Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 071156

Area Forecast Discussion
National Weather Service Spokane WA
356 AM PST Wed Dec 7 2016

A cold air mass will settle over the Inland Northwest this morning
through Thursday. Some localized light snow flurries will be
possible this morning, otherwise conditons will be dry.
Widespread light snow will overspread the region Thursday
afternoon and last through Friday, and this snowy pattern will
likely continue through early next week.



...Chilly today and tonight...followed by widespread snow Thursday
night and Friday...

Today and tonight...Northerly flow over the region this morning
as the upper level trough exits the region and high pressure noses
in from the west. This puts the area in a very chilly northerly
flow. Winds down the Okanogan valley and to a lesser degree the
Purcell trench are still gusting and expect that to linger through
at least the morning hours until the surface gradient fills. Some
moisture is moving into the region resulting in a mid level deck
and will result in some isolated snow flurries over the area
through the morning. But the big story will be the cool
temperatures. Areas that were clear through the late evening our
within a few degrees of the forecasted lows, however the ares the
clouded up earlier this evening will have temperatures several
degrees warmer then forecasted. Model guidance shows this moisture
moving out of the area early in the afternoon and temperatures
will have a hard time rebounding. Afternoon highs will be in the
low teens to mid 20s and 10-12 degrees below average. Clearing
skies and lighter winds will result in strong radiational cooling
again tonight. The cooling will be more pronounced in areas that
have decent snow cover. As a result temperatures will drop into
the single digits to the lower teens fro another frigid night.

Thursday and Thursday night...A flat ridge of high pressure will
track across the region Thursday and Thursday night. This will
switch the flow to southwesterly and tap into an impressive
atmospheric river. It does look like the deepest moisture stays
south of our area, but still there will be plenty of moisture to
deal with. Models have been hinting at a later start of the
precipitation for the past few runs and this forecast follows that
thinking. So light precipitation will begin across the southwest
in the afternoon, then increase in intensity Thursday night and
Friday. The lifting will be initiated by strong isentropic upglide
along the 290-300k surface. In addition the winds back around to
the east-southeast for orographic enhancement into the east slopes
of the Cascades and moving east into the northern mountains
Thursday night and Friday. With the very chilly air in place snow
levels will be at valley/basin floors so precipitation will be as
snow. The 00z models have come in with less QPF with this run.
With the stratiform precipitation the average snow ratios are
around 12 to 14:1, but with the cold air in place these were
bumped up to 13 to 18:1. As such snow accumulations through Friday
afternoon will be a bit less than previous thinking. But it is
early yet and this will likely change over the next 36 hours or
so. A Winter storm Watch is in place and that will continue, just
with the later start and slightly lower numbers. Tobin


Friday and Friday night: The warm front will shift and
potentially stall out over the northern portion of the forecast
area. The amount of warm air upglide and isentropic lift will
decrease a bit through the morning and into the afternoon. This is
expected to result in lighter snow accumulations than what will
be observed through Thursday night and into Friday morning. Models
are in pretty good agreement with warm air topping over the
Northeast Blue Mtns and raising snow levels up off of the L-C
Valley by the time snow winds down or with snow transitioning over
to rain. A trailing cold front boundary will cross east of the
Cascades Friday afternoon. This boundary does generate precip
across southeast WA with precip returning for southeast WA and
into the L-C Valley and Camas Prairie. The colder air behind this
front will drop snow levels back down into the L-C Valley.
Westerly flow behind the front will create an opportunity for a
precip shadow in the lee of the Cascades. Best precip chances
Friday night will be up across the Cascade crest and over the
Central Panhandle Mtns where orographics will play a larger role.

Saturday through Sunday night: The upper level low pressure
system that brings through the moist overrunning event Thursday
into Friday is progged to move in on Saturday. Models show subtle
differences on the track of the surface low, which will impact
where the axis of heaviest snow will be and how strong winds will
get off of the Northeast Blue Mtns. The atmosphere will also be
more unstable with colder air aloft in place with higher intensity
snow bands possible compared to the stratiform snow expected
Thursday into Friday. The general consensus off of the models is
to track the low across the southern portions of WA state. This
would place best chances for snow across the central portions of
the WA Cascades, over southeast WA and into the southern to
central ID Panhandle. Flow pattern will become northwest behind
the exiting low Saturday night. Moist and unstable upslope flow
will keep snow showers going over the Central Pahandle Mtns that
look to linger into Sunday. Heavy snow will be possible along the
Cascade crest and over the Central Panhandle Mtns with between
8-16 inches possible. Confidence in exact snowfall accumulations
are low since they will be dependent on track of surface low.
Highest snow accumulations are expected along the Cascade crest
and in the Central Panhandle Mtns where heavy snow will be
possible. /SVH

Monday and Tuesday: Models still disagree on the beginning of next
week. The GFS keeps moist zonal flow until Tuesday with precip
hanging around into Tuesday morning while the EC`s drier
northwest flow ends the precip by early Monday, save for
orographically forced showers in the panhandle mountains. Went
with a compromise taking the snow showers out of the lowlands by
late Monday with just a slight chance on Tuesday. Tuesday night
into Wednesday looks like the driest period before the next system
arrives late Wednesday in an atmosphere likely cold enough for
snow in most places. I lowered temps a few degrees but have not
seen enough agreement to go with the frigid temps the GFS is
calling for. /bwilliams


12Z TAFS: Mid level moisture is moving through the region down the
back side of the exiting trough...and this is resulting in a
widespread stratus deck bkn-ovc 040-060. There is also some very
localized...but light snow showers/flurries mainly in the up-
sloping areas vcnty of KGEG-KCOE corridor and possibly south to
KPUW. Little if any accumulations are expected. This moisture
should exit it out of the region this afternoon. Conditions will
remain VFR through 12z Thursday. Otherwise well below normal
temperatures through Thursday. Tobin


Spokane        20  10  24  22  28  23 /   0   0  30  90  90  50
Coeur d`Alene  20   7  24  23  29  24 /   0   0  10  90 100  60
Pullman        24  13  25  25  34  28 /   0   0  60  90  70  80
Lewiston       25  14  30  29  38  29 /   0   0  80  90  60  90
Colville       21   8  27  24  27  21 /   0   0  10  90  80  30
Sandpoint      16   7  23  18  25  22 /   0   0  10  90 100  50
Kellogg        15   4  21  19  29  25 /  10   0  10 100 100  90
Moses Lake     26  12  29  21  30  20 /   0   0  80  90  40  50
Wenatchee      25  14  24  21  29  21 /   0   0  90  90  50  40
Omak           23  10  26  21  28  18 /   0   0  30  80  60  30


ID...Winter Storm Watch from Thursday evening through Friday
     afternoon for Coeur d`Alene Area-Idaho Palouse-Northern

     Winter Storm Watch from Thursday evening through Saturday
     morning for Central Panhandle Mountains.

WA...Winter Storm Watch from Thursday evening through Friday
     afternoon for Moses Lake Area-Northeast Mountains-Okanogan
     Highlands-Okanogan Valley-Spokane Area-Upper Columbia Basin-
     Washington Palouse-Waterville Plateau-Wenatchee Area.

     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for East Slopes Northern Cascades.


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