Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 161208

Area Forecast Discussion
National Weather Service Spokane WA
408 AM PST Tue Jan 16 2018

The weather turns unsettled for the rest of the work week. The
first frontal system will move through Tuesday with a band of
light rain or snow. A second, stronger and wetter frontal system
arrives Wednesday night into Thursday with a rain/snow mix in the
valleys with snow in the mountains. A cooler and potentially snowy
pattern is possible for the weekend.


Today and Tonight...A stretching and somewhat negative tilt trough
continues to move east today. Recent observations show rain and
higher elevation snow as far east as Ephrata stretching into the
northern Cascades early this morning. Surface observations have
shown moderation of near freezing temperatures in the warm tongue
ahead of the front. Where showers have occurred there has been
actual warming of temperatures. With this trend the threat of snow
for most of the Basin to Wenatchee looks pretty low. Farther north
toward Omak and the northern Cascade valleys there is still hope
for a little light snow maybe an inch or two with the higher
amounts in the upper Methow valley. Heavier rain and snow showers
look to develop coalescing along the frontal band later this
morning. The slow progression of precipitation doesn`t look to
reach the WA/ID border until around 10 am. By that time the
Cascades and west Basin will begin to shut off precipitation
behind the frontal band. The front continues to weaken toward late
afternoon with only some lingering showers in the Idaho Panhandle
before sunset. Overnight expect some fog to redevelop with the
recent precipitation and cool nighttime temperatures. TC

Wednesday through Monday: The Inland NW will be in an active
weather pattern with several storm systems poised to move through
the region. In between organized storm systems, weaker
disturbances will keep precipitation chances elevated. The first
storm system to arrive Wednesday-Thursday will be the warmest and
wettest with good ties to subtropical moisture. By Thursday night,
we start to cool the atmosphere and snow returns to the forecast
at different intervals, especially locations outside the Columbia
Basin. The next two organized storm systems look to arrive on or
around Friday and again Sunday. The pattern gets rather messy
behind the first system and I would anticipate moderate changes
from day to day as we dial in timing and precipitation amounts.
There has been decent consistency over the last few days that the
region will become positioned on the north side of the Polar jet
suggesting increased odds for snow. The only caveat for each of
the system Friday and Sunday systems will be a steady southwest
wind across the Columbia Basin and toward the Spokane Area which
may be enough to warm the boundary layer above freezing and keep
snow accumulations limited to nighttime hours. Plume diagrams via
the North American ensembles support the idea of several rounds of
light to moderate snow late week into the weekend for communities
of NE WA and N ID so despite the spring like conditions that were
around the last few days, do not put the shovels away quite yet.
A bit more details regarding the two larger systems can be found

Wednesday-Thursday: A warm front lifts through during the day Wednesday
with light precipitation mainly for the North Cascades. GFS/NAM
wavered again and came in a bit wetter with potential for an inch
of snow. There will be a break through much of the day then
precipitation picks up in the evening and overnight with deep
moisture transport ahead of a cold frontal occlusion. Very mild
air will be drawn into Eastern WA and N ID ahead of this front
with this evening`model runs showing 850mb temperatures over the
Palouse near 11C or 52F and around 3C (37F) at Usk, WA. The trend
with yesterday`s models was to bring this warm surge further north
and this appears consistent with tonight`s runs. There still
looks to be enough cold air dammed into the Methow Valley eastward
toward Republic and Chesaw to allow precipitation to be, or at
least start as snow. Per NAM bufkit data, the thermal profile at
Republic and Omak will be nearly isothermal from the surface to
3-4k ft AGL and hovering very close to the 0C isotherm. This leads
to the low confidence how long snow will persist and resultant
accumulations. This evening`s 00z run via the NAM was the first to
introduce a few hours of rain at Republic and Omak indicative of
the warmer trends. If it does remain all snow, expect snow to be
extremely wet and heavy. The cold air will be deeper in the Methow
Valley and with southeast flow in place, see little evidence for
this to change around Winthrop and Mazama and this will be area
that carries the highest probabilities for moderate snow
accumulations. A lot will rely on what happens to these locations
over the next 36 hours. Many of these locations are 33-34F right
now. Will warm into the mid to upper 30s today, then we should see
get some cooling and a break in the clouds Tuesday evening prior
to the warm front arrival but if temperatures continues to rise
after the warm frontal passage, we may be looking at less snow
accumulations and more rain or rain/snow mix by the time the
deeper moisture arrives Wednesday evening. Sherman Pass and
Selkirks are also looking at the potential for 5-8 inches of wet
snow above 5000-6000 feet. Cannot rule out a few pockets of wintry
mix between the rain/snow line, at this time, opted to leave it
out. The punch of warmer, subtropical air will not stick around
long and snow levels will lower Thursday afternoon and night. Not
necessarily to all valley floors immediately but low enough to
switch precipitation back to accumulating snow for Lookout Pass,
Deary, Clarkia, and Camas Prairie around Winchester. Snow will
also continue across the Cascade Crest and at Stevens Pass...with
moderate snow possible. A wave arriving in the wake of the frontal
occlusion will renew the chance for light snow showers across the
Upper Columbia Basin and northern valleys with a slushy inch
possible by Friday morning.

