Winter Storm and Winter Weather in Montana, December 18, 2015

Area Forecast Discussion
National Weather Service Missoula MT
237 PM MST THU DEC 17 2015

...Snow tonight transitioning to freezing rain by Friday
morning...

.DISCUSSION...A complex weather scenario is shaping up for the
next 24 hours as a weather system and associated warm front move
through the region. Initially, this system will bring widespread
snow to north central Idaho and western Montana beginning this
afternoon in Idaho and this evening across western Montana.
Generally light snow accumulations are expected during this time
frame, allowing roads to become snow covered for many locations.

Late tonight into Friday morning, the warm front will begin to
warm the mid levels of the atmosphere. While warming occurs aloft,
cold air will remain trapped in the valleys. This sets up a
scenario where freezing rain can develop. The lower valleys of
north central Idaho (including the Highway 12 and Highway 95
corridors), and valleys of west central Montana (Interstate 90
from Haugan to Garrison, Highway 200, Highway 93 from the Mission
valley south through the Bitterroot valley) currently look most
susceptible to this freezing rain threat. Extremely slippery roads
are possible for the Friday morning commute in these areas.
Therefore, winter storm warning/winter storm watches have been
issued for many of these areas due to the potential impact to
travel.

Meanwhile, further north across northwest Montana (generally north
of the Highway 200 corridor), the warm air aloft is not expected
to be nearly as prominent. Therefore, many locations will likely
stay snow through this entire event. Anywhere from 2-5" of snow
will be possible, with locally heavier amounts possible in Glacier
National Park, and the Seeley and Swan valleys. Numerous winter
weather advisories have been issued for the expected slick and
snow covered roads.

A cold front will swing back through the region Friday night,
largely ending the threat of freezing rain across the region.
There are indications this front may stall somewhere across west
central Montana, which could lead to another period of light to
moderate snow accumulations for valleys with higher amounts in the
mountains and over mountain passes. Travel will continue to be
difficult into Saturday for those leaving for the holiday.

Monday night through Wednesday morning: A strong jet stream is
expected to bring continued moisture to the region. With the upper
level jet expected to track from Oregon towards southern Idaho,
this will mean a chance for snow in both mountains and valleys.
There is still a chance for gusty winds across north- central
Idaho which could combine with snow and cause hazardous travel
impacts. There will also be above normal chance for precipitation
across western Montana through this period.

Wednesday through Christmas Day: A rather active period in terms
of winter weather. Temperatures will generally be near to below
normal and there could be several disturbances that bring periods
of snow to both mountain and valley locations.

&&

.AVIATION...Cloud cover is lowering throughout the region this
afternoon, and mid-level cloud cover has already reached north
central Idaho as seen on the KGIC observation. Precipitation
associated with a warm front is also widespread in Washington and
Oregon and high resolution radar forecasts (HRRR) show this
precipitation to spread through all of north central Idaho by
17/2300z. Snow is likely to be the predominant precipitation
type tonight. The snow will move into western Montana around
18/0100z. Widespread obscured terrain and lowered visibility can
be expected with this warm front. The peak of warm air will be
over north central Idaho near 18/1200z and western Montana by
18/1500z. Valley locations are forecast to remain stable and cold
tonight and Friday morning, so there is a strong concern that
freezing rain will develop with this warm air. Highest chances for
freezing rain will be near the Interstate 90 corridor and south,
which includes KMSO. Winter Storm Warnings and Watches have been
issued because of this freezing rain threat. The cold front will
move into the region during Friday afternoon which will help to
lower snow levels. There are indications on recent model guidance
that the cold front will stall and slowly pass through north
central Idaho and western Montana on Friday night and Saturday.
This slow moving cold front will keep low ceilings, snow, and
lowered visibilities through at least Saturday morning.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 11 PM this evening to noon MST
     Friday Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region.

     Winter Storm Watch from late tonight through Friday morning
     Missoula/Bitterroot Valleys.

     Winter Storm Warning from 2 AM to noon MST Friday Lower Clark
     Fork Region.

     Winter Weather Advisory from 2 AM to noon MST Friday
     Flathead/Mission Valleys...Kootenai/Cabinet Region...West
     Glacier Region.

     Winter Storm Watch from late tonight through Friday morning
     Potomac/Seeley Lake Region.

ID...Winter Weather Advisory from 5 PM this afternoon to noon MST
     Friday Eastern Lemhi County...Western Lemhi County.

     Winter Weather Advisory from 4 PM this afternoon to 11 AM PST
     Friday Lower Hells Canyon/Salmon River Region...Northern
     Clearwater Mountains...Orofino/Grangeville Region...
     Southern Clearwater Mountains.

&&