FXUS66 KPDT 171750

Area Forecast Discussion
National Weather Service Pendleton OR
940 AM PST Sun Dec 17 2017

.UPDATE...A weak shortwave in northwest flow aloft is crossing
eastern WA/OR, producing areas of rain with pockets of freezing
rain across the Columbia Basin and the Blue Mountain Foothills.
Radar trends show the band of precip is weakening as it pushes
east toward the eastern OR mountains. Have increased PoPs through
mid morning to chance/low end likely. The pockets of residual
cold air/freezing rain will likely scour out with the passage
of this wave. Have issued a short term forecast to address
potential minor impacts in freezing rain as a minor glaze
cannot be ruled out. Additional weak waves will cross the area
late today/tonight, keeping upslope precip chances for the
foothill/mountain locations. Snow levels will rise to 3500-4500
feet for eastern sections and 4500-6000 feet for western sections
by tonight. Highs today will range from the upper 30s to mid 40s
for the lower elevations to mainly 30s for the mountains.


.AVIATION...18Z tafs. Columbia basin taf sites will have ceilings
mostly bkn-ovc 030-070 today with light rain showers at times.
Some lower ceilings and visibility at dls ykm with bkn-ovc 010-030.
Higher level cloud deck at rdm bdn with bkn 100-150. Winds 5-10kt.
Little change for the overnight period although southwest winds will
increase a bit into the 5-15kt range. Front moving into the region
from the north on Monday with increasing chances of rain. 94


.PREV DISCUSSION... /issued 650 AM PST Sun Dec 17 2017/

SHORT TERM...Today through Tuesday night...A weak weather
disturbance is currently moving across the CWA from northwest to
southeast with a weak cold front. This has produced some isolated
light sleet and snow from the Washington Cascade east slopes into
the Lower Columbia Basin. This precipitation will work its way
across the Blue Mountains this morning and then exit the region by
this afternoon. Then a series of weak weather disturbances will move
over the top of a flat upper ridge over the Pacific Northwest
through Tuesday...followed by a stronger system Tuesday night into
the extended period. Precip type will be mainly rain...with snow in
the mountains above snow levels which will rise to about 5000 feet
MSL. The rain will change to mostly snow by Wednesday as an upper
trough moves into the region with a colder air mass before
conditions dry out in the extended period. The wettest period will
be Tuesday night into Wednesday. May need winter weather highlights
later this week as that system moves into the region. For now will
not issue anything that far into the future due to model
uncertainty. Winds will be generally light through the short term
period. There is patchy to areas of freezing fog in the Lower
Columbia Basin and adjacent valleys. The freezing fog is locally
dense in the Washington Basin and may need a freezing fog advisory
there for this morning. For the time being do not believe the fog is
widespread enough for any freezing fog advisories, but will monitor
it this morning and issue an advisory by daybreak if needed. 88

LONG TERM...Wednesday through Saturday...Models show a strong upper
level trough moving through the region Wednesday with snow levels
nearly to the floor of the Lower COlumbia Basin resulting in a mix
of rain/snow in the Basin and snow in the mountains. The trough
exits forecast area Wednesday evening with snow showers tapering off
and ending before midnight in the mountains. Then a large longwave
mid/upper level ridge of high pressure returns to the interior
Pacific Northwest for the next period of dry conditions through
Saturday. Polan


PDT 41 35 50 36 / 20 20 20 20
ALW 43 37 49 38 / 60 30 30 30
PSC 39 35 49 37 / 30 10 10 20
YKM 37 31 46 33 / 20 10 20 30
HRI 40 36 51 35 / 30 10 10 20
ELN 37 32 44 32 / 40 10 20 30
RDM 44 30 49 28 / 10 10 10 10
LGD 38 35 42 35 / 20 30 30 30
GCD 40 35 46 33 / 10 20 20 20
DLS 44 39 50 38 / 20 20 30 30




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