Winter Storm and Winter Weather in Montana, December 12 to 14, 2015

Area Forecast Discussion
National Weather Service Missoula MT
320 PM MST SAT DEC 12 2015

...Hazardous Winter Travel Conditions Tonight through Monday...
...Unsettled Weather with Much Colder Temperatures Next Week...

.DISCUSSION...

The next weather system is right on our doorstep as of this
afternoon, and will begin to move through Idaho and western
Montana by this evening. This storm will bring several rounds of
winter weather; one this evening and tonight affecting areas
generally west of Missoula, and another later Sunday into Monday,
impacting areas generally south of I-90. The first round will have
the biggest travel impacts along I-90 over Lookout Pass to Haugan
and Highway 12 over Lolo Pass, where over a foot of snow is
expected. Wet westerly flow will favor all west-facing slopes from
the Bitterroot Crest westward, with gusty winds in addition to the
snow.

As the trough deepens Sunday, the area of heaviest precipiationwill
shift into a southwest-to-northeast oriented band from the central
mountains of Idaho through southwest Montana, and slowly progress
eastward through Monday. Areas such as Highway 93 over Lookout
Pass and I-90 from Butte over Homestake Pass and east of the
Continental Divide will likely see well over a foot by the time
the storm exits the area. In addition, a cold front will push
across Lemhi County Idaho and southwest Montana Sunday night,
resulting in reduced visibilities with blowing snow. Elsewhere,
northwest Montana and populated valleys from Kalispell to Missoula
may see snow mix with rain at times Sunday, but will likely see
travel impacts as well, especially towards the Monday morning
commute for the Bitterroot Valley. Northwest Montana will be
spared the heaviest snow Sunday night. Lastly, the Camas Prairie
region of Idaho may see upslope snow increase in intensity by
Monday as snow levels lower, impacting Grangeville to Whitebird
Summit.

Bottom line: expect difficult travel at times this weekend,
especially in the higher terrain and over mountain passes, due to
both snow and wind.

Light snow will linger over the Northern Rockies Monday night
through Tuesday morning as the wrap around moisture from the
exiting low pressure interacts with a northerly upslope flow.
Most of the accumulating snowfall will be in the Butte/Blackfoot
region, with an additional 2-3 inches of snow. Locations west and
south of I-90, including central Idaho, will only see light
accumulations, around a trace to 1 inch of snow. Most of the area
will see a break in the snow Tuesday afternoon and evening as the
upper-level flow transitions to a northwesterly flow. A shortwave
trough will move through the flow, allowing light widespread snow
to once again impact the region in a 24 hour period starting
Wednesday morning. Typically in this scenario snowfall remains
fairly light. Though, 1 to 3 inches of snow will be possible for
the valleys of western Montana and central Idaho. The mountain
passes could see accumulations up to 5 inches. Snow levels will be
low enough for all valleys in the locations mentioned above, to
see at least a trace of snow.

Temperatures will remain fairly cold, and below normal for the
rest of the week, with no new snow impacting the region. Patchy
fog will however impact some valley locations of western Montana
and central Idaho, creating slick roads due to below-freezing
temperatures. The short break in the active weather could come to
an end by the start of next weekend, with another potentially
strong winter storm entering the region. Stay tuned!

&&

.AVIATION...
Light snow showers will continue off and on for KMSO and KGPI,
which could temporarily reduce visibility and obstruct mountains
from brief low ceilings. A strong winter system approaches this
evening, bringing moderate to heavy snow and low
visibility/ceilings to western Montana and north-central Idaho.
KGPI, KMSO, and KSMN will start to see impact around 13/0600Z and
continue through Sunday. Snow impacts at KBTM will begin generally
after 13/1200Z. Moderate to strong westerly winds aloft will
increase tonight through Sunday morning, with 40-50 kts right
above mountain ridges. Southerly winds of 10-15 kts are possible
near the surface for all terminals overnight into Sunday morning,
as well. Cold frontal passage from the northwest is expected to
pass through northwest Montana and KGPI between 13/1500Z and
13/1800Z, and reach KMSO between 13/2100Z and 14/0000Z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 8 AM Sunday to 11 PM MST Monday
     Butte/Blackfoot Region...Potomac/Seeley Lake Region.

     Winter Storm Warning until 11 AM MST Monday Bitterroot/Sapphire
     Mountains...Lower Clark Fork Region.

     Winter Weather Advisory until 11 AM MST Monday
     Missoula/Bitterroot Valleys.

     Winter Weather Advisory until 11 AM MST Monday Flathead/Mission
     Valleys...Kootenai/Cabinet Region.

     Winter Weather Advisory until 11 AM MST Monday West Glacier
     Region.

ID...Winter Storm Warning from 8 AM Sunday to 11 PM MST Monday
     Eastern Lemhi County.

     Winter Storm Warning until 11 AM MST /10 AM PST/ Monday Lower
     Hells Canyon/Salmon River Region...Northern Clearwater
     Mountains...Southern Clearwater Mountains...Western Lemhi
     County.

&&