Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 180029
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
429 PM PST Sun Dec 17 2017

.SYNOPSIS...
Light snow will continue this evening with the potential for
moderate accumulations on area mountain passes. A warmer and
windier weather system impacts the region Tuesday into Wednesday
bringing the potential for heavy mountain snows and mix of rain
and snow in the lower elevations. The weather will then be dry and
very cold for the remainder of the week.


&&

.DISCUSSION...
Tonight through Monday: Subtropical moisture is being transported
into the region on the backside of a flat ridge. The west to
northwest trajectory of this moisture has resulted in steady light
precipitation along the Cascade Crest then redeveloping over
Eastern WA and N Idaho where there is persistent warm advection
and isentropic ascent. There is still enough subsidence aloft from
the flat ridge to keep precipitation intensities on the light
side due to the sinking air within the favorable ice growth layer.
However we expect this will change overnight as moisture
continues to deepen and lift becomes enhanced, first along a warm
front tonight then cold front Monday. As such, look for snow to
increase for a few hours this evening before migrating east into
the Idaho Panhandle overnight. Additional snow amounts of 1-3
inches will be possible in the valleys of NE WA and N ID with
higher amounts in the mountains. Snow should begin to re-stick to
roads as the sun sets but do not expect temperatures to fall much.
In fact, most locations will cool near the freezing mark with
snow falling but will warm slowly by Monday morning near to above
freezing as southerly winds increase and warmer air surges north.
With this in mind, snow advisories for NE WA and N ID expire at
10PM this evening. Winds will remain breezy on Monday keeping
temperatures mild for mid December standards. This will push snow
levels up between 3000-4000 feet and when the cold front comes
through Monday, precipitation will sag south into Southeastern WA
and lower two-thirds of the Idaho Panhandle. We will need to keep
a close eye on rainfall amounts and snow melt into the Paradise
Creek Basin which is quite flashy in these quick snow melt
regimes. As for travel, the subtropical moisture fetch has pushed
snow levels just above Stevens Pass this afternoon with rain
currently falling. Snow will continue for Lookout Pass throughout
the next 24 hours with moderate to heavy accumulations possible.
Road temperatures may rise above freezing during the late
afternoon leading to mainly wet roadways for a few hours but it is
always recommended to check pass conditions before venturing out.
/sb

Monday night through Wednesday night...A weak cold front sags south
through the northern portion of the Inland Northwest Monday
afternoon, then pushes back north as another warm front Tuesday
night and Wednesday. After a brief break in the precipitation
late in the day Monday, or in the case of the central Panhandle a
minor slow down, the spigot will be turned back on. Surface to
850mb winds become southeast to south for a good period Monday
night then southerly and this along with moderate to strong
isentropic upglide will provide plenty of lift through through the
day on Tuesday in the Cascades and to the east across the northern
mountain zones. A robust cold front will follow Tuesday evening
and will be east of the forecast area by early Wednesday morning.
This will bring an end to the precipitation from west to east.
However upsloping into the Cascades and the Panhandle mountains
will keep fairly high chances for precipitation for continued
snow showers into Wednesday morning. Moisture increases to well
over 200 percent of normal as deep sub tropical moisture moves
into the region. This pattern is a good set up for an extended
period of moderate to heavy precipitation. Lastly we look at
winds with and behind the cold front. Winds will increase on
Tuesday with gust 20-30 mph lingering around the region through
Wednesday. This is not real bad, but enough for the potential of
localized blowing snow for the mountains.

Snow levels across the south will be 3500-5000 feet Monday night
and increase from there, then decrease behind the cold front, but
by that time we lose the deep moisture tap, so no major concerns
as precipitation will be as rain. Although some of the smaller
streams may see some rises.

North of about highway 2 snow levels are between 1200-3000 feet
with snow or a mix of rain and snow down to the valley floors.
Snow in the mountains above 3000 feet will pile up pretty good
with anywhere from 8-14 inches for the Okanogan Highlands to 1-2
feet or more in the Cascades and the north Panhandle mountains,
and slightly less for the central Panhandle mountains. The valleys
will get there share as well before the snow turns to a rain/snow
mix.

