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fxus66 kotx 201147 
afdotx

Area forecast discussion
National Weather Service Spokane Washington
347 am PST Sat Jan 20 2018

Synopsis...
a cool and moist westerly flow will bring snow showers to the
mountains and northern valleys today. A stronger system looks to
bring another round of snow Sunday. An active weather pattern
should continue into next week with moderate snow possible
Tuesday into Wednesday.



&&

Discussion...
today and tonight...satellite indicates a tight but potent little
short wave trough over the west side early this morning set to
eject through eastern Washington today while it weakens. This is a
set up for another unsettled round of mainly hit-and-miss snow
showers across the region this morning becoming mainly mountain
snow showers and valley/basin rain or wet non-accumulating snow
showers during the afternoon. By this evening most of this
activity will be limited to the Idaho Panhandle mountains with the
remainder of the region experiencing a short break period before
the next and more potent storm system for Sunday.

Sunday and Sunday night...clearly visible on satellite in the Gulf
of Alaska off the Queen Charlotte islands is a deep and cold
closed low pushing a feed of deeper Pacific moisture out ahead of
it. Models are similar in weakening the parent closed low into an
open trough which will remain off the coast...but will serve to
push the moisture feed into the forecast area and enhance it into
mainly snow along a spur trough ejecting through the region on
Sunday. Models all depict some good and sustained southeasterly
upslope flow into the Cascades as this wave passes...and it is in
this area as well as the Okanogan Highlands and River Valley where
a sustained period of snow is expected from early Sunday morning
through early evening. At least moderate accumulations in the
valleys and heavy mountain accumulations are expected...but there
is a distinct possibility that the valleys from the Methow to the
Okanogan and Kettle River/Republic area may receive heavy
accumulations as well. This will be dependent on how warm or near
freezing these lower elevations achieve during the day which will
have a marked effect on snow/water ratios in these lower
elevations. This uncertainty mandates issuing a Winter Storm
Watch for these areas while punting to the day shift the final
call as to whether a warning or mere advisory is justified in
these valleys...although substantial and probably heavy snow
accumulations are likely in the mountains above 3000 to 4000 feet
elevation. Travel approaching and across the area passes from
Stevens to Loup Loup to Sherman will be difficult and snow laden.

Sunday afternoon through Sunday night the focus of snow will be
over the northeast Washington and north Idaho mountains and
valleys with moderate valley accumulations and potentially heavy
accumulations in the mountains near the Canadian border. Winter
weather advisories my be needed in these valleys with slippery
conditions in moderate snow across Lookout Pass Sunday night.

In the basin the best chance for accumulating snow of 1 to 3
inches will be over the Waterville Plateau and Columbia River
valley tucked against the Cascades during the day Sunday...as well
as on the Palouse and Camas Prairie area during Sunday night wit 1
to 2 inches possible. Mainly rain or non-accumulating wet snow is
expected in the Central Basin south of Highway 2. /Fugazzi

Monday through next weekend: little has changed in the medium range
models which continue to maintain a progressive jet stream aimed
at the Pacific northwest resulting in several rounds of
precipitation and near to slightly warmer than normal conditions.
Precipitation will largely fall as snow in the mountains and be
more variable from system to system in the lower elevations.
Impacts will be minor Monday and Monday night with the weather
mainly featuring mountain snow showers in the Idaho Panhandle and
Cascade crest. Passes may experience bursts of snow under these
showers and slick travel at times. Otherwise, the focus will be on
a deeper trof passing Tuesday night through Wednesday night
bringing the next bout of moderate precipitation. The trof will
carve into the eastern Pacific as far south as central California
drawing a rich plume of moisture northward into the inland northwest.
Precipitable water values via the GFS increase near 0.60" or
higher suggesting a good chance for rain for much of the Columbia
Basin and potential for moderate snows followed by a transition
toward rain in the northern valleys. Exactly how high snow levels
become in the warm sector Wednesday night is a tough call at this
time with midlevel temperatures differing by as much as 5c on this
evening's deterministic models. The forecast largely followed a
blend of the gefs and European but could be warmer if the 00z GFS
is accurate. Nonetheless, cannot rule out the potential for a
messy Wednesday morning commute for locations along and north of
Highway 2. Once the trof crosses inland Thursday, the air mass
will begin to cool and dry and weather will transition back to
mountain snow showers. 850mb temperatures within the trof will be
the coldest we see all week dipping into the -4 to -5c range
suggesting a good shot for freezing surface temperatures each
Friday and Saturday morning.

By next weekend, models are hinting at an atmospheric river slamming
into the northwestern US. The exact trajectory and duration is
uncertain at this time (ie, southwest or west) but the potential
for a mild and wet pattern exist. Gusty winds, moderate to heavy
precipitation, increasing snow levels, and flooding are all items
we must consider during these atmospheric highways of subtropical
moisture and forecast trends should be closely monitored in the
coming week. /Sb

&&

Aviation...
12z tafs: a compact but vigorous short wave trough over the
Cascades at 12z will pass through the region between 12z and 21z
today promoting morning scattered snow showers with generally MVFR
ceilings and visibilities mainly over the eastern taf sites while
the western sites will benefit from some downslope off the
Cascades for isolated snow showers with only a marginal chance of
the keat and kmwh taf sites being impacted. A break period
tonight ahead of the next storm system will bring dry conditions
but patches of stratus will develop overnight mainly impacting the
kgeg area taf sites. Ceilings will also lower overnight at keat
with an onset of -sn possible by the end of the taf period. /Mjf



&&

Preliminary point temps/pops...
Spokane 37 29 36 29 37 27 / 70 10 60 100 20 20
Coeur D'Alene 37 28 36 30 36 27 / 70 40 20 100 30 40
Pullman 37 28 39 32 39 29 / 40 20 30 80 20 20
Lewiston 43 30 44 33 44 32 / 30 10 20 60 10 10
Colville 38 28 34 28 36 24 / 30 20 70 100 20 20
Sandpoint 35 28 34 30 35 26 / 60 40 30 100 50 40
Kellogg 34 26 34 29 34 28 / 60 60 20 90 50 50
Moses Lake 43 30 40 29 42 28 / 20 10 90 30 10 10
Wenatchee 37 28 33 27 37 26 / 10 20 90 40 10 10
Omak 35 29 33 25 35 24 / 10 30 100 60 10 10

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...Winter Storm Watch from late tonight through Sunday evening for
east slopes northern Cascades-Okanogan Highlands-Okanogan
Valley.

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