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Area forecast discussion 
National Weather Service Spokane Washington
558 PM PST Friday Dec 19 2014

Synopsis...
a more active weather pattern will continue into early next week.
Another weather system will bring snow to the mountains with
mainly rain for the valleys tonight into Friday. A stronger storm
Saturday into Sunday will bring mainly rain for the valleys...as
well as mountain snow with rising snow levels. Temperatures are
expected to remain above average through early next week...before
dropping down towards normal values by Christmas day.

&&

Discussion...
tonight: a cold front and supporting upper trough will continue
to push east this evening, but the warm front ahead of the next
system will begin to enter from the west overnight. This will
support an overall drying trend across much of the region, with
the main snow risk shifting into Panhandle mountains through the
evening and overnight. The approach of the next warm front will
renew the precipitation threat toward the Cascades and western
Columbia Basin overnight. Otherwise look for mostly cloudy skies
east and partly cloudy skies in the west this evening, with more
clouds expanding across the region late this evening and overnight
with the aforementioned warm front. /J. Cote'

..heavy snow expected over the higher mountains and Cascade
valleys Saturday into early Sunday...

Saturday through Sunday...very moist atmospheric river is still
poised to impact the region this period. As of 2pm...the leading edge
of the river was located near 133w and streaming rapidly
eastward. Although some of the moisture from this river could
produce some light precipitation later tonight...it looks like the
brunt of the moisture and ascent associated with it will push into
the Cascades early tomorrow morning and slide across the remainder
of the inland northwest during the day. The main burnt of this
precipitation will be delivered via a fairly robust warm
front...which should move into the Cascades by midday and into the
Idaho Panhandle by late afternoon. The ascent will be strong
enough that all locations receive precipitation...the big question
is where will most of it fall and what will it fall as. It still
looks good that the heaviest precipitation will fall near the
Cascades with amounts ranging from 1.5-3.0 inches near the crest
and anywhere from 1-2 inches over The Blues and Idaho Panhandle
mountains. Meanwhile totals in between should generally range from
30-.80 inches. These are very impressive amounts...and would be
higher if the atmospheric river decided to remain fixed over our
forecast area. However chances are good that the core of the
moisture drift toward the Washington-Oregon border by evening so
much heavier precipitation will be possible south of our area. As
far as what precipitation type to expect...this is the big
question...and a not very confident answer...at least for the
Cascade valleys. Our confidence is highest that the Cascade
mountains...will see the largest amounts of snow...especially
north of Lake Chelan and at elevations above 3000 feet. 1-2 feet
of snow still looks like a good possibility. Moderate snow amounts
are also quite likely for the northern mountains near the Canadian
border...with totals ranging from 4-8 inches mainly at elevations
above 3000. Similar totals also look possible for the central
Panhandle mountains...however snow levels will be about 4000 feet
or higher. This will likely impact travel conditions both at
Sherman and lookout passes and we will address via a weather
story. Snow levels will rise rapidly at both sites after the front
moves through. In fact by early Sunday morning...snow levels could
rise to 6k feet or higher south of I-90 and up to 4k feet near the
Canadian border. The toughest question to answer is what to do
with the Cascade valleys. There is no question that when the
precipitation starts it will do so as snow. However how long will
it remain that way? The GFS and European model (ecmwf) models have been too cold of
late at locations such as Mazama and plain and the temperatures at
both sites will be critical in determining how long they can get
accumulating snow. The NAM was a little better but even it was too
cold compared to current readings. The key will be how cold will
temps get tonight. At least there are some partly cloudy skies
headed that way so there is potential for some good radiational
cooling this evening. It remains to be seen if temps in those
areas can drop well into the 20s tonight like the models are
suggesting...but we have some doubts. If we believe those model
solutions...it would never changeover to rain at Mazama and would
only do so at plain by tomorrow evening. The NAM has a better
handle on the situation...and switches plain over by mid-
afternoon...and Mazama by evening. But based on cold bias suspect
this changeover could occur a little sooner. Nonetheless that
leaves a good amount of time for accumulating snow with 4-8 of
snow a possibility. We will issue a Winter Weather Advisory for
snow...and word some uncertainty into the product.

While most locations won't experience snow...most should see
breezy conditions develop Saturday night and continue into Sunday
as the surface low passes through southeast British Columbia. Since we won't really see
strong cold air advection it isn't a prime setup for mixing strong
winds aloft to the ground. Nonetheless we expect to see breezy
conditions...especially over the Palouse...eastern Columbia
Basin...Blue Mountains and Lewiston area. These winds will deliver
very warm temperatures with highs surging into the middle 40s to
middle 50s. Fx

Sunday night through tuesday: timeframe will be a significant
transition period with a shift from the very moist atmospheric
river to a drier and more stable ridge pattern by Monday
afternoon. Activity level then ramps up again by Tuesday with
another approaching wave of energy. Pattern splits late Sunday
with the general storm track and associated energy shifting to
the north while the subtropical plume of moisture shifts to the
south. Both the Euro and GFS currently depict this quite well
ending the threat of widespread precipitation Sunday for the
inland northwest. With the continued westerly component of the upper
level winds and limited remnant moisture streaming in,
topographically induced showers (mostly in the form of snow)
remain into Monday. Best chance for precip will reside in the
Cascades and Panhandle mountains, but only lighter amounts are
expected. Coupled with this mountain precip, stronger
northwesterly flow allows cooler maritime Pacific air to
penetrate dropping our snow levels early in the period. For now
I feel the cooler air will penetrate deep enough to promote snow
in the northern and central mountains with the southern locations
taking a bit longer to transition.

