Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 031213
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
413 AM PST Sat Dec 3 2016

.SYNOPSIS...
Deep low pressure will bring rain, wind, and heavy mountain snow
to the Inland Northwest tonight into Sunday. The heaviest snow
accumulations will occur over the Cascade Crest and the mountains
of the Idaho Panhandle. Much colder temperatures will arrive
Sunday night and remain through Thursday. Light valley snow will
be possible with a weak weather system on Monday. Additional snow
will be possible late next week.


&&

.DISCUSSION...

...HEAVY MOUNTAIN SNOW FOR THE CASCADE CREST AND THE CENTRAL
PANHANDLE MOUNTAINS TONIGHT INTO SUNDAY FOLLOWED BY COLDER
TEMPERATURES...

Today: Precipitation will decrease over the Idaho Panhandle this
morning. For much of the night, we have been monitoring a mix of
snow showers and drizzle with the passage of a weak upper trough.
The moisture with this feature has been most abundant below 10
thousand feet. Temperatures within this layer are a bit on the
warm side for efficient production of snow, so we have seen a
combination of wet snow and drizzle. Much of our snow will be
coming to an end by sunrise as mid-level dry air spreads over the
region. We will be canceling the Winter Weather Advisory for the
north Idaho Panhandle around Bonners Ferry and Sandpoint.
Additional light accumulations are expected through sunrise over
Lookout Pass, so we will retain the Advisory for the Central
Panhandle for locally slick conditions on I-90.

It will be a breezy day over the Palouse and along the Foothills
of the Blue Mountains in SE Washington. Sustained winds of 15 mph
with gusts to 25 mph are expected for Pullman, Pomeroy, and
Washtucna. Afternoon temps in the L-C Valley and Columbia Basin
will likely reach the mid to upper 40s. Elsewhere upper 30s to
lower 40s will be common.

Tonight through Sunday: A deep polar low over the Gulf of Alaska
will interact with a fetch of subtropical moisture late tonight
into Sunday. A strong westerly jet will produce heavy mountain
snow over the Cascade Crest and portions of the Idaho Panhandle.
Temperatures will likely be warm enough that most of our populated
areas will experience rain or a rain/snow mix through the end of
this evening Sunday evening.

* Snow Amounts: The strong westerly component of the mid/upper
  level jet will favor snow over the Cascade Crest and mountains
  of the Idaho Panhandle. A foot or more of snow is likely over
  the Cascade Crest including the Stevens Pass area. The mountains
  of Shoshone County (above 3000 feet) in the Idaho Panhandle will
  likely receive 8 to 12 inches by Sunday evening, and the high
  terrain around Sandpoint and Bonners Ferry will likely get 5 to
  10 inches. A pronounced central Washington rain shadow will
  limit the amount of precipitation in the lee of the Cascades.
  Places like Wenatchee, Omak, and Moses Lake will receive very
  little precipitation tonight into Sunday.

* Winds: Look for southwest winds of 15 to 20 mph with gusts to 35
  mph on Sunday morning over the Palouse and West Plains as low
  pressure moves through southern B.C. and emerges east of the
  Continental Divide. By Sunday afternoon, a cold front plunging
  southward out of B.C. will produce a wind shift to the north or
  northwest. The Okanogan Valley and Waterville Plateau may
  experience a few hours of sustained north winds around 20 mph
  with gusts to 30 mph in the afternoon.

* Impacts: At this time, travel over the mountain passes looks to
  be the main impact on Sunday. Stevens Pass in the Cascades and
  Lookout Pass on I-90 in the Panhandle will receive heavy
  accumulations. Most valley communities in the Panhandle and in
  the Cascades will likely be too mild to receive much appreciable
  snow during this event. The Camas Prairie is a tough call.
  Elevations above 3000 feet like Nez Perce and Winchester have
  the potential for 2 to 5 inches of wet snow on Sunday. However,
  daytime temperatures on Sunday may be a couple degrees too warm
  for much snow to stick on the roads. /GKoch

