Winter Storm in Idaho, December 18 to 19, 2021

Area Forecast Discussion
National Weather Service Missoula MT
225 PM MST Sat Dec 18 2021

.DISCUSSION...Temperatures have been very slow to warm up this
afternoon as cloud-cover this morning locked in the cold dense
air in the valleys. Most locations remain in the upper teens to
low 20s, and will likely remain there for the rest of the day.

The onset of our winter storm this weekend has already begun
across northwest Montana, where light to moderate snow has fallen
most for of the day. The snowfall is still expected to pick up in
intensity over northwest Montana this evening and overnight, as an
Atmospheric River overrides the cooler denser air at the surface.
The plume of moisture will slowly slide south through tomorrow
morning, while at the same time an arctic boundary will spill over
the Continental Divide. At the intersection of these two
boundaries, moderate to heavy snowfall is expected to develop for
locations north of Interstate 90. As the arctic boundary moves
west, gusty west winds of 40 to 50 mph will become gusty east
winds, that combined with snow, will create hazardous travel
conditions from Marias Pass to West Glacier. As the moisture moves
to the south tomorrow afternoon, a light freezing drizzle/rain is
possible for the valleys of Lincoln and Flathead counties.

By late in the day tomorrow, the moisture plume and arctic
boundary will have arrived to west-central Montana. Most model
guidance and their ensembles agree that the boundary will stall
somewhere in the vicinity of the I-90 corridor through Monday
morning. If this does occur, then a prolonged period of moderate
to heavy snow is possible along the interstate from the
Idaho/Montana border, east through Missoula. Forecast confidence
has increased over the last 24hrs for this scenario to occur.
Locations from Saint Regis to Drummond could see around 4-8
inches of snow by the time the event is over late Monday.

North-central Idaho valleys will not be left out in the snow
either. A foot or more of snow will be possible for elevations
above 4000 feet, with several inches possible for the Grangeville
area. The lowest elevations around Orofino to Kooskia will be very
close to the rain/snow line; although, precipitation intensities
are expected to be high enough for snowlevels to lower enough to
where a very wet 2 to 4 inches of snow is possible through mid
morning Sunday.

During the day Monday, the arctic boundary will retreat back to
the north, which will bring another round of moderate snowfall to
northwest Montana.

Weak high pressure is forecast to build to the west during
Tuesday, leaving central Idaho and northwest Montana under
northwesterly flow. Showers are expected under this flow pattern
with a decreasing trend as the weak ridge passes.

Low pressure is expected to takeover the Pacific Northwest for the
second half of next week, placing the Northern Rockies under an
active southwesterly flow. This flow pattern will promote a
continuation of mountain snowfall and changeable snow levels with
periods of valley snow accumulations.

&&

.AVIATION...

An atmospheric river of moisture continues to bring snow to
northern Idaho and northwest Montana. The relatively warm air
aloft is causing increased stability in most valley locations,
which continues to promote widespread snowfall with low visibility
and lowered ceilings. The main area of moisture will slowly shift
southward overnight bringing the decreased visibility and lowered
ceilings to north-central Idaho and west-central Montana. Some
drying will take place on the north side of the moisture plume
which may lead to some freezing rain in places like KGPI. The high
terrain and places along the Divide will continue to have gusty
southwest to west winds through tonight. An Arctic airmass is
still expected push south across the region on Sunday to around
the I-90 (KMSO) corridor. Expect winds to shift from southerly to
east northeasterly and become gusty, potentially lowering
visibility with blowing snow.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 5 AM to 11 AM MST Sunday
     Butte/Blackfoot Region...Potomac/Seeley Lake Region.

     Winter Storm Watch from Sunday morning through late Sunday night
     Butte/Blackfoot Region...Potomac/Seeley Lake Region.

     Winter Storm Warning from 5 AM Sunday to 5 AM MST Monday
     Bitterroot/Sapphire Mountains.

     Winter Storm Warning until 5 AM MST Monday Lower Clark Fork
     Region.

     Winter Weather Advisory from 5 AM to 11 AM MST Sunday
     Missoula/Bitterroot Valleys.

     Winter Storm Watch from Sunday morning through late Sunday night
     Missoula/Bitterroot Valleys.

     Winter Weather Advisory until 5 AM MST Monday Flathead/Mission
     Valleys.

     Winter Storm Warning until 5 AM MST Monday West Glacier Region.

     Winter Storm Warning until 5 AM MST Monday Kootenai/Cabinet
     Region.

ID...Winter Weather Advisory until 10 PM PST this evening Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday
     Orofino/Grangeville Region.

     Winter Storm Warning from 10 PM this evening to 4 AM PST Monday
     Northern Clearwater Mountains.

     Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday
     ABOVE 3500 FEET for the Lower Hells Canyon/Salmon River
     Region.

     Winter Storm Warning from 4 AM Sunday to 4 AM PST Monday
     Southern Clearwater Mountains.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
520 PM MST SAT DEC 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0520 PM     SNOW             1 SW COLUMBIA FALLS     48.36N 114.20W
12/18/2021  M3.1 INCH        FLATHEAD           MT   TRAINED SPOTTER

&&

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
547 PM MST SAT DEC 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0546 PM     SNOW             LIBBY                   48.39N 115.56W
12/18/2021  M4.0 INCH        LINCOLN            MT   PUBLIC

&&

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