Winter Storm in Montana, December 19, 2016

Area Forecast Discussion
National Weather Service Missoula MT
403 AM MST Mon Dec 19 2016

.DISCUSSION...Today will bring wind and snow for many locations.
Idaho county will be hardest hit from Elk River through Pierce and
Powell with 3 to 7 inches of new snow today. Wind will also be a
concern for those at higher elevations in the back country, but
should be as much of an impact along the mostly sheltered Hwy 12.
A winter weather advisory has been issued for this event.

Further west on the Camas Prairie down to Grangeville and Riggins
it will be very windy, and winds have already started gusting this
morning over 35 mph at some locations. Later today and this
evening gusts near 50 mph are possible at times. Down sloping with
the strong winds is expected to prevent significant snow in these
areas today.

In northwest Montana, snow has already started falling with 2 to 5
inches of new snow expected in Lincoln county and 3 to 7 inches
from West Glacier to Marias Pass. Much less snow is expected in
the Flathead and Mission valleys, but these areas are exposed to
enough wind that some difficulties are expected due to blowing and
drifting snow today. Advisories have also been issued in these
areas.

Elsewhere in western Montana, wind will be an issue. Winds are
already gusting this morning near 40 mph from Garrison Junction
through Deer Lodge and Anaconda. Blowing and drifting snow has
been a concern for this area, but so far none has been observed,
so we opted for a wind advisory for today in this area where
little to no new snow is expected.

Just as the advisories for today's weather are winding down
tonight, a winter weather watch goes into effect. A strong, warmer
system with a lot of wind and moisture impacts the region. The
Clearwater and Bitterroot Mountains of central Idaho and western
Montana will get pummeled with between 1 and 2 feet of new, wet
snow. This will be accompanied by a lot of wind, which will have
huge impacts for avalanche risks. In the valleys, the cold air
currently in place will get scoured out with temperatures rising
to around freezing for the first time in several days. The strong
winds will downslope, causing the valleys to get much less
moisture. Still, the southern and western portions of the
Bitterroot Valley could see 4 to 10 inches of new snow, while the
northern Bitterroot and Missoula valleys may only see a couple
inches of new snow and a lot of wind. The Mission and Swan
Mountains will also see a lot of new snow, with around 4 inches
in the lower elevations of Northwest Montana. Lookout and Lolo
Passes will both be hit with over a foot of snow. Travel over
mountain passes, especially over Lolo Pass into central Idaho will
very likely be dangerous Tuesday. As a result several watches
have been issued for this period. The area from Garrison Junction
through Anaconda and Butte during the Tuesday system is expected
to be windy again, with less than an inch of snow in the
forecast.

Wednesday through Thursday morning currently looks like a mostly
dry period in the wake of Tuesday's snowy system. Some showers
will remain in the terrain of northwest Montana, but these are
not expected to be impactful. Thursday evening through Friday
brings increasing moisture in westerly flow. Valleys may be
somewhat sheltered, but will still likely see light accumulating
snow.

Friday into Saturday and Sunday, models have come into slightly
better agreement regarding the next impactful weather system.
Current trends focus most of the energy to the south.
Nevertheless, west central Montana, southwest Montana, and Lemhi
county currently look to be the areas most likely to see
significant snow. That being said, model uncertainty is still
high, so stay tuned if planning to travel on or around Christmas.

&&

.AVIATION...Snow showers will likely affect visibility at KGPI and
KMSO this afternoon as a quick-moving system moves through. By
19/2000z, many sites could begin seeing gusty winds ahead of the
large trough of low pressure that comes in Tuesday. Snow totals
for today are forecast to be light, roughly 1 to 2 inches for
most valleys, with winds potentially adding a blowing component
to the wintry weather. Expect widespread mountain obscurations
with lowered ceilings and snow showers in the terrain.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 8 PM MST this evening
     Bitterroot/Sapphire Mountains.

     Winter Storm Watch from this evening through Tuesday afternoon
     Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

     Winter Weather Advisory until 8 PM MST this evening
     Flathead/Mission Valleys...Kootenai/Cabinet Region.

     Winter Weather Advisory until 8 PM MST this evening West Glacier
     Region.

     Winter Weather Advisory until 8 PM MST this evening
     Potomac/Seeley Lake Region.

     Wind Advisory until 5 PM MST this afternoon Butte/Blackfoot
     Region.

     Winter Weather Advisory until 8 PM MST this evening Lower Clark
     Fork Region.

ID...Winter Weather Advisory until 7 PM PST this evening Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     Winter Storm Watch from this evening through Tuesday afternoon
     Lower Hells Canyon/Salmon River Region...Northern
     Clearwater Mountains...Orofino/Grangeville Region...
     Southern Clearwater Mountains.

&&