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Area forecast discussion 
National Weather Service Spokane Washington
528 am PST Thursday Dec 25 2014

Synopsis...
much of central and eastern Washington will experience sunshine
and seasonably chilly temperatures today. Relatively quiet weather
will persist into Friday. Confidence is increasing that
widespread accumulating snow will occur Saturday into Sunday over
the Cascades, Idaho Panhandle, and eastern third of Washington.
Much colder conditions will likely arrive Monday and Tuesday.



&&

Discussion...

..strong winter storm to bring snow across much of the region
over the weekend...Holiday travel likely to be impacted...

Today: today's weather will be relatively tranquil and seasonably
cold. Snow has largely ended over the Idaho Panhandle with the
departure of the upper trough responsible for the heavy snow over
the Camas Prairie and Panhandle mountains last night. The Winter
Weather Advisory for Winchester and Craigmont will be allowed to
expire at 6 am. Deep layer dry advection has overspread central
and eastern Washington overnight. Only the sheltered valleys of
northeast Washington around Colville, hunters, Priest Lake, and
Newport have experienced fog development. The northerly gradient
and subsequent dry push will weaken by sunrise. The expansive low
stratus over the Idaho Panhandle will likely persist through the
day. The patchier low clouds and fog in northeast Washington
should dissipate by mid-day.

Tonight and friday: a weak upper level disturbance embedded in the
northerly flow will increase the potential for light snow over
Shoshone County and the Camas Prairie tonight into Friday morning.
Snow amounts should be very light. Moisture availability will be
limited with this system. Model forecast soundings suggest
marginal moisture within the dendritic layer (-10c to -20c). Look
for accumulations of an inch or less for the mountains around
Deary, Clarkia, and over the Camas Prairie near Winchester and
Craigmont. /Gkoch

Saturday through Sunday...a strong shortwave trough will drop
south out of British Columbia Saturday with the potential to bring
significant snowfall to much of the inland northwest. Models are
in rather good agreement that the heaviest snowfall will take
place Saturday night but there remains some spread as to the
location. The general picture brings a surface low across the
forecast area but there remains a decent amount of spread in the
exact track of the low center, which will determine where the
deformation bands set up as the low slides southeast across the
region. The 00z model run of the GFS and European model (ecmwf) show surprisingly
similar locations of the low center and associated areas of
heaviest precip which include the Spokane-Coeur D'Alene areas,
Shoshone County and southwest to include portions of the Palouse,
the NE blues and Camas Prairie. The NAM and sref bring the low
center farther east which would keep Spokane to the west of the
deformation bands. Consensus on the timing of heavy snow is
Saturday evening into the overnight hours.

* Snow levels: down to valley floors for most areas. The
exception will be across the southern portion of the upper
Columbia Basin over into the l-c valley on Saturday. These areas
are expected to start out as rain or a rain/snow mix; however,
colder air pushing in from the north should change precip type
to all snow through Saturday night.

* Snow amounts: best chances for heavy snow will be across the
central Idaho Panhandle, Palouse, northeast blue mtns and Camas
Prairie. Confidence level: moderate and increasing as model
consensus increases.

* Winds: southwest winds could be breezy along the southern
periphery of the surface low, affecting the Palouse, NE blues
and Camas Prairie Saturday night. This would also set up the
potential for blowing snow. Gusty north winds will increase down
the northern valleys Sunday with very cold wind chill
temperatures expected Sunday night.

* Potential hazards: heavy snow will create treacherous travel
conditions during the busy Holiday travel season. Primary time
of concern will be from Saturday afternoon through Sunday
morning. Mountain Pass may be impacted, including Stevens Pass
and Lookout Pass. This may also adversely affect travel across
much of the eastern half of the forecast areas.

Sunday night and Monday...as the surface low pulls away to the
southeast a northerly surface pressure gradient will strengthen,
with increasing winds channeled down the northern valleys. Cold
dry air will spill into the forecast area and steady snow will
diminish to isolated snow showers over the southeast zones.
Temperatures will drop into single digits Sunday night. Combine
that with brisk winds and wind chill temperatures could drop to
ten below. Any locations with fresh snow on the ground will be
hard pressed to warm up past the teens on Monday. /Kelch

Monday night through Wednesday night: as we push towards the
middle of the week, the general trend in cooler and drier
conditions continues. Longer range models currently appear to have
a good handle on this timeframe with a building ridge off the West
Coast pushing the active storm track well to our north. With the
ridge just off the coast, our steering flow will come from
directly from the north allowing for cold and dry Continental
polar air to feed into the region. This will lead to a significant
drop in cloud cover which in some respects is nice as the sun
should be out, but this sunshine will come at a significant price.
The continued lack of cloud cover overnight will allow for very
efficient radiational cooling sending our lows crashing into the
teens and single digits for most with even some scattered areas
likely falling below zero for Tuesday morning. Even during the day
on Tuesday our highs will struggle to surpass 20 due to the strong
northerly flow.

Moving to the middle of the week our temperatures start to
moderate a bit as we shift to more of a northwesterly flow pattern
bringing maritime polar air which is a bit milder. This will also
allow for limited amounts of moisture to stream in, but at this
point an increase in mid and high clouds looks to be the only
result. Overall this looks to be a rather event-less period with
the big story being the very cold temperatures both daytime and
into the overnight hours. /Fliehman

&&

Aviation...
12z tafs: ragged stratus and patchy fog will be problematic at
kcoe, ksff and kpuw this morning. Dry northerly flow in the
boundary layer has elimated much of the cloud cover around the
region, but a stratus remains banked against the rising terrain of
the Idaho Panhandle. The NAM suggests that northerly winds within
the boundary layer will weaken between 12-15z which may allow the
Panhandle stratus deck to build back into kpuw, ksff, and kcoe.
The ground is wet following yesterday's rain. Sheltered spots may
also experience a bit of shallow fog this morning as well. /Gkoch



&&

Preliminary point temps/pops...
Spokane 33 22 32 23 34 24 / 10 10 0 20 90 80
Coeur D'Alene 33 24 33 23 33 25 / 10 10 10 30 90 80
Pullman 34 24 34 24 34 27 / 10 20 10 10 90 90
Lewiston 39 28 36 27 38 30 / 10 20 10 10 60 90
Colville 33 20 30 22 33 21 / 0 0 0 30 80 50
Sandpoint 33 23 32 23 33 21 / 10 10 10 30 90 70
Kellogg 34 25 31 23 30 23 / 20 50 40 30 90 90
Moses Lake 38 22 36 22 38 26 / 0 0 0 10 20 50
Wenatchee 39 25 36 23 39 25 / 0 0 0 10 20 50
Omak 33 19 29 20 32 21 / 0 0 0 20 20 40

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

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