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Area forecast discussion 
National Weather Service Spokane Washington
944 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: throughout this timeframe we will be
in a bit of a transition period as we go from the very moist
atmospheric river event to more of a ridging pattern, back to an
active pattern. Beginning Sunday night we will start to see the
general storm track shift to the north while the moisture from
the atmospheric river shifts to the south. With the jet
continuing to have a westerly component, the Cascades will allow
significant rain shadowing for the east slopes into the basin
leading to a significant drop in precipitation chances. With the
main moisture tap also deviating away from the region, topography
will play a key role in driving the higher precip chances into the
mountains. Dropping snow levels will also add to the complexities
of the forecast as we once again return to a northwesterly flow
pattern with the approach of the ridge. How fast these drop will
determine precip type for the mountains, but at this point the
northern and central mountains should see a transition back to
snow. Breezy winds continue mainly for the southern portions of
the region late Sunday before tapering off Monday.

Moving into Monday we will continue to see the ridge approach from
the west leading to the previously mentioned drying trend. Early
Monday we could see some areas of fog as cool morning
temperatures coupled with abundant low level moisture leads to its
development. Confidence is not high enough at this point to add it
to the forecast, but do not be surprised if some is added later
on. The key in this will be if any clearing occurs and the
timing. Throughout the day the activity level really diminishes
with dropping winds and less chaotic skies due to the stable
ridge.

Monday night would be another night where fog development is
possible as overnight temperatures continue the general cooling
trend. Once again the 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 a bit 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 would bring a
good shot for precipitation to much of the region minus the
basin. Cold air advection in the northwesterly flow allows snow
levels to further drop bringing mountain and even some valley snow
along the Canadian border. 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 for this short
period 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...
06z tafs: vlifr conditions at klws until about 11z when it may
lift as thicker clouds move in...confidence isn't high. Elsewhere
-ra at kmwh and -rasn at keat will start aft 09z. For eastern taf
locations the -ra will start aft 12z. Tafs don't go out much
further than 13z- 16z for some locations. Believe -ra with periods
of ra/+ra will continue for nearly the entire remainder of the
valid taf time with little to no change in cigs or vis.

&&

Preliminary point temps/pops...
Spokane 33 40 39 46 35 40 / 0 90 90 30 20 20
Coeur D'Alene 33 38 37 45 35 40 / 0 80 90 40 50 30
Pullman 34 42 41 50 39 46 / 0 90 100 80 50 20
Lewiston 33 45 43 57 42 50 / 0 70 90 70 70 30
Colville 32 37 36 41 30 36 / 0 100 100 20 20 10
Sandpoint 32 36 35 40 33 37 / 10 90 100 60 60 30
Kellogg 31 38 36 42 35 38 / 0 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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