Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 091042

Area Forecast Discussion
National Weather Service Spokane WA
242 AM PST Fri Dec 9 2016

Multiple weather systems will produce periods of snow through the
weekend and into early next week. Brisk north winds are expected
on Monday down the Okanogan Valley the will spread out over the
western basin. Temperatures will become very cold by Tuesday that
will continue through next week. Many areas will see overnight
temperatures fall into the single digits with some locations
seeing temperatures below zero for early to mid week.


Today...The warm front that brought light to moderate snow to the
region overnight is rapidly moving northeast and out of the area.
Some light snow will still be possible across the northeast
mountains and the central and north Panhandle in up-sloping flow
for the next few hours. And snow will continue for the Cascades as
well. Otherwise the snow is pretty much done, at least for this
morning. With that I will take down the snow advisory with the
morning forecast. A quick moving weak front will tap into some
lingering moisture for a chance of light snow for most places this
afternoon. The focus will again be across the Cascades and the
Panhandle mountains, but just about anywhere could see some light
snow at times through the day. We will still see some elevated
winds down the Purcell Trench this morning.

Tonight and Saturday...The upper level low currently off the B.C.
coast will begin to move into the Pac NW this evening and move
through the region overnight. Another warm front will swell up
from the south with a cold front passage expected to track from
west to east Friday night. This will bring another round of light
to moderate snow to the forecast area. The westerly flow will
shadow out the lower east slopes and a majority of the Columbia
basin. But the remainder of the area should get another shot of
light to moderate snow. At this time it looks like the heaviest
accumulations will be across the eastern zones over night tonight
then decreasing rapidly after sunrise from west to east.

* Snow: We are looking at 12-18 hour accumulations of 7-10 inches for
  the Cascades, 2-4 inches for the Cascade valleys. Further to the
  east we could see 2-4 inches near the West Plains-Spokane-Coeur
  D`Alene areas and south towards the Palouse.  For the central
  Panhandle valleys 3-6 inches and for the mountains 6-8 inches will
  be possible. For the remainder of the area 1-2 inches or less will
  be possible .

* Impacts: Moderate snow will result in hazardous driving
  conditions Saturday morning. By ate morning temperatures will be
  warming enough for compaction and melting. Tobin

Saturday Night through Monday Night: The Inland Northwest will be
sandwiched between two air masses. There is the much colder and
drier polar continental air masses trying to push in from the
north and the more mild and moist Pacific Maritime air mass
pushing up from the south. Models are in general agreement with
these two air masses colliding over the region without one taking
over the other. The northern branch of the jet directed out of
Canada will eventually win out, but this will happen after the
weekend. Until then, we will continue to see chances for snow over
the region.

An upper level low pressure system will swing through Saturday
night. It looks like models are beginning to come into agreement
with the track of the upper level low. It will start out as a
closed low northwest of Vancouver Island today. It will fill and
swing through as an open wave with the best dynamics aloft
occurring in the evening hours. Lower levels across eastern WA
will see its best isentropic ascent at lower to mid levels during
the evening as well with snow expected across much of the eastern
WA. The Cascade crest will likely see snow, but will not as
favorable in the lee of the Cascades due to downsloping flow.
Snow will pick up across the eastern basin and much of extreme
eastern WA. The Panhandle will see snow as well with this wave and
will continue longer into Sunday as orographics remain in play.
The Central Panhandle Mtns will likely see snow showers continue
into at least Monday. The flow pattern will turn more
northwesterly Sunday night through Monday. Not only is this a
favor flow pattern for the Central Panhandle Mtns, but it will
turn a bit more favorable for the Camas Prairie as well.

Snow ratios figure to be above climatology (13:1); the air mass
is colder than normal and lapse rates between 700-500 mbs will be
at around 8 C/km with unstable sounding profiles. I wouldn`t be
surprised to see snow ratios up near 20:1, especially for the
Central Panhandle Mtns. The high snow ratios could be a
determining factor for heavy snow across this area. The Cascade
crest will likely see heavy snow as well. All other areas are more
likely to see advisory level snow amounts; although, the Camas
Prairie could see heavy accumulations as the flow pattern turns
northwesterly, which can be a very favorable patter for this area
in this kind of set up.

Tuesday and Wednesday: A surge of cold Canadian air will
infiltrate across much of the region by mid week. There are some
timing differences amongst the 00Z model guidance. The GFS, DGEX
and Canadian solutions are in pretty good agreement with each
other that this will occur by Tuesday. The ECMWF model holds off
until Wednesday. The forecast will discount the much slower ECMWF
resulting in drier weather and a trend toward some bitterly cold
temperatures. The cold air advection is expected to be accompanied
by some breezy winds. The cold Canadian area of high pressure
that develops will be centered over BC. This will create a strong
pressure gradient down the Okanogan Valley with gusty north winds
possible. We will continue to portray the strongest winds
occurring on Monday, but will be delayed until later in the week
if the ECMWF solution pans out. Temperatures will become very cold
with much drier air in place by Tuesday. /SVH

Wednesday night and Thursday: Dry northerly flow between the
exiting low in Canada and the approaching ridge over the E.
Pacific brings even colder temps in by mid-late week with temps
looking to be 10 to 15 degrees below normal. All but the EC have
dropped any hint of another wet system for Wednesday night-
Thursday, so for now keeping pops low through the end of the week.
This of course is in the very extended period so confidence is
low at this point. /bw


06Z TAFS: A warm front moving southwest across the aviation area
is weakening as it does so but is still managing to produce light
snow. This same frontal zone will almost clear the northern
portion of Washington but then change course and come back down
and spread more snow over the aviation area after 00Z Saturday.


Spokane        26  24  33  27  32  22 /  30  80  40  70  40  50
Coeur d`Alene  26  24  33  28  33  23 /  30  80  70  80  70  50
Pullman        30  27  34  29  34  26 /  40  80  70  70  70  70
Lewiston       36  30  39  31  39  28 /  50  80  30  60  50  50
Colville       27  24  32  26  33  21 /  30  60  30  70  30  20
Sandpoint      26  25  32  28  32  25 /  30  70  60  80  60  30
Kellogg        27  25  31  27  31  24 /  50  90  90  90  90  90
Moses Lake     27  19  32  24  34  21 /  30  60  20  30  10  10
Wenatchee      26  22  30  25  32  24 /  50  70  30  30  10  10
Omak           25  21  30  23  31  19 /  50  70  20  50  10  10


ID...Winter Storm Watch from this afternoon through late Sunday night
     for Central Panhandle Mountains.

WA...Winter Weather Advisory until 4 PM PST Saturday for East Slopes
     Northern Cascades.

     Winter Weather Advisory until 10 AM PST this morning for
     Waterville Plateau-Wenatchee Area.


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