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Area forecast discussion 
National Weather Service Spokane Washington
342 PM PST Friday Dec 26 2014

Synopsis...
a vigorous low pressure system will produce widespread
accumulating snow Saturday into Sunday. The heaviest snow
amounts will likely occur over the Idaho Panhandle where amounts
of 6 to 12 inches will be a good bet but far eastern Washington
and the Cascades will also receive significant accumulating snow.
Bitterly cold northeast winds will develop by Monday morning.
Temperatures will be well below average most of next week as
Arctic high pressure settles over the inland northwest.

&&

Discussion...

..heavy snow expected for portions of eastern Washington and the
much of the Idaho Panhandle this weekend...

Tonight through Saturday night: satellite imagery shows a
vigorous shortwave trough of lower pressure pushing into northern
British Columbia. This disturbance will dive south along the British Columbia coastline over
the next 24 hours. Increasing moist isentropic accent on Saturday
morning will result in widespread precip across the eastern two-
thirds of the forecast area and along the Cascade crest.
Precipitation intensity will start out light early Saturday, but
will increase in intensity through the afternoon into Saturday
night. This will be due to strong dynamical forcing aloft with the
region under a moderate strong 120 kt upper level jet streak and
strong positive vorticity advection at upper levels. This forcing
aloft will also result in cyclogenesis at the surface with an area
of low pressure tracking across northwest Washington through eastern Washington
and across the central Idaho Panhandle. This will result in good
level forcing along the occluded front and along the backside of
the low as moisture wraps around into the region. The good news is
that models have been more consistent showing a fairly progressive
passage of the low pressure system. Even so, westerly to
northwesterly flow will result in good orographics across the
Palouse and into the Idaho Panhandle. A tightening pressure gradient
is also expected to result in increasing winds, especially across
the Palouse and over the higher terrain over the Cascades and the
southeast portion of the forecast area.

* Snow levels: valley floors. The exceptions will be in the
southern portions of the upper Columbia Basin and in the l-c
valley. These areas will start out with rain Saturday afternoon
but will transition over to snow Saturday night, especially the
l-c valley.

* Snow amounts: for the 24 hour period from Saturday through
Saturday night, we are generally looking at 2 to 5 inches from
the upper Columbia Basin to the Spokane area and across the
Okanogan Highlands over to the NE mtns. Higher amounts are
anticipated over the Idaho Panhandle with 4 to 10 inches possible
in the valleys and closer to 7 to 12 inches in the mountains.
The Palouse is also expected to see the potential for heavy snow
with primarily along the Highway 195 corridor and points
eastward. Heavy snow is also anticipated for the Cascade crest,
NE blue mtns and on the Camas Prairie.

* Winds: breezy Saturday afternoon into Saturday night. Strongest
winds at lower elevations will be across the Palouse and on the
Camas Prairie. Expect sustained wind speeds of 15 to 20 mph with
gusts as strong as 30 mph. Gusts will be as strong as 40 to 50
along ridge tops in the Cascades, NE blue mtns, central
Panhandle mtns and on the Camas Prairie.

* Hazards: the winter storm watches in the Idaho Panhandle will be
upgraded to winter storm warnings for heavy snowfall. We are
also including a Winter Storm Warning for heavy snow across the
eastern portion of the Washington Palouse, northeast blue mtns and the
Camas Prairie. A Winter Weather Advisory is being issued for the
east slopes of the Cascades to highlight the potential for heavy
snow at the crest. Main concern with the storm system will be
for snow covered roads that will make travel treacherous. Winds
are also expected to result in areas of blowing snow across the
Highway 195 corridor and Highway 95 corridor on the Palouse.
This could result in drifting snow and poor visibility down to a
1/4 mile at times. Heavy snow is expected along I-90 between the
Spokane area to Lookout Pass and over Stevens Pass. /Svh

