Ice Storm in Montana, December 3 to 4, 2015

Area Forecast Discussion
National Weather Service Missoula MT
931 PM MST THU DEC 3 2015

.UPDATE...The Winter Weather Advisories have been extended through
Friday morning due to the potential for freezing rain and very
slick roadways overnight.

&&

.DISCUSSION...The next front is moving into Idaho this evening and
will pass through the region tonight. Precipitation is expected to
expand into western Montana by early Friday morning while most
valleys in west central and northwest Montana remain below
freezing. Some clearing occurred before the front that allowed
places like Kalispell to get even colder with freezing fog. The
combination of cold temperatures, freezing rain, and patchy
freezing fog will extend the threat for very slick roadways into
Friday morning.

&&

.AVIATION...A return of reduced visibility and lowered ceilings
are likely overnight in the stable valleys of north central Idaho
and western Montana. There is also a threat for freezing rain into
Friday morning in the valleys of north central Idaho and west
central/northwest Montana with the next front passing. KGPI and
KMSO are included in the freezing rain threat. KBTM and KSMN are
likely to remain free and clear of any fog and/or stratus impacts
overnight with only light rain falling in over the higher terrain
in the vicinity.

&&

.PREV DISCUSSION... /ISSUED 236 PM MST THU DEC 3 2015/

DISCUSSION...The first in a series of weak disturbances will
continue moving through the Northern Rockies this evening. This
feature will produce very light precipitation across the region
and where valleys continue to deal with fairly strong valley
inversions, pockets of freezing rain are still a possibility for
a few locations across north central Idaho and western Montana.
Winter Weather Advisories have been issued to highlight the
potential public impacts attributed to freezing rain. The
inversions will continue to erode this afternoon and evening so
the precipitation should transition to all rain across most
valleys while light snow continues in the mountains.

A second disturbance will impact the Northern Rockies on Friday
into Friday night. This feature will bring in cooler air with the
snow level dropping to around 3000-4000 feet. The mountains along
the Montana/Idaho border will receive the bulk of the
precipitation and Lookout, Lolo and Lost Trail passes could
receive 2-4 inches of new snow by Saturday morning. Meanwhile valley
locations are anticipated to receive more of a rain/snow mix with
snow accumulations being generally less than a half of an inch.

The overall pattern in the long term is changing to a more stormy
and active. Most of the systems next week will be fairly weak.
However, one system Tuesday and Wednesday will have an inland
penetrating atmospheric river. The biggest impacts from this
system will be west of our area in eastern Washington, but some of
the heavy rain will spill over into northwest Montana and central
Idaho. Some locations in the Cabinet Mountains and Clearwater
Mountains could see around an inch of rain in 24 hours mid-week.
Snow levels will shoot up quickly as the moisture moves into the
area, peaking at around 7,000 feet before falling just as fast
behind a cold front. Thus most of the precipitation will fall as
rain before the cold front. Beyond mid-week expect a cool and
unsettled weather pattern with good potential for some mountain
snow.

AVIATION...The persistent fog/stratus which have impacted KGPI
and KMSO through much of the day will continue to dissipate as
precipitation moves into the region. However a return of reduced
visibility and lowered ceilings are likely after sunset and
through the overnight. Though conditions should not be nearly as
detrimental to aviation operations as those experienced this
morning. KBTM and KSMN are likely to remain free and clear of any
fog and/or stratus impacts overnight with only light rain falling
in over the higher terrain in the vicinity.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 11 AM MST Friday
     Bitterroot/Sapphire Mountains...Flathead/Mission Valleys...
     Kootenai/Cabinet Region...Lower Clark Fork Region...
     Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory until 10 AM PST Friday Northern
     Clearwater Mountains...Orofino/Grangeville Region...
     Southern Clearwater Mountains.

&&