Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 132209
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
209 PM PST Wed Dec 13 2017

.SHORT TERM...Tonight through Saturday...A ridge of high pressure
continues to reside just offshore WA/OR, amplifying slightly this
afternoon. Low clouds persist across the lower elevations mainly
under 3500 feet, with mostly clear skies above that. The base of the
low cloud deck is about 2500 feet above sea level, thus the deck is
reaching the ground as freezing fog across portions of north-central
OR as far south as RDM, as well as across the southern foothills of
the Blue Mountains. With coverage of the thickest fog a bit more
limited than the past few days, the freezing fog advisory was
allowed to expire at noon today. Think the areal coverage of fog
across the aforementioned areas will increase some tonight, but
with some uncertainty of dense fog formation, will not reissue the
advisory for now. Low temperatures tonight will range from the
mid 20s to lower 30s under the inversion, with mainly teems to
lower 20s above the inversion. High temperatures Friday will
remain in the lower 30s for the lower elevations, with mainly 40s
for the higher elevations. Conditions will change little through
Thursday Night as the ridge will only slowly weaken with the
approach of the next system.

Friday/Friday Night...Models are in general agreement with the
approach/passage of a trough of low pressure/associated surface cold
front. The frontal boundary will move onshore coastal WA/OR early
Friday morning, crossing eastern WA/OR Friday afternoon, and exiting
to the east Friday Night. The front will not be particularly strong,
and given the W/NW trajectory of the system, enough precip
shadowing will take place for PoPs to be limited to chance for the
Columbia Basin/Blue Mountain foothills, with little/no precip for
the Yakima/Kittitas Valleys as well as for most of Central Oregon.
Upslope flow across the eastern mountains will yield likely PoPs
Friday afternoon/evening. With a relatively weak front, the potential
exists for a limited scouring of the low level cold dome. Thus, have
trended high temepratures down for the lower elevations, nudging
toward the colder ECMWF guidance, which yields mid 30s for the lower
elevations. In addition, with the slightly slower movement of the
frontal boundary, have delayed the temperature warmup until late
afternoon for the lower elevations. This will result in the
potential for the initial precip in early/mid afternoon being in the
form of freezing rain. With moisture limited, think any ice
accumulations will be minor. Snow levels will lower to around 4000
feet Friday afternoon and to around 3000 feet Friday Night, so
expect an inch or two of snow across the eastern mountains. The
system will exit to the east overnight, with most of the precip
ending.

Saturday...Ridging will rebuild over the eastern Pacific as the
trough exits well to the east. This will result in a chilly NW flow
regime across the local area. Enough residual moisture will be
present in the upslope flow to wring out a few snow showers across
the eastern mountains with snow levels of 1500-2500 feet. Highs
Saturday will range from the lower/mid 40s for the lower elevations,
to mainly 30s for the mountains.

.LONG TERM...Saturday night through Wednesday...A series of weak
waves will move through an offshore upper level ridge Saturday
through Monday. This will weaken the ridge with time. These waves
will keep mostly cloudy skies with rain and snow along the Cascades
Saturday night and Sunday. By Sunday night a few showers may develop
in the northeast OR mountains. Uncertainty increases for Tuesday and
Wednesday. Over the past day GFS runs have varied between deep
troughing and flat ridging. The 12Z GFS moves a warm front through
WA and OR Tuesday then has a trough/cold front Wednesday. The 12Z
ECMWF moves the upper level ridge over eastern WA and OR Tuesday
then slowly develops an upper level low offshore with most of the
precipitation along and west of the Cascades Wednesday. I kept a
slight chance of rain and snow in the mountains Tuesday then
increased pops and coverage Tuesday night through Wednesday night as
the tendency toward troughing increased in both models. Coonfield

&&

.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

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  25  29  26  35 /   0   0   0  40
ALW  30  33  30  37 /   0   0   0  50
PSC  31  34  30  36 /   0   0   0  30
YKM  31  32  30  35 /   0   0   0  10
HRI  29  32  29  36 /   0   0   0  30
ELN  29  30  28  36 /   0   0   0  10
RDM  15  31  14  47 /   0   0   0  20
LGD  24  35  27  37 /   0   0   0  40
GCD  27  44  28  44 /   0   0   0  30
DLS  32  35  32  41 /   0   0   0  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for ORZ041-044-
     050-505-507>511.

WA...Air Stagnation Advisory until 10 AM PST Friday for WAZ024-
     026>029-520-521.

&&

$$

80/76


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