Rainfall amounts of half an inch to three quarters of an inch will be
possible across the Idaho Panhandle and eastern third of
Washington with local amounts near an inch possible in the Blue
Mountains, Camas Prairie, Palouse, and southern Shoshone County.
This will create the potential for minor hydrologic issues such as
sharp rises on area streams, ponding water, and potential for mud
and rock slides within the steep terrain of Nez Perce and Lewis
Counties. Cannot rule out some problems extending into Northern
areas of the Panhandle but there is much more snow on the ground
which should act as a sponge to the incoming rainfall and continue
to insulate the ground from the roller coaster of temperatures, a
contributor to rock slides.

Winds will be gusty in the foothills of the Blue Mountains, Palouse,
Camas Prairie, and around the L-C Valley. Sustained winds of 15
to 25 mph will be possible with local gusts to 40 mph as
south/southeast winds channel along the Snake River Valley and
across the Blue Mountains. Temperatures Wednesday evening
accompanying these winds could warm into the 40s to 50s.

Sunday/Monday: Sunday will be the next threat for moderate to
heavy precipitation. At this point, the air mass looks cold
enough to support snow for all locations north of I-90. Model
trends have been toward a deeper fetch of moisture which has
resulted in slightly warmer air punching into the lower Basin and
lower confidence for precipitation type across the southeastern
Basin and L-C Valley. Communities across Northern WA and North
Idaho will stand a decent shot of light to moderate snow
accumulations with snow showers persisting into Monday. /sb


12Z TAFS: Variable IFR/MVFR conditions will spread from west to
east as moisture and precipitation associated with a passing
front brings low clouds and visibility. KEAT and KMWH TAFs will be
affected from 12-17Z, other eastern TAFS will be impacted after
17Z including KGEG, KSFF, KCOE. MVFR CIGS will likely continue
after after 22Z with light southwest winds and upslope conditions
continuing into the evening hours. Patch fog will likely develop
after 02Z through the remaining TAF period to 12Z as moisture
lingers. TC


Spokane        37  30  38  37  41  30 /  60   0  10 100  80  40
Coeur d`Alene  37  30  39  36  40  31 /  60   0  10  90  90  50
Pullman        42  32  44  39  44  31 /  80   0  10  70  90  30
Lewiston       47  33  47  41  46  34 /  80   0  10  40  90  30
Colville       35  31  37  34  40  29 /  60  10  20 100  60  40
Sandpoint      36  30  36  34  40  31 /  60  30  10 100  90  60
Kellogg        36  29  38  34  38  30 /  60  10  10  80  90  50
Moses Lake     42  31  39  36  45  30 /  90   0  10  70  30  10
Wenatchee      37  31  35  31  40  28 /  90   0  30  80  20  20
Omak           34  29  34  32  37  26 /  80   0  60 100  40  30




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