By Monday morning the ridge continues to amplify off the coast
further strengthening the mentioned cool northwest flow. Stability rises
which lowers the precip chances regionwide and allows for winds
from previous days to diminish. Overall Monday appears to be a
rather lackluster day with the ridge quickly moving through the
region. With stable conditions, recent precipitation, light
surface winds and falling overnight temperatures, Monday night
looks to bring a decent chance for fog around portions of the
region. Exact locations at this point brings low confidence so it
was left out of the forecast. The Main Key with fog will likely
be the amount of breaks in the cloud cover and timing as well.

Early Tuesday is where models start to diverge as the Euro would
bring another wave through the region whereas the GFS holds off
until late Tuesday. For now I have sided with the more aggressive
Euro bringing the moisture in throughout the day. This brings a
good shot of precipitation to much of the region. Continued cold
air advection in the northwesterly flow allows snow levels to
further drop bringing more mountain snow with even some valleys
along the Canadian border seeing the potential for snow as well.
The further south we go, the lesser the chance for snow and
better the chance for rain. With a bit of model uncertainty at
this point, snow levels and the timing of precipitation will
likely need to be fine tuned once better consistency is achieved.
Overall a real roller coaster ride of activity is expected for
this short period of time at the beginning of next week. /Fliehman

Tuesday night through thursday: there is a high degree of
confidence that a rich plume of subtropical moisture will interact
with an incoming trof...yielding a deepening low pressure system
within the boundaries of the northwestern US during the Tuesday
night- Thursday morning time-frame. Exactly where this sets up
continues to carry high uncertainty and the last 24 hours of model
runs have brought US no closer to pin pointing the area. For
several days, model trends were pushing the axis of heavier precip
to the south...which at times was even displaced south of the
Washington/or border. Last night, models began reversing and the axis of
heavier precip was brought back into northern Washington/Idaho then with this
morning's runs, some guidance remained north while others split
the difference. There is moderate confidence that the air mass
will remain too mild initially to support any snow in the valleys
except along the immediate Canadian border but cooler air will
move in during the time of cyclogenesis Christmas eve and snow
levels will fall. So at this time, we made little changes to the
forecast which puts more weight on the slightly more stable European model (ecmwf)
but rather than go into details pertaining to this model, which
could change drastically over the course of 12 hours, here is a
break down of what could be the high impacts from this period of
busy Holiday travel.

- There is a moderate potential for snow to impact mountain passes
starting early Wednesday morning and continuing through Christmas
morning. Depending where the heaviest precip sets up...travel may
be hazardous at times.

- If the storm takes a more northern track, all region mountain
passes could experience heavy snow and a small potential other
lowland locations could receive some periods of snow . This
solution also presents the possibility for stronger winds across
the Columbia Basin and mountains.

- If the storm takes a more southern track, Stevens Pass could see
much less snow as could the I-90 corridor but this corridor will
be right on the border. Winds will also be significantly weaker a
and most of northern Washington/Idaho could see very little precipitation.

- The Camas Prairie, blue mtns, and central Panhandle mtns of Idaho
continue to carry the highest probabilities for snow
accumulations into lower elevations (at this time...roughly 2000
feet and higher) for Christmas eve into Christmas morning. This
holds true for most model solutions whether or not they supported
a northern or southern storm track, a northern track would delay
timing some.

This developing storm/potential for moderate precip will be through
Wednesday night then northwest flow will begin to take over
through the day on Christmas bringing in cooler and drier from
north to south with light snow gradually coming to an end over the
far southeastern zones (blue mtns, Camas Prairie, lower Idaho
panhandle). Gusty north winds will channel through the Okanogan
Valley but should be well below advisory limits with the air mass
lacking any Arctic origins. Large scale northwest flow will
continue into Friday with the potential for a weak wave or two
passing through the region keeping some mention of snow showers
into the weekend. /Sb



&&

Aviation...
00z tafs: look for a return to IFR/LCL LIFR conditions later this
evening...aft 06z for kgeg...ksff...kcoe...kmwh. Then the threat
of rain re-develops after about 11z for keat and kmwh and moves east
through the morning hours.

&&

Preliminary point temps/pops...
Spokane 33 40 39 46 35 40 / 10 90 90 30 20 20
Coeur D'Alene 33 38 37 45 35 40 / 20 80 90 40 50 30
Pullman 34 42 41 50 39 46 / 20 90 100 80 50 20
Lewiston 33 45 43 57 42 50 / 10 70 90 70 70 30
Colville 32 37 36 41 30 36 / 10 100 100 20 20 10
Sandpoint 32 36 35 40 33 37 / 30 90 100 60 60 30
Kellogg 31 38 36 42 35 38 / 40 90 100 90 90 40
Moses Lake 34 41 40 48 34 44 / 20 90 80 10 10 10
Wenatchee 32 36 36 45 35 42 / 20 100 90 20 20 10
Omak 30 35 34 39 28 36 / 20 100 80 10 10 10

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...Winter Weather Advisory from 4 am to 11 PM PST Saturday for east
slopes northern Cascades.

&&

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