Monday through Wednesday night: This period will feature a small
round of snow followed by some of the coldest temperatures of the
season thus far. The region will be under the influence of a quite
cold upper level trough on Monday. A surface high will be over the
Canadian Rockies, meanwhile a secondary Pacific low will pivot
along the Washington coast. Colder continental air will filter in
from the north, although milder and moist Pacific air will
gradually spread inland with a weak warm front that slices from
northwest to southeast across Washington. The overunning of this
milder air over the colder air will give way to a band of light
snow. The current timing looks for snow showers to increase in the
Cascades and Wenatchee valley Monday morning, spread into the
southern Columbia Basin and Blue Mountains Monday afternoon and
expand over the Palouse and southern ID panhandle Monday night.
The cutoff of light snow looks to be along and south of Highway 2
with 1-2 inches possible from Wenatchee, Ritzville to Pullman. So
far confidence on timing is moderate, but the bulk of the snow
should fall in the late afternoon and evening. Daytime
temperatures in this area may peak in the mid 30s, complicating
snow accumulations initially but better chance of accumulations
are possible in the evening hours. This surface low and
accompanied front will slowly slip south of the region late Monday
night into Tuesday. The surface high in the Canadian Rockies will
build southward. Drier and colder air will filter into the region
with clearing aloft with an upper level ridge. Low level north to
northeast winds will increase while low level temperatures remain
colder than normal. Some low clouds may bank up into the Cascades,
but most areas should be mostly clear. Daytime highs will struggle
to reach the 20s for Tuesday and Wednesday. The coldest night
looks to be Tuesday night with overnight lows in the single digits
to mid teens. Some colder temperatures are possible in areas with
fresh snowfall, clearing skies and light winds. In addition,
patchy fog is possible in the sheltered river valleys. The next
weather system will arrive from the Pacific and breakdown the
ridge aloft while mid and high level clouds spill into the region.
Temperatures will moderate, although the chances of precipitation
look to be delayed for Wednesday night with the best chance of
snow being over the Cascades Wednesday night and mostly dry
conditions further east. /rfox.

Thursday through Saturday...The ridge is flattened and pushed east
as our next system overspreads the region. A strong moisture tap
and good isentropic ascent will lead to widespread precipitation.
Models are in generally good agreement with these details with the
ECMWF a little slower than the GFS with the onset of the
precipitation. Where model consistency and agreement diverges is
with how much warm air is pulled in and thus if snow may change to
rain/mix in some locations for a period. Current thought is that
the GFS is too aggressive with the warm tongue and that with cold
air already entrenched across the region, most locations will
remain all snow...and accumulating snow at that. /Kalin

&&

.AVIATION...
12Z TAFS: Precipitation over the Idaho Panhandle will decrease
this morning as moisture moves eastward. Low stratus in the
Spokane, Coeur d`Alene, Sandpoint, and Pullman areas should lift
and gradually disperse during the late morning and afternoon
hours. Warming over the Columbia Basin should help to erode the
low cloud deck. The arrival of our next round of precipitation
tonight will be accompanied by a strong mid/upper level westerly
jet. Much of central and eastern Washington will be under the
influence of the rain shadow as our mountains get hammered by
valley rain and mountain snow. Low level wind shear was added to
the Wenatchee TAF as the jet increases overnight. /GKoch

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        40  34  38  19  28  15 /  10  30  40  10  10  30
Coeur d`Alene  40  34  38  18  26  14 /  20  70  70  10  10  20
Pullman        43  35  40  20  28  17 /  20  70  90  20  10  30
Lewiston       47  38  46  25  33  22 /  10  30  90  30  10  40
Colville       39  34  37  17  27  13 /  10  30  20  10  10  20
Sandpoint      40  33  38  17  25  13 /  20  90  80  10  10  10
Kellogg        37  33  36  17  23  12 /  60 100 100  40  10  20
Moses Lake     46  34  45  20  35  19 /   0  10  10  10  30  30
Wenatchee      43  35  44  23  32  21 /  10  10  10  10  50  40
Omak           40  31  38  17  30  14 /  10  10  10   0  20  30

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 10 AM PST this morning above 2500
     feet for Central Panhandle Mountains.

WA...None.
&&

$$



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