Winter Storm and Avalanche in Montana, December 28 to 30, 2017

Area Forecast Discussion...CORRECTED
National Weather Service Missoula MT
414 AM MST Thu Dec 28 2017

......PROLONGED HEAVY SNOW EVENT BEGINS TODAY...

.DISCUSSION...The next few days worth of weather is as textbook as
it gets when it comes to a favorable snow-producing pattern for
the Northern Rockies. Such factors necessary for efficient,
intense snow production are present in our upcoming winter storm,
with the likelihood of generating crippling amounts of snowfall
across western Montana and north central Idaho, in addition to
other dangerous impacts.

So what are the impacts and timing? Let's hit them one by one:

FREEZING RAIN/WINTRY MIX:
Freezing rain is expected to impact valleys of north central Idaho
and west central Montana (particularly the mid-to-southern
Bitterroot valley, the general Highway 12 corridor, and the
Orofino-Grangeville-Riggins-Salmon corridor). Our confidence that
freezing rain could become a major factor is high, but timing the
freezing rain remains difficult. We estimate freezing rain to
affect the aforementioned areas any time this afternoon through
early Friday morning, before turning back over to all snow
Saturday. There is also a chance for the Bitterroot valley to see
a short period of freezing rain Friday morning before changing
over to, and remaining, all-snow thereafter. The threat of
freezing rain falling on top of already-snowpacked road surfaces
will create additional travel complications.

INTENSE SNOWFALL RATES = HEAVY SNOW ACCUMULATIONS:
North of the Highway 12 corridor, including the northern
Bitterroot valley, the Missoula valley, the I-90 corridor and all
points north of there, will remain snow for most of the duration
of this storm. In fact, the general I-90 corridor will be the
region of best snow production, beginning today, increasing in
intensity Friday, and then lasting through Saturday. Snowfall
rates could approach the 1 to 2 inch per hour rate Friday
afternoon through Friday night, making travel extremely dangerous
for a prolonged period of time.

BLOWING SNOW FROM ARCTIC WINDS:
At the same time, though northwest Montana will receive
respectable snow accumulations, blowing snow and near-zero
visibility could become their greatest impact Friday through
Saturday as the arctic front punches through and heads south
towards I-90. Winds accompanying this front may cut down snow
totals across the NW, but will create hazardous travel as a result
of low visibility and snow drifts over roads. Wind chills will
also be quite low during this time.

BACKCOUNTRY DANGER:
And lest we forget the impact to the mountainous terrain: aside
from several feet of new snow expected, intense snowfall rates
with gusty winds (particularly in NW MT) will make for extremely
dangerous backcountry conditions. Avalanche danger will creep
upwards with time, so please be sure to check with your local
avalanche center before heading out to enjoy all the new powder
when it is all said and done.

IN SUMMARY: Dangerous travel, power outages from snow loading and
gusty winds, blowing and drifting of snow, potential aviation
impacts due to snowfall rates and low visibility, and hazardous
backcountry conditions will make for a very treacherous winter
conditions Thursday through Saturday!

Expect some lingering snow showers with little accumulations
Saturday evening through Sunday morning behind the exiting front.
Clear skies Sunday night combined with fresh snow cover will
result in cold temperatures Monday and Tuesday mornings, with
overnight lows in the single digits.

A ridge of high pressure along the PAC NW will result in a
northwesterly flow pattern over the Northern Rockies through the
middle of next week. Outside of continuing cold temperatures with
lows in the teens, there doesn't appear to be any big weather
impacts. Late next week, models are trying to move a high
pressure ridge over the Northern Rockies, which would result in
strengthening inversions, cold pools, and possibly valley fog.

&&

.AVIATION...Light snow will impact all terminals through this
afternoon with a couple of inches of snow possible at KGPI and
KMSO. Little accumulations are expected at KBTM and KSMN. However,
wherever it is snowing there will be reduction to visibility and
ceilings. Snow intensities will pick up arund 29/0000z at KGPI,
and 28/0060 at KMSO, persisting through 29/1200z. Snow
accumulations of 2-4" are expected in this 12 hour period. Expect
just light snow with around 1" of snow from 29/0000z-29/1200z at
KBTM and KSMN. Friday afternoon through Saturday morning: the
winter storm will strengthen and produce significant snow
accumulations, especially at KGPI and KMSO. Significant impacts to
aviation are anticipated.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 5 AM Friday to 5 PM MST Saturday
     Butte/Blackfoot Region.

     Winter Storm Warning until 5 PM MST Saturday Bitterroot/Sapphire
     Mountains.

     Winter Storm Warning from 11 AM Friday to 5 PM MST Saturday
     Missoula/Bitterroot Valleys.

     Winter Storm Warning until 5 PM MST Saturday Lower Clark Fork
     Region.

     Winter Storm Warning from 5 AM Friday to 5 PM MST Saturday
     Flathead/Mission Valleys.

     Winter Storm Warning until 5 PM MST Saturday West Glacier Region.

     Winter Storm Warning until 5 PM MST Saturday Kootenai/Cabinet
     Region.

     Winter Storm Warning from 11 AM Friday to 5 PM MST Saturday
     Potomac/Seeley Lake Region.

ID...Winter Weather Advisory from 5 AM Friday to 5 PM MST Saturday
     Eastern Lemhi County...Western Lemhi County.

     Winter Storm Warning until 4 PM PST Saturday Northern Clearwater
     Mountains...Southern Clearwater Mountains.

     Winter Weather Advisory until 10 AM PST this morning
     Orofino/Grangeville Region.

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