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Area forecast discussion 
National Weather Service Spokane Washington
414 am 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...
today through Saturday night...this will be an active and very wet
period as a pair of storm systems move through the forecast area.
The first today followed by the second Saturday. This will result
in measurable precipitation for all locations, with valley rain
and mountain snow.

For today and tonight a vigorous frontal system will move through
the forecast. The warm over-running precipitation moved into the
region last evening and satellite imagery shows the cold front
moving onto the Cascades early this morning. This front will cross
the Cascades around 12z and should be east into Montana by this
afternoon. So expect drying from the west beginning near sunrise
then traversing the forecast area through the day. Temperatures
will be on the warm side with highs around 10 degrees above normal

* precipitation: yes and for just about all locations. With an
upper level jet paralleling the front and southerly low level
flow early this morning even the normally shadowed out lower
east slopes and deep basin will see measurable precipitation. As
the cold front moves through the region later this morning the
flow will shift the focus of the heaviest precipitation across
the northern mountains and finally the Panhandle mountains this
afternoon and this evening. Precipitation amounts will range
from .10-.20 for the low lands with anywhere from .25-.50 for
the mountains, and possibly as much as .75 near the crest of the
Cascades. Some showers will linger across the Panhandle
mountains this evening, otherwise there will be a brief break in
the wet weather.

* Precipitation type: warm air advection and southerly flow will
result in snow levels at or above 3-4k feet for valley rain and
mountain snow. The area of concern will be for the Methow Valley
where the cooler air remains trapped, but even there
temperatures are around freezing or a little more and any snow
that falls will be wet. So a couple of inches for the Methow
Valley this morning and 3-6 inches for the mountains above 3-4k
feet.

..a strong winter storm expected to result in heavy rain and
mountain snow across the northwest this weekend...

Moving on to Saturday and Saturday night...this storm system is
shaping up to be much wetter as the westerly flow will tap into
an atmospheric river of very deep moisture. Model guidance has
been pretty consistent the past several runs showing this moisture
reaching the Cascades early Saturday morning and over-running the
remainder of the region during the day Saturday. Isentropic up-
Glide will be quite impressive and combined with the deep moisture
means that everyone will get pretty wet.

* Precipitation: yes and quite a bit. Through the entire event
the Cascades can expect anywhere from 1-3 inches of liquid and
perhaps more near the crest. The northern and Panhandle
mountains .50-1.5 inches and for the low lands around a half
inch from Saturday morning through Sunday morning.

* Precipitation type: behind the first cold front temperatures
will cool down with snow levels dropping down to 2.5-3.0k feet
across the south and 1.5-2k feet across the north. Strong warm
air advection will result in snow levels increasing through the
day Saturday and finally rising above 6-7k feet across the
south, but still only around 3.5-4k feet across the north. The
challenge will again be the Methow Valley where the cooler air
will be tougher to scour out. The models continue to be a little
too cold for the Methow and I chose to go above guidance in
these areas, and secondly with this much precipitation and warm
air advection I would think these areas should mix out pretty
well. Still there should be an extended period of snow
accumulation for the Methow Valley with moderate snow
accumulations possible. For the remainder of the mountain
valleys it looks like they might start out with a snow/rain mix
but quickly transition over the rain. Snow amounts in the
Cascades could be in the 1-2 foot range above 4k feet and up to
a foot or maybe slightly more for the northeast and Panhandle
mountains by Sunday morning. The Methow could see as much as 5-7
inches.

* Winds: 850 mb winds will climb up to around 40-45kt. The
question will be if these winds can mix to the surface. Without
a robust cold front this will be tough. So winds breezy to even
windy in locations but likely not advisory speed.

