Blizzard and Winter Storm in Idaho, December 27, 2016

Area Forecast Discussion
National Weather Service Missoula MT
637 PM MST Mon Dec 26 2016

.DISCUSSION...Added a winter weather advisory for the
Flathead/Mission valleys beginning late tonight and for the
Seeley/Swan valleys beginning early tomorrow morning. Snow
accumulations of several inches are expected to occur, which will
make for a slick morning commute on Tuesday. While snow will
likely decrease across the Flathead valley late Tuesday morning,
it will likely continue in the Seeley/Swan. Breezy westerly winds
by Tuesday afternoon will make for difficult travel due to
blowing and drifting snow along the Highway 200 corridor from
Clearwater Junction to Ovando.

We also started the winter weather advisories for the Clearwater
Mountains in Idaho and far northwest Montana about 6 hours earlier
than the previous forecast. It looks as though light to moderate
snow will begin sooner than previously thought (beginning
overnight vs early tomorrow morning).

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.AVIATION...As our next complex winter system moves in from the
west/northwest tonight, KGPI will see snow begin to fall as early
as 27/0300Z, with reductions in visibility and lower ceilings soon
to follow. KMSO, KBTM, and KSMN will also experience similar
operation impacts from snowfall, as well as elevated wind speeds,
by 27/1200Z and onward. Expect impacted operations from snow
through Wednesday morning at the earliest. In addition, an 80-130
knot jet will be draped from west to east over the Intermountain
West, and will create increased turbulence.

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.PREV DISCUSSION... /ISSUED 248 PM MST Mon Dec 26 2016/

DISCUSSION...As drier air moved over the Northern Rockies today,
many locations were able to see a little sunshine through the
clouds. On the other hand, several other locations held on to a
stratus deck that developed last night, which caused our morning
lows to be warmer than anticipated. Whether you saw some sun today
or held on to some clouds, we'll see an overall change to much
wetter, windier conditions come Tuesday.

Our next winter storm begins early Tuesday morning and persist
through Wednesday morning. Several highlights have been issued for
various impacts, mostly for travel. IMPACTS FOR THE TUESDAY
STORM:

1. Strong winds and blowing snow in Grangeville/Camas Prairie.
South winds gusting 40-50 mph at times tonight through Tuesday
will produce blowing snow thus reduced visibility, possibly to
less than 1/4 mile at times. Winds should subside a bit Tuesday
afternoon however snow accumulations of 1-4 inches are possible to
further hamper travel. In addition in central Idaho 6" of snow is
anticipated over Whitebird grade and 2-4 inches along Highway 95
Slate Creek to Riggins.

2. MOUNTAIN SNOWS - A strong westerly jet streak will settle over
the region Tuesday. This will be favorable for mountain snows and
strong winds. Areas vulnerable to a west wind could receive
sustained wind 15-20 mph with higher gusts. Blowing snow reducing
visibility to near white out at times will be the main impact.
Marias and Lookout passes are the most vulnerable passes for
blowing snow so winter storm warnings have been issued. Snow
amounts of 8-14 inches are likely in most mountain locations.
Lesser amounts of snow, only 1-3" is expected over MacDonald pass,
but because they will receive some pretty strong winds local,
blowing snow with reducing visibility is possible.

3. Snow Banding potential. Winds at the mid levels will turn to
the northwest Tuesday afternoon and evening. These winds combined
with moisture and instability will be favorable for snow bands to
develop across western Montana. Snow bands could produce a quick
1-3 inches of snow which will impact traffic quickly reducing
visibility and hampering travel especially as timing of the snow
will be around commute time. Stay tuned for updates and follow us
on twitter for real time updates on where these bands start to
setup and move.

WEDNESDAY...Weak northwest flow will persist and initiate
mountain snow showers and perhaps a few snow bands in the
afternoon.

High pressure looks to briefly build over the region for Thursday
morning, which could result in a very cold morning in some
valleys, pending cloud cover. Model agreement on the next trough
is improving, so another snow producing trough could arrive as
early as Friday morning. This weather system has the potential to
bring a swath of moderate to heavy intensity snow during the day
on Friday.

Another snow producing weather system is possible for Sunday, and
this weather system may also have an arctic front with wind and
colder temperatures. These weather systems will make for an
active weather period with continued snow accumulations to finish
2016.

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.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 11 PM this evening to 5 AM MST
     Wednesday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 11 PM this evening to noon MST
     Tuesday Flathead/Mission Valleys.

     Winter Storm Warning from 5 AM Tuesday to 5 AM MST Wednesday
     West Glacier Region.

     Winter Weather Advisory from 11 PM this evening to 5 AM MST
     Wednesday Kootenai/Cabinet Region.

     Winter Weather Advisory from 5 AM Tuesday to 5 AM MST Wednesday
     Potomac/Seeley Lake Region.

     Winter Storm Warning from 5 AM Tuesday to 5 AM MST Wednesday
     Lower Clark Fork Region.

ID...Winter Weather Advisory from 10 PM this evening to 4 AM PST
     Wednesday Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Weather Advisory from 5 AM Tuesday to 11 AM MST Wednesday
     Eastern Lemhi County...Western Lemhi County.

     Winter Weather Advisory from 10 PM this evening to 10 PM PST
     Tuesday Lower Hells Canyon/Salmon River Region.

     Winter Weather Advisory from 10 PM this evening to 10 PM PST
     Tuesday Orofino/Grangeville Region.

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