension of the advisory that is currently in
effect until noon today.


.AVIATION...18Z TAFs...Low level inversions remain across the
Interior Northwest this morning. These will persist through the next
24 hours. As such, ceilings at 015-025 will persist through the
period at KDLS KPSC and KYKM. IFR conditions at KALW and KPDT will
lift to MVFR during the afternoon and decrease again this evening.
VLIFR conditions will improve at KRDM and may return again early
tomorrow morning. LIFR conditions at KBDN will improve this
afternoon...but may not return tonight since the inversion will
decrease and push the FG/ST layer back north.  76


.PREV DISCUSSION... /issued 344 AM PST Wed Dec 13 2017/

SHORT TERM...Today through Friday...An upper ridge will remain over
the Pacific Northwest for a couple more days which will continue the
current pattern of low clouds and freezing fog below 2500 to 3000
feet and generally mostly clear to partly cloudy above that.
However, this morning a weak wave is crossing through northern
Washington and influencing the northern portion of the area. Have
kept snow flurries in the forecast for this morning and this seems
prudent since so far, aside from a few high elevation snotel sites,
there haven`t been any reports of measurable precipitation. In the
last hour, Hanford is reporting trace amounts of precipitation, so
there are a few very light showers out there. By later this morning,
the wave will clear out and we will go back to our quiet pattern
with little change until Friday morning. On Friday morning, a cold
front will begin crossing the Washington Cascades. Initially there
will be a slight chance of rain, snow and a slight chance of
freezing rain as far east as Ellensburg and Yakima. The front will
cross the entire area in the afternoon with generally light
precipitation mainly in the mountains. With cold air in place in the
Columbia Basin, the concern is how fast the cold air will get
scoured out. Until it does, freezing rain will be a possibility.
Think that it will get scoured out in most areas by the evening
except the Washington Columbia Basin and Washington Blue Mountain
Foothills. So have a chance of rain and a slight chance of freezing
rain in the afternoon for the Columbia Basin and North Central
Oregon and a chance of rain and freezing rain for the Tri-Cities and
Walla Walla area. Precipitation amounts will be a few hundredths of
an inch at most. In the rest of the area there will be a slight
chance of rain in the lower elevations and a chance of rain with
snow above 5000-6000 feet In the mountains. Temperatures will
continue to be in the mid 20s to lower 30s under the inversion with
upper 30s and 40s in the mountains during the day and teens and
lower 20s  at night. On Friday highs will be in the mid 30s to mid
40s. Perry

LONG TERM...Friday night through Tuesday...The Friday system will
continue to move east into Idaho Friday evening. As such will see
some lingering precip over our eastern zones. Also should see a few
slop over showers near the Cascade crest. Should also see an
increase in winds and a final mixing of the low level air mass
across the region. Snow levels will lower to between 1500 and 2500
feet across the region. A northwesterly flow looks to become
established Saturday. Moisture within this flow could result in a
few mountain snow showers. Uncertainty increases for the Sunday
through Tuesday period. Latest data indicates that there could be
some sort of flat ridging over the region. This would limit precip
chances to near the Cascade crest. For now will follow this idea for
Sunday. Meanwhile will trim back precip chances for Monday and
Tuesday, but will continue to indicate a somewhat broader area with
a potential for precip than the latest solutions show. 90


PDT  30  25  29  26 /   0   0   0   0
ALW  33  30  33  31 /   0   0   0   0
PSC  33  32  34  31 /   0   0   0   0
YKM  33  31  32  31 /   0   0   0   0
HRI  31  29  32  30 /   0   0   0   0
ELN  33  28  30  29 /   0   0   0   0
RDM  39  12  39  13 /   0   0   0   0
LGD  37  24  35  29 /  10   0   0   0
GCD  44  26  44  29 /   0   0   0   0
DLS  37  32  35  31 /   0   0   0   0


OR...Air Stagnation Advisory until 10 AM PST Friday for ORZ041-044-

     Freezing Fog Advisory until noon PST today for ORZ508-510.

WA...Air Stagnation Advisory until 10 AM PST Friday for WAZ024-



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