Area Forecast Discussion
Issued by NWS Pendleton, OR

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494
FXUS66 KPDT 141745 AAA
AFDPDT

Area Forecast Discussion...Update
National Weather Service Pendleton OR
945 AM PST Thu Dec 14 2017

Updated Aviation Discussion

.UPDATE...An upper level ridge remains near the coast with areas
of low stratus persisting over the Lower Columbia Basin and
adjacent areas. I made minor adjustments in temperatures for the
morning update. The focus for the afternoon forecast will be on
possibility of freezing rain with the front tomorrow. Coonfield

&&

.AVIATION...18Z TAFs...Strong low level inversions with cloud tops
between 2500 and 4500 feet will persist across the Interior
Northwest through the next 12-18 hours. An approaching system will
begin to force the inversions to lower late in this period. As such,
widespread low clouds and fog will continue under the inversions.
Expect MVFR and IFR cigs/vis to persist at KDLS, KYKM, KPSC, KALW
and KPDT. KRDM and KBDN will see VLIFR or LIFR conditions, though
there is some chance for some break out this afternoon (could be very
brief at KRDM). Winds will remain less than 10 kts. 90

&&

.PREV DISCUSSION... /issued 300 AM PST Thu Dec 14 2017/

SHORT TERM...Today through Saturday...An upper level ridge remains
centered along the coast this morning though it will be moving
eastward today as a trough approaches the area. The ridge will keep
another 24 hours of the current weather pattern. The lower
elevations will continue to have an inversion and a stratus layer
between 2500 and 3500 feet. This will keep temperatures in the
narrow range from the mid 20s to lower 30s through tonight. While
freezing fog will be possible between 2500 to 3500 feet, it will be
limited to central and north central Oregon and the Southern Blue
Mountain Foothills. Above the inversion, mostly clear skies will
continue, perhaps becoming partly cloudy tonight. Temperatures will
mainly be in the mid 30s to mid 40s today and in the mid teens to
mid 20s tonight.

By late tonight, the ridge will be flattened and the axis will be in
Idaho. The trough will be just offshore and a front will be
approaching the Cascades. Through mid morning precipitation will be
restricted to the Washington Cascade crest. In the afternoon
precipitation will proceed across the area. The Cascades will have a
chance of rain and snow and perhaps some pockets with a slight
chance of freezing rain. There will be significant rain shadowing
east of the Cascades and in the Yakima and Kittitas Valleys. The
Columbia Basin west of the Tri-cities and central and north central
Oregon will have little if any precipitation. Further east, a wintry
mix of rain, freezing rain, mountain snow is expected through early
evening before switching over to rain and mountain snow. The best
chance of freezing rain will be in the Blue Mountain Foothills and
the Grande Ronde and Wallowa Valleys. The front is not a strong one
and precipitation amounts will be generally under a tenth of an inch
with up to a quarter inch in the mountains. Ice accumulations will
be a few hundredths of an inch but will be sufficient to create
difficult driving conditions. Do expect that the cold air in the
lower basins and valleys will mix out late in the afternoon or early
evening and temperatures will rise to the mid 30s while further
south temperatures will be in the lower to mid 40s.

The trough will exit the area by Saturday morning and an offshore
ridge will develop a northwesterly flow over the area. An upslope
flow will keep a chance of snow showers in the mountains in the
morning and a slight chance in the afternoon while the rest of the
area will be dry. Temperatures will be in the upper 30s to mid 40s.
Perry

LONG TERM...Saturday night through Thursday...Models are in good
agreement with the pattern in the extended period. An upper ridge
will weaken as an upper low pressure system forms in the four
corners region on Saturday and Sunday. Moisture will move over the
top of the ridge into the CWA for mainly light precipitation in the
mountains. Snow levels will lower to pass levels with light snow
over the passes of the Cascades and eastern mountains. The lower
elevations will remain mostly dry at first. However the atmosphere
will gradually moisten up during the extended period with
precipitation...mainly light rain...becoming possible in the lower
elevations. Late in the extended period an upper low will drop down
out of the gulf of Alaska and bring colder air and lowering snow
levels by Wednesday and Thursday. There may be enough cold in the
lower elevations for snow or mixed snow and rain in the lower
elevations by the end of the extended period. Models are in good
agreement with this upper trough so will increase pops late in the
extended period over all areas of the CWA with increasing coverage
of rain and snow. These systems will keep the atmosphere mixed with
better ventilation to prevent fog and low stratus. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  30  27  35  31 /   0   0  40  30
ALW  33  30  37  34 /   0   0  50  40
PSC  34  29  36  32 /   0   0  30  20
YKM  33  29  35  29 /   0   0  10  10
HRI  32  29  36  31 /   0   0  30  20
ELN  33  28  36  29 /   0   0  10  10
RDM  33  16  47  23 /   0   0  20  20
LGD  35  27  37  30 /   0   0  40  50
GCD  43  29  44  29 /   0   0  30  50
DLS  35  32  41  36 /   0   0  10  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.

&&

$$

76/90/90



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