Winter Storm in Montana, December 29 to 30, 2018

Area Forecast Discussion
National Weather Service Missoula MT
247 PM MST Sat Dec 29 2018

...Difficult travel tonight through Sunday due to snow, blowing
snow and slick roads...

.DISCUSSION...A plume of subtropical moisture will interact with
an approaching cold front to create a host of wintry impacts this
evening into Sunday. Initially, snow levels will be on the rise,
with most valleys seeing a transition to rain or a rain snow mix
late this afternoon and evening. The one exception is in the
Missoula valley and portions of the I-90 corridor east of Missoula
where persistent cold air may allow for a period of freezing rain.
Meanwhile in the higher terrain and over mountain passes, moderate
to heavy snow will develop. Increasing winds will allow for
blowing and drifting snow, which could cause travel to become
particularly difficult over passes such as Marias, Lookout and
Lolo into Sunday.

A strong cold front will pass through the region late tonight,
quickly crashing snow levels down to valley floors. A transition
from rain/freezing rain to all snow will occur, even down into the
lowest elevations. Although overall snow accumulations in most
valleys should be relatively light, the combination of slushy snow
and wet roads freezing up as colder air and gusty winds move into
the region will create icy and slick travel generally area wide.

By Sunday morning, precipitation will become more showery.
Forecast models are indicating the potential for some moderate
snow bands. Depending on where these set up, this could bring more
snowfall to valleys. These bands would also be accompanied by
gusty winds, which could cause areas of blowing snow and low
visibility. Travel across southwest Montana, particularly along
I90 from Missoula to Butte and over MacDonald Pass could become
particularly difficult. Numerous winter weather highlights are in
place across the region. Bottom line: Be prepared for difficult
conditions if traveling tonight into Sunday morning.

By Sunday evening, a secondary cold front from Canada will sneak
southward and over the Continental Divide. This will allow for
even colder air to filter into the region and bring a shift in
winds to a more northerly direction. An area of light to moderate
snow will likely develop across western Montana Sunday evening as
this occurs, prolonging wintry conditions into Monday morning.

Very light snow will gradually decrease on Monday with much colder
air settling in. This cold air along with decreasing winds
presents on opportunity for lake effect snow on the south and east
side of Flathead Lake Monday afternoon and evening. High pressure
ridging will nose into the Northern Rockies Tuesday into
Wednesday leading to drying, but occasional light snow will
continue at times on the terrain as weak disturbances pass
through. Patchy fog and stratus are also possible during the
morning hours each day. Daytime highs will only reach the teens
and 20s, while overnight lows drop into the single digits and even
below zero in the favored cold spots.

Late next week into next weekend, the general pattern supports
several weak disturbances passing through the Northern Rockies.
These disturbances will lead to a wintry mix in the valleys and
periodic light to moderate snow in the mountains but a major
winter storm is not expected.

&&

.AVIATION...An active weather pattern will continue across the
Northern Rockies as a significant weather disturbance moves into
the area Saturday night. Snow levels will rise close to 4,000
feet this evening with a transition to rain expected at KMSO and
KGPI. Cold sub-surface temperatures in the Missoula Valley could
cause a thin layer of ice build up at KMSO. Snow levels will come
crashing down after 30/0600z as a vigorous cold front moves
through the Northern Rockies. Snow showers will align into bands
Sunday morning and will lead to periods of MVFR/IFR visibility at
area TAF sites. KMSO and KSMN will have greater chances of being
impacted by these bands. Snow shower activity will continue Sunday
across the area under a cold, moist, and unstable northwesterly
flow.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 11 PM this evening to 2 PM MST
     Sunday Butte/Blackfoot Region.

     Winter Storm Warning from 8 PM this evening to 2 PM MST Sunday
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 11 PM this evening to 2 PM MST
     Sunday Missoula/Bitterroot Valleys...Potomac/Seeley Lake
     Region.

     Winter Storm Warning until 2 PM MST Sunday Lower Clark Fork
     Region.

     Winter Weather Advisory from 11 PM this evening to 2 PM MST
     Sunday Flathead/Mission Valleys...Kootenai/Cabinet Region.

     Winter Storm Warning until 2 PM MST Sunday West Glacier Region.

ID...Winter Weather Advisory from 2 AM to 2 PM MST Sunday Eastern
     Lemhi County...Western Lemhi County.

     Winter Storm Warning from 7 PM this evening to 1 PM PST Sunday
     Southern Clearwater Mountains.

     Winter Weather Advisory from 10 PM this evening to 1 PM PST
     Sunday Lower Hells Canyon/Salmon River Region...
     Orofino/Grangeville Region.

     Winter Weather Advisory until 1 PM PST Sunday Northern
     Clearwater Mountains.

&&