* Temps: very warm for the season with highs in the 40s on
Saturday and 40-50 on Sunday. This will be 10-20 degrees above
seasonal normals. Tobin



Sunday and Sunday night...a cold front plows across the inland
northwest on Sunday, helping to shunt the bulk of the moisture
from the atmospheric river south and east of the region. Gusty
westerly winds will develop by Sunday afternoon and evening with
gusts to 30 kts. Under the west to northwest flow, precipitation
will still be confined to the orographic favored areas like the
Cascades crest, the Blue Mountains and the central Idaho Panhandle.
Snow levels will be relatively high with mainly valley rain and
mountain snow. The only exception would be some High Valley snow
in the far northern valleys Sunday morning, but it looks the
combination of the winds from the frontal passage and arrival of
the drier air, will help mix out that colder air and help shut off
the additional low elevation snow accumulations in the northern
valleys. Also temperatures look to be warmest of the week on
Sunday.

Monday and Monday night..cooler and drier air to filter in from
the north, giving the region a break from the precipitation. An
upper level ridge will slowly migrate across the region. A few
lingering showers are possible by early Monday morning across the
northern mountains. Expect temperatures to cool late Monday
night.

Tuesday through the Christmas Holiday...the ridge aloft flattens
allowing for a more active period. One weather disturbance will
drop down from the northwest with a frontal band pushing through
the region on Tuesday, skipping across the Columbia Basin but
targeting the mountains. Chances for snow look good for Tuesday
morning across the northern valleys and mountains, then snow
levels will jump giving way to valley rain and mountain snow again
for Tuesday afternoon and evening. This frontal band will slide
south and stall across the Blue Mountains and southern Idaho
Panhandle. Meanwhile the models show more energy dropping down
from the Gulf of Alaska and strengthening the northern jet stream,
but what they show happens to the frontal band are two differing
scenarios. The GFS continues the trend that this northern
disturbance will carve out a deeper trough which digs further
south and pushes the boundary further into Oregon, and leaving
much of the inland northwest drier. The 00z European model (ecmwf) bounced back its
solution and shows the frontal band lifting northward and bringing
a return of precipitation, mostly in the form of snow, back into
the inland northwest. To avoid bouncing forecasts around, opted to
lean more toward climatology gives most areas a chance of snow for
Christmas eve day, then tapering off by Christmas eve night,
although confidence is marginally poor. A drier northwest flow
aloft will move into the region with cooler, more seasonal
temperatures into the end of the week. /Rfox.

&&

Aviation...
12z tafs: a frontal system is moving through the eastern portions
of the area this morning. Light to moderate precipitation will
effect the eastern taf sites for a few more hours...otherwise
expect drying from the west. Current VFR/MVFR conditions for the
eastern zones will begin to decrease MVFR/IFR after 15z as the
rain comes to an end and fog/stratus develops. Improvement to VFR is
expected after 22z. Stratus/fog will reform after 06z lowering
conditions back to IFR. Current IFR/LIFR conditions for kmwh/keat
may not see as much improvement. Cigs/vsby may increase to MVFR
at keat after 21z...but the fog/stratus is expected to reform
around 06z again tonight. Tobin

&&

Preliminary point temps/pops...
Spokane 41 33 40 38 46 36 / 100 10 90 100 50 20
Coeur D'Alene 41 33 39 36 44 36 / 100 20 90 100 70 50
Pullman 44 34 42 40 50 41 / 100 10 90 100 70 60
Lewiston 46 33 45 42 54 43 / 90 10 80 100 70 70
Colville 39 32 36 34 41 31 / 100 20 100 90 50 20
Sandpoint 37 32 37 34 40 34 / 100 30 80 100 70 60
Kellogg 38 31 38 34 41 36 / 100 40 80 100 100 90
Moses Lake 44 34 41 39 47 36 / 50 10 90 50 30 20
Wenatchee 42 32 37 37 45 36 / 30 20 90 60 40 20
Omak 38 30 34 34 39 32 / 80 20 90 80 30 20

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...air stagnation advisory until noon PST today for east slopes
northern Cascades-Moses Lake area-northeast mountains-
Okanogan Highlands-Okanogan Valley-upper Columbia Basin-
Waterville Plateau-Wenatchee area.

&&

$$

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