Winter Storm Watch 3, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
421 AM MST MON NOV 2 2015

...First snow of the season for SW Montana & Lemhi County
tonight...

.DISCUSSION...A weak mid-level atmospheric disturbance will
produce some mountain snow/valley rain showers over central Idaho
and across parts of NW Montana this afternoon. However, the more
significant snow begins to impact Lemhi County this evening, & SW
Montana early Tuesday morning.

We have issued a Winter Storm Watch for locations above 6000ft in
southern Lemhi County (beginning this evening), as well as for
the benches around Butte and the higher elevation valleys of
Philipsburg & Georgetown Lake (both beginning early Tuesday
morning). A Winter Weather Advisory has been issued for the
Bitterroot/Sapphire Mountains (focusing on the southern ranges &
Lost Trail Pass) starting early Tuesday morning. Snow will begin
to fall over southern Lemhi County late this afternoon, with
accumulations impacting traffic traveling south of Salmon this
evening and continuing through much of tomorrow. A total of 4 to
6 inches of accumulation appear likely by Tuesday evening.

Farther north, models still struggle a bit in handling the wrap
around moisture, but we believe sufficient snowfall will fall
near/around Butte, and especially in the higher terrain
surrounding the Butte region (Georgetown Lake, Philipsburg &
Homestake Pass) - enough to pose a significant impact to travel.
4 to 6 inches (locally up to 8 inches near the Continental Divide)
of snow are expected across the higher terrain, with a slushy 2
to 4 inches of snow in Butte proper. However, the presently shown
precipitation intensity in Butte appears light enough to prevent
much of this snow from piling up and thus, the impacts to
travel/commerce within and around the city appear limited at this
time (hence the Winter Storm Watch for elevations just above
Butte). Snow will become showery and gradually taper off Tuesday
night, with perhaps some brief clearing possible by Wednesday
afternoon.

The second of the two main weather systems impacting the Northern
Rockies will occur Thursday. Both mountains and valleys have the
potential to pick up some snowfall. This will create some
unpredictable icy/slushy/snowy/wet road surface conditions. The
snow intensity is projected to pick up for most locations above
2500 feet by Thursday evening, with light snow falling through
Friday morning. Mountain passes will see more snow than valleys,
however given the early debut of snowfall for valley drivers, it
is advised to exercise caution while traveling Friday morning.
Quickly-changing and resultant dangerous conditions will be
common.

A brief reappearance of high pressure will bring drier, tamer
conditions Saturday into Sunday, but a more active weather returns
early next week. Given continued differences between forecast
models, we will keep an eye on any impacts that develop in the far
extended period.

&&

.AVIATION...Areas of dense freezing fog will impact operations at
KBTM through much of this morning before dissipating during the
afternoon hours. Persistent visibility reduction to 1/4mi are
likely to continue through mid morning before conditions begin to
show signs of improving. Patchy fog is likely within the vicinity
of KMSO this morning, but with little impact to aviation
interests and with conditions quickly improving after sunrise.
Occasional showers are expected this afternoon near KGPI, though
minimal impacts to aviation interests are expected at this time.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from Tuesday morning through Tuesday
     afternoon ABOVE 6000 FEET for the Butte/Blackfoot Region.

     Winter Weather Advisory from 6 AM to 5 PM MST Tuesday ABOVE 6000
     FEET for the Bitterroot/Sapphire Mountains.

ID...Winter Storm Watch from this evening through Tuesday afternoon
     ABOVE 6000 FEET for the Eastern Lemhi County.

&&