*Impacts: As of this writing there are several winter highlights
 that are currently in play. The advisories for The North and
 central Panhandle were upgraded to a warning. The central
 Panhandle likely will not see much of a break, if any, in the
 precipitation. The north Panhandle will likely see about a 6-8
 hour break as the cold front passes through late Monday
 afternoon, but with more snow expected tonight and Monday and to
 avoid confusion it was decided A Winter Storm Warning was
 warranted. A snow advisory is still in affect for the Northeast
 mountains for snow through 1000 PM this evening. But then that
 area does see a pretty good break in the snow tonight and Monday
 and a Winter Storm Watch was also issued for the the additional
 snow Monday night and Tuesday. For the Okanogan Highlands and the
 Cascades a Winter Storm Watch has been issued for Monday night
 and Tuesday for heavy snow. Driving through the Idaho mountains
 will be difficult tonight and all of the passes and many of the
 valleys will have hazardous driving conditions for the Tuesday
 morning commute. Tobin

Thursday through Sunday: A strong ridge along the 140W meridian
and reaching into Alaska will for a decent push of Arctic air into
the region during this period. A trof will preempt this system
bringing snow showers to the region on late Thursday and Friday.
Breezy wind down North/South oriented valleys and the Camas
Prairie in the range of 10 to 15 MPH with gusts in the mid 20s. By
late Friday, dry cold air will begin to move into the region.
High for the period will start in the upper 20s to low 30s and
drop into the low 20s and teens for the weekend. The lows will
start lows 20s and upper teens and drop into the low teens and
single digits for the weekend. /JDC

&&

.AVIATION...
00z TAFS: ISOLD -shrasn will be possible for KEAT/KMWH but not
likely with cigs/vsby VFR/MVFR at KEAT. KMWH has been struggling
all day to break out of the stratus and confidence that they do is
low so expect IFR/LIFR conditions. Things are much messier
further to the east. A warm front is moving slowly through the
eastern portions of the area. Expect mountain obscurations along
the ID/MT border the next 24 hours. -SHRASN will result in
fluctuating cigs/vsby through 06-08z with mainly MVFR/IFR
cigs/vsby and locally LIFR. Behind the precipitation warm air over
snow would normally result in conditions dropping to IFR/LIFR.
However SW winds 15-20kts with gust 25-30kts will result in some
mixing so stayed with MVFR/IFR cigs/vsby but forecast confidence
is rather low. Tobin



&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        35  41  34  40  30  33 / 100  10  90 100  80  30
Coeur d`Alene  34  40  34  39  30  33 / 100  70 100 100  90  40
Pullman        35  41  36  40  31  33 /  90  70  90  90  70  30
Lewiston       37  45  38  45  33  38 /  40  30  70  40  50  20
Colville       33  39  31  37  29  33 /  40  10  90 100  80  30
Sandpoint      33  39  32  37  31  33 / 100  80  90 100 100  40
Kellogg        32  36  33  36  30  30 / 100  90 100 100  90  60
Moses Lake     34  45  33  44  30  38 /  10   0  60  90  10  10
Wenatchee      33  43  32  39  28  36 /  10  10  50  70  20  10
Omak           33  38  31  37  27  32 /  30  10  60  80  50  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Storm Warning until 7 AM PST Wednesday for Central
     Panhandle Mountains.

     Winter Weather Advisory until 10 PM PST this evening for Coeur
     d`Alene Area.

     Winter Storm Warning until 7 AM PST Wednesday for Northern
     Panhandle.

WA...Winter Weather Advisory until 10 PM PST this evening for
     Northeast Mountains.

     Winter Storm Watch from Monday afternoon through late Tuesday
     night for East Slopes Northern Cascades.

     Winter Storm Watch from Monday evening through late Tuesday
     night for Northeast Mountains-Okanogan Highlands.


&&

$$



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