Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 221149
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
349 AM PST MON DEC 22 2014

.SYNOPSIS...
Expect dry conditions and cooler but still above normal
temperatures today and tonight. Light mountain snow and valley
rain or snow will return late Tuesday. An organized storm system
will impact the region around Christmas Eve. This will bring the
potential for moderate to heavy snow in the mountains and light
accumulations to the valleys.

&&

.DISCUSSION...
Today through Tuesday...High pressure will build off the coast
today then move through the region on Tuesday. This will result
in a quiet period in the forecast compared to the weekend and what
is expected by the middle of the week.

We have lost the deep moisture tap but with northwest flow there
will still be the possibility of rain and snow showers for the
central Idaho Panhandle southward today...then those come to an
end overnight. The upper level flow is in a good position for the
Puget Sound convergence to set up. This is sending bands or small
rivers of moisture across the Cascade crest and slopping over into
the east slopes. Radar and satellite imagery show that these bands
are quite transient both north and south. As such I chose to just
cover the entire crest with higher pops. Some of this moisture may
make it into the valleys of the lower east slopes at times through
the day, resulting in a short burst of rain and or snow. Otherwise
the forecast will be dry and cooler. High pressure...light winds
and a moist boundary layer will likely set the stage for fog and
stratus development tonight and into Tuesday morning.

By late Tuesday morning the leading edge of moisture from the next
upstream weather system will begin to move into the Cascades. This
will result in increasing chance of precipitation for the
Cascades...then pushing across the northern mountains near the
Canadian border through the afternoon. Precipitation amounts will
remain on the light side during this period. Snow levels will drop
to near the valley floors across the northern zones by this
time...and the valleys may pick up a dusting of snow with an inch
or two possible near the crest of the Cascades. Tobin

Tuesday night through Friday...The flat ridge and westerly flow
will be replaced by an upper trough passage that will bring
mountain snow and a mix of rain and snow for the valleys during
the busy holiday travel period. There is good model agreement
among the latest ECMWF/NAM/SREF solutions in a warm front Tuesday
night into Wednesday followed by the upper trough passage and
falling snow levels late Wednesday into Christmas Day. The GFS
remains the outlier with slower timing of the trough passage and a
more northerly track of the main area of precipitation. The
forecast will trend toward the ECMWF/NAM/SREF consensus.
Confidence is above average for the precipitation event as a fetch
of deep Pacific moisture is directed into the northern tier of
counties Tuesday night then sags south across the Inland Northwest
for Christmas Eve and Christmas morning. The type of precipitation
in the valleys and the amount of accumulating snow comes with a
lower confidence level due to changing snow levels and also the
possibility of areas of low level convergence forming as the
trough moves across the forecast area Wednesday night. Snow levels
will start off low in the north Tuesday night with light valley
snow possible but then surge to 4-6K ft during the heaviest
precipitation before crashing back near valley floors Wednesday
night. Christmas Day the region will be in unstable northwest flow
which will be favorable upslope for the southeast zones but by
then the moisture tap will be lost and precip will be more showery
in nature. Snow showers will continue to diminish Friday as the
trough dynamics exit to the southeast. Isolated snow showers will
linger across the Camas Prairie and Shoshone County with little
additional accumulations.

I like the format of the previous discussion so I am borrowing it.

...MODERATE THREAT FOR SNOW TO IMPACT MOUNTAIN PASSES WEDNESDAY...

* Snow: Most mountain locations will receive some snow. The
  heaviest mountain snow will be along the Cascade Crest and Idaho
  Panhandle where 6-12 inches will be possible. Confidence
  increases with elevation as snow levels will be in flux. The
  high peaks will see all snow while the mid slopes and valleys
  will see a mix of rain/snow as snow levels initially rise
  Tuesday night with the arrival of the warm front and southerly
  winds. The transition to snow will take place late on Christmas
  Eve as the upper trough moves overhead and winds shift to the
  northwest. The rain/snow line will sag south overnight. A few
  inches of snow will be possible in the valleys of northeast WA
  and north ID but confidence in exact amounts is low at this time
  due to uncertainty with the timing of changing snow levels. 2-4
  inches will be possible in the Blue Mtns, Camas Prairie, and
  Idaho Palouse Wednesday night into Christmas morning following
  the transition from rain to snow. Confidence is moderate for at
  least an inch but lower for additional accumulations as it is
  difficult to pin down the timing of the changeover. Best chances
  for any accumulations will be above 2000 feet and east of a line
  from Republic to Pomeroy but this also carries low confidence
  since lowest snow levels will occur as precip ends.

* Mountain Passes: Snow is likely to impact mountain passes along
  I- 90, Hwy 2, and Hwy 20 starting Tuesday night and continue
  through Wednesday.

Snoqualmie: will start off as rain Tuesday evening but should switch
to snow by early Wednesday morning. Amounts could be light to
moderate.

Stevens Pass: will be a mix of wet snow and rain Tuesday evening but
should quickly switch over to snow overnight and remain snow on
Wednesday. Amounts could be moderate.

Lookout Pass: will start off as snow Tuesday night, may briefly mix
with rain or become heavy wet snow overnight, then become all snow
for Wednesday. Amounts could be heavy.

Sherman Pass: Snow Tuesday night into Wednesday morning then
tapering off through the day. Light to moderate accumulations.

* Confidence levels: Large changes are possible with the storm if
  model trends switch toward the GFS. Consequently, confidence is
  moderate to low. Confidence is moderate to high for significant
  mountain snow, and low accumulations for valleys. If model
  trends go toward the GFS, winds will be stronger, the
  precipitation will be slower to traverse the region, and snow
  levels will be higher. (sb)/Kelch

Friday night through Monday night: This period will involve one
or more cold weather systems affecting the Inland Northwest. These
weather disturbances will help create a larger trough across the
region, allowing chilly air to funnel south. Any precipitation
that falls will fall as snow due to the cold air, but the flow
pattern should keep moisture on the limited side. The trough
doesn`t stick around for long, with a ridge of high pressure
trying to push the trough east by midweek. ty

&&

.AVIATION...
12Z TAFS: Northwest winds over the Cascades has created a
convergence zone with bands of precipitation slopping over into
the east slopes this morning. This may result in short periods of
-shra at KEAT through 15-17z...showers should be brief but could
be intense...Otherwise VFR conditions at KEAT. High pressure
building into the region has resulted in drying through the
lower atmosphere and will inhibit stratus formation this morning
for the TAF sites. As such expect VFR conditions through roughly
06z. After 06z the inversion will strengthen and fog/stratus
formation will be possible at KGEG/KSFF/KCOE with possible
IVR/LIFR cigs. Moisture ahead of the next incoming Pacific storm
system will begin to lower the cigs at KEAT/KMWH after 06z with
VFR cigs BKN-OVC100. Tobin

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        41  29  40  33  37  26 /   0   0  20  60  80  40
Coeur d`Alene  40  29  40  33  37  26 /  10   0  20  70  80  50
Pullman        43  29  43  34  40  29 /   0   0  10  30  80  70
Lewiston       47  33  45  37  43  32 /  10   0  10  20  80  70
Colville       38  27  36  31  34  25 /   0  10  30  70  50  20
Sandpoint      38  28  36  32  35  27 /  10  10  20  80  80  40
Kellogg        38  27  36  33  35  29 /  40   0  20  80  90  70
Moses Lake     45  31  42  34  40  28 /   0   0  10  20  60  20
Wenatchee      44  34  42  34  39  29 /   0   0  20  20  50  10
Omak           38  28  36  29  34  25 /   0   0  30  40  30  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$



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