Sunday through Monday...the organized winter storm system passing
through on Saturday and Saturday night will be moving off to the
southeast into central Idaho Sunday morning. However...a dense
area of residual accumulating snow showers will continue to
harass the Palouse and points south...in particular the Camas
Prairie and the Blue Mountains. In the wake of the main storm
system an unstable but colder air mass will exist over the region
in the cusp of a broad upper level trough settling into the
region. On Sunday and through Monday no location will be safe from
further light accumulation random snow showers...and in fact the
latest models agree on dropping a second short wave disturbance
into the forecast area from the north Sunday night and
Monday...with this next wave potentially impacting the deep basin
and Cascades east slopes with a further general 1 to 3 inches of
snow. This second wave will not be as organized or moist as
saturday's system and no further winter highlights beyond the
current suite are expected...although the Camas Prairie is under
serious threat of further heavy snow that may necessitate
extending the current warning further into Sunday evening.

The second serious weather issue during this period concerns the
potential for breezy and gusty north and northeast winds
developing across the region as the passing upper level trough
draws a shot of modified Arctic air into the region from the
north...promoting areas of blowing and drifting fresh snow
particularly in the Purcell Trench and the northeast basin
including northern Whitman County along the Highway 195 corridor.
Also these winds will create very raw and brisk conditions with
wind chills reaching dangerous levels on exposed locations Monday.
/Fugazzi

Monday night through Friday...flop-over ridge with axis placement
to the west continues a cold dry northerly flow for most of this
forecast interval void of shortwaves until perhaps Thursday and
beyond. With the expectation there will be snow on the ground from
this weekends expected storms the forecast temperatures remain on
the very cold side of normal, especially the overnight lows, due
to expected efficient radiational cooling and advective cooling
from the cold north/northeast wind. Minor nudges to increase sky
cover and pops slightly to hint that a disturbance or two may drop
down in the northwest flow and bring US more snow perhaps late
Thursday and on through Friday remain but confidence not too high
this far out in the extended. /Pelatti



&&

Aviation...
00z tafs: stubborn stratus and fog will linger over or very near
the kgeg taf site through this evening...and persist at the ksff
and kcoe taf sites through the night under a strong low level
inversion. Stratus may form in the deep Columbia Basin at kmwh and
keat after 06z as well but this potential is low confidence. A
winter storm descending from British Columbia will spread light
snow over the eastern taf sites after 15z Saturday and persist
through 00z Sunday with IFR conditions common for the eastern taf
sites. Kmwh and keat will be on the edge of the precipitation
shield for the potential for MVFR or brief IFR conditions in
occasional snow. /Mjf



&&

Preliminary point temps/pops...
Spokane 26 32 27 31 19 22 / 20 100 100 50 30 10
Coeur D'Alene 27 31 28 31 19 20 / 20 100 100 60 20 10
Pullman 25 33 28 30 16 23 / 10 100 100 80 50 10
Lewiston 27 37 31 36 24 28 / 10 50 100 60 50 10
Colville 25 32 24 32 20 24 / 30 100 60 40 30 10
Sandpoint 26 31 27 31 19 21 / 30 100 100 60 20 10
Kellogg 22 30 26 30 15 17 / 20 100 100 80 40 20
Moses Lake 27 36 27 36 25 29 / 10 30 10 10 30 20
Wenatchee 30 39 28 35 26 30 / 10 20 10 10 60 30
Omak 24 30 25 30 21 25 / 10 20 10 20 40 10

&&

Otx watches/warnings/advisories...
Idaho...Winter Storm Warning from 4 am Saturday to 4 PM PST Sunday for
central Panhandle mountains.

Winter Storm Warning from 7 am Saturday to 4 PM PST Sunday for
Lewis and southern Nez Perce counties.

Winter Storm Warning from 4 am Saturday to 10 am PST Sunday for
Coeur D'Alene area-Idaho Palouse-northern Panhandle.

Washington...Winter Weather Advisory from 4 am Saturday to 4 am PST Sunday
for east slopes northern Cascades-Okanogan Highlands.

Winter Storm Warning from 7 am Saturday to 4 PM PST Sunday for
northeast Blue Mountains.

Winter Storm Warning from 4 am Saturday to 10 am PST Sunday for
northeast mountains-Washington Palouse.

Winter Weather Advisory from 4 am Saturday to 10 am PST Sunday
for Spokane area.

&&

$$

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