Winter Weather Advisory 63, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
236 PM MST Fri Dec 10 2021

...HEAVY SNOW AND GUSTY WINDS OVER NORTHERN ROCKIES MOUNTAIN PASSES
TONIGHT THROUGH WEEKEND...

.DISCUSSION...

A surge of moisture will combine with a powerful jet stream over the
Pacific Ocean today and move inland over the Northern Rockies
tonight through the weekend. Considering the cold air already in-
place across much of the region, the environment ahead of the storm
is primed for efficient snowfall production (particularly across
high terrain of northwest Montana and north central Idaho). The
strong winds aloft will not only enhance snow rates (1-2 inches per
hour at times over area passes), but also combine with the snow to
decrease visibility and drift snow over pass roadways in a short
amount of time. Lookout, Lolo, and Marias Pass will experience high
snow rates, however Marias Pass will be particularly impacted with
that blowing/drifting snow and high snow rates combination. Thus,
travel may become quite hazardous over these passes over this
weekend, especially during the evening through morning hours. If you
do travel over passes this weekend, remember to pack emergency food,
water, and a blanket in case your route of choice suddenly becomes
impassible.

The strong winds aloft this weekend will also translate to low
elevation population centers of the Northern Rockies on Saturday.
The Camas Prairie of Idaho, including Grangeville, will see a
combination of south winds gusting to 50 mph and brief bouts of snow
creating episodic hazard winter travel conditions. Lesser wind
gusts, but also variable snow intensity, will also be present for
much of the lower elevation population centers of northwest and west
central Montana. Currently, the wider valleys (e.g., Flathead,
Mission, Missoula, Bitteroot) will have episodic snow and wind on
Saturday, however the overall winter weather impact timing is still
fluid through Sunday. Also, there is a bit of uncertainty with
respect to snow/wind impacts for the Butte/Blackfoot Region and
Lemhi County Idaho (including mountain passes along the Continental
Divide, such as Bannock, Lemhi, MacDonald, and Homestake Passes).

The pattern stays active next week as a large scale trough
gradually works onto the West Coast. Southwest flow ahead of the
trough will warm up the lower and mid levels of the atmosphere
which could allow for a light wintry mix in valleys as cold air
remains settled in some sheltered valleys. On Tuesday, there is
decent agreement on a cold front passage with a round of minor
pass level snow, potentially a light dusting in the valleys. A
general northwest flow is favored for late next week with
seasonably cold air settling in and highs only in the 20s to near
30F. Subtle shortwaves will leave open the opportunity for
occasional snow showers late next week as well.

&&

.AVIATION...Strong west winds aloft will move into the Northern
Rockies air space after 11/0600Z. Expect gusts to 60 knots above
high terrain, as well as strong low level shear within area
valleys. Terrain obscurations will become commonplace by 11/1200Z.
Heavy snow will be locked onto the high terrain thereafter,
particularly true for air space north and west of KMSO (including
KGPI, KMLP). Snow will intermittently fall into low elevations
and valleys, as the winds will have a localized downslope (drying)
effect at times. Much of the terrain obscurations will extend
into Sunday, as will the snowfall. However, winds will begin to
diminish after 12/0000Z, except for air space along the
Continental Divide.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 2 AM Saturday to 11 AM MST Sunday West
     Glacier Region.

     Winter Storm Warning from midnight tonight to 5 AM MST Sunday
     Kootenai/Cabinet Region.

     Winter Weather Advisory from 8 AM Saturday to 11 AM MST Sunday
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 8 AM Saturday to 11 AM MST Sunday
     Potomac/Seeley Lake Region.

     Winter Storm Warning from midnight tonight to 5 AM MST Sunday
     Lower Clark Fork Region.

ID...Winter Weather Advisory from 4 AM to 7 PM PST Saturday
     Orofino/Grangeville Region.

     Winter Storm Warning from 4 AM to 4 PM PST Saturday Northern
     Clearwater Mountains.

     Winter Weather Advisory from 4 AM Saturday to 10 AM PST Sunday
     Southern Clearwater Mountains.

     Winter Weather Advisory from 4 AM to 7 PM PST Saturday Lower
     Hells Canyon/Salmon River Region.

&&

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