Area Forecast Discussion
Issued by NWS Spokane, WA

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643
FXUS66 KOTX 142243
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
243 PM PST Thu Dec 14 2017

.SYNOPSIS...
Many of the lowlands across eastern Washington and north Idaho
will continue to be covered by low clouds. Chances for light
precipitation increase Friday with snow over northeast Washington
and north Idaho and a chance of a wintry mix in the Cascade
valleys, and Columbia Basin. It will be drier for Saturday before
another weak weather system brings light snow on Sunday. A
stronger and wetter system arrives next Tuesday into Wednesday.

&&

.DISCUSSION...
Tonight through Friday night: An occluded front will push across
the region on Friday. Precipitation type will be somewhat of a
challenge as the atmosphere moistens down from the top. Near the
surface, we will continue to see a blanket of low stratus clouds
over the region. Weak upslope flow with mid levels moistening in
the lee of the Cascades is expected to result in flurries and/or
freezing drizzle overnight. Chances for freezing drizzle will
shift across the northern basin and into the northern mountain
valleys Friday morning. This light freezing precipitation may
result in a light glaze of ice with slick conditions for the
morning commute. The front will cross the Cascades by the late
morning and afternoon hours with the vertical atmospheric column
becoming saturated and below freezing for precipitation type then
to transition over to snow for most areas. Best chances for snow
will be across extreme eastern WA and into the ID Panhandle. A
northwest trajectory to the trof will favor the southeast portion
of the region where orographic enhancement will take place in the
Central Panhandle Mountains in the afternoon/evening and then over
the Camas Prairie Friday night. Once the atmosphere saturates and
much of the region will wet bulb down below freezing. The warm
nose aloft will generally be as far north as about Moses Lake to
La Crosse, so mainly right along State Route 26 and points
southward. These areas will see predominately freezing rain. Ice
accumulations as far north as Washtucna will be light up to around
a tenth of an inch possible. For the most part, impacts from ice
accumulation will remain south of our area with snow impacts more
likely. Winter Weather Advisories for snow will be issued with
between 1 to 3 inches of snow expected on the Idaho Palouse to
the Central Panhandle Mountains and Camas Prairie. /SVH

Saturday through Monday: Saturday will generally be a dry day as
Friday`s wave departs to the southeast and a moist laden warm
front shifts into Western WA. The warm front will spread light
snow into the Cascades by the afternoon hours and continue to
expand east into the remainder of Ern WA and N ID Saturday night
and Sunday. This will not be a heavy precipitation event but
steady and widespread with a good influx of moisture and
isentropic ascent ahead of a warm front. Precipitation will start
off as snow for most elevations Saturday night and Sunday morning
but a punch of warmer air arriving on Sunday will lead to the
possibility for a brief mix or switch to rain south of Hwy 2. A
cold front front swings through Sunday night swinging midlevel
winds to the west/northwest. This will help shut off precipitation
in the lee of the Cascades and Western Basin but continue for
orographically favored areas such as the Cascade Crest and Idaho
Panhandle, at times expanding into the eastern third of WA. Snow
accumulations through Sunday will be light with the potential for
1-2 inches in NE WA and N ID, heavier amounts near the Cascades
Crest, and lightest in the Columbia Basin. For Sunday night, light
to moderate snow amounts will be possible for the Idaho Panhandle
and moderate to heavy along the Cascade Crest. Snow levels in
North Idaho will waver between 3500-4000 feet near the Camas
Prairie to 2000 feet or lower north.

Monday night through Wednesday: A shortwave dropping out of the
Gulf of AK will swing into the region. This wave will track along
a tight thermal gradient leading to deep cyclogenesis off the
coast with the potential for the cyclone to remain deep or even
deepen slightly on its track inland. We have seen this advertised
in the models in one form or another for several days and they
continue to dial in the track and impacts from this storm. At this
time...loose agreement amongst the North American and ECMWF
models is for the surface low to track along the WA-British
Columbia Border Tuesday afternoon/Tuesday night and depart on
Wednesday. ECMWF is a bit further north while the Canadian is
further south. GFS is a compromise. Strong midlevel winds
accompanying the system will draw warmer air from the south and
given the current track, suggest most lowlands south of Hwy 20
will have a potential for rain until Tuesday night when the low
reaches N Idaho and colder air wraps into the system. Weather
hazards are likely to shift but current guidance suggest moderate
to heavy mountain snow accumulations across the north (North of
I-90) through Tuesday evening accompanied by wide variety in
accumulation potential for in the lower elevations. Snow will
shift into the lower Idaho Panhandle Tuesday night and early
Wednesday with the potential for moderate accumulations before the
system departs Wednesday night.

Wednesday night through Friday: High pressure amplifies off the
Coast placing the Inland NW under a dry and colder northerly flow
regime. If the ridge axis remains just west of 130W as currently
predicted, the northerly jet will align along the ID/MT border
promoting enough mixing to yield sunny skies, cool temperatures,
and periods of breezy north to east winds. However, if the ridge
axis sets up further east, the midlevel subsidence could have more
influence leaning toward stronger inversions. /sb

&&

.AVIATION...
18Z TAFS: Low clouds continue to blanket the area. The cigs in
general are MVFR with the exception being KPUW. These conditions
should persist for the next 24 hours. There is the potential that
the low clouds may clear out of KPUW in the afternoon. Confidence
is low as to when this would happen if it does. /SVH


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        26  32  25  30  25  35 /   0  80  20  10  20  40
Coeur d`Alene  26  33  26  31  24  35 /   0  90  40  10  20  50
Pullman        27  34  27  31  24  35 /   0  90  80  30  10  40
Lewiston       29  37  29  37  25  39 /   0  70  70  20  10  20
Colville       29  32  25  34  26  32 /   0  40  10   0  20  40
Sandpoint      26  33  26  32  24  33 /   0 100  30  10  30  60
Kellogg        26  32  25  29  21  32 /   0 100  70  30  30  50
Moses Lake     28  33  26  36  28  36 /   0  20   0   0  10  30
Wenatchee      29  34  28  38  29  36 /   0  10   0  10  10  30
Omak           29  33  25  34  28  33 /   0  10   0   0  20  30

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 10 AM PST Friday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern
     Panhandle.

     Winter Weather Advisory from 1 PM Friday to 4 AM PST Saturday
     for Central Panhandle Mountains-Idaho Palouse.

     Winter Weather Advisory from 4 PM Friday to 7 AM PST Saturday
     for Lewis and Southern Nez Perce Counties-Lewiston Area.

WA...Air Stagnation Advisory until 10 AM PST Friday for East Slopes
     Northern Cascades-Lower Garfield and Asotin Counties-Moses
     Lake Area-Northeast Blue Mountains-Northeast Mountains-
     Okanogan Highlands-Okanogan Valley-Spokane Area-Upper
     Columbia Basin-Washington Palouse-Waterville Plateau-
     Wenatchee Area.


&&

$$



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