Heavy Snow in Idaho, February 4 to 5, 2021

Area Forecast Discussion
National Weather Service Missoula MT
300 PM MST Thu Feb 4 2021

...Sound the Trumpets, Winter Has Returned to the Northern
Rockies...

.DISCUSSION...Light snow has overspread Western Montana along the
I-90, MT-200, HWY-12 corridors. However, surface temperatures are
at or around freezing and any impacts are unlikely until late
afternoon or overnight. The moisture feeding into the Northern
Rockies is originating from east of Hawaii, so this is expected to
be a very moist system. The best moisture is forecast for central
Idaho and west central Montana, to include the Mission, Swan, and
southern Flathead ranges. Snowfall rates of 1 to 2 inches an hour
can be expected in the Clearwater Mountains of Idaho. Therefore,
we have issued a winter storm warning for that location. Expect
travel to be very difficult overnight through late morning Friday.
From Lookout Pass to Garrison Junction along I-90, a couple hours
of moderate snowfall is expected, from after midnight through 6
am, travel may be impacted. Snow will gradually wind down from
northwest to southeast during the morning hours, however, snow
showers will likely continue all day in the mountains in a moist
northwest flow. There will be a bit of a lull in the active
weather Friday night, though light snow will persist.

A second push of moisture in the jet stream, presently coming on
shore near Juneau, AK, will bring yet another round of heavy snow
to the Northern Rockies Saturday into Sunday. Similar to tonight,
the Seeley/Swan Region, West Glacier Region & Bitterroot/Clearwater
Mountains will all be favored for another 1 to 2 feet of intense
mountain snow over the course of 24 hours. Several inches of
valley snow are also expected as well. The "x factor" at play for
the valleys will be the westward progression of a deepening mass
of bitterly cold, dense arctic air banked up against the
Continental Divide. At present we anticipate a brief slosh of this
air to spill over Marias Pass late Friday night and flood across
the West Glacier Region and Flathead Valley. Winds gusting to 40
mph are possible and the combination with intensifying snow will
reduce visibility.

The initial slosh will likely be short lived with moist westerly
flow taking over on Saturday. This will lead to enhanced mountain
snowfall production and steady light snow in most valleys. Valley
snow production will increase late Saturday and especially
overnight as a flood of arctic air ultimately pours over the
Continental Divide. An area of VERY impressive snowfall is
expected as strong snow squalls are typical to form in the dynamic
environment out ahead of the arctic boundary. Timing this area of
intense snow will ultimately be key to determining when and where
our most intense valley accumulations will occur and is something
we'll be diligently evaluating these next 48 hours. That being
said, there is high confidence that the combination of intense
snowfall, wind, and cold will create dangerous conditions in the
West Glacier Region, Flathead Valley, and Seeley/Swan Region and a
Winter Storm Watch has been issued. Additional Watches and
Advisories are expected as confidence in this weekend storm
evolves, so please stay tuned for additional details.

Steady light to moderate snowfall will continue through Sunday and
into Monday as the arctic air mass takes up residence this side
of the Continental Divide. Several more inches of accumulation
are to be expected, both in the valleys and terrain. It looks like
by Tuesday the moisture feed will be cut off and the atmosphere
starts to show signs of drying. Bitterly cold temperatures are
expected to develop with many locations across Western Montana
experiencing several days with lows below zero and highs not
climbing out of the teens. Dangerously cold wind chill readings
are also to be expected with even the lightest of breeze,
particularly early next week. Also, there is still a chance we
could see dangerous cold as some models actually bring down a
reinforcing shot of even colder arctic air late next week.

&&

.AVIATION...Light snow is currently falling at KMSO, precipitation
will continue to spread over the entire forecast throughout the
afternoon. Ceilings in most locations will lower to 2500 feet or
lower after 05/0000z tonight. Moderate to heavy rates of snow late
this evening into Friday morning for central Idaho and west
central Montana, reducing visibilities to a 1/2 mile or lower.
Snow will begin to dissipate after 05/1200z from northwest to
southeast. Showers are expected to continue all day in the
mountains creating terrain obscurations at times.

&&

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

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Friday Bitterroot/Sapphire Mountains...Potomac/Seeley Lake
     Region.

     Winter Weather Advisory until 11 AM MST Friday Lower Clark Fork
     Region.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Friday Missoula/Bitterroot Valleys.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Friday Flathead/Mission Valleys...West Glacier Region.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Friday Kootenai/Cabinet Region.

ID...Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Friday Lower Hells Canyon/Salmon River Region.

     Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Friday Northern Clearwater Mountains...Southern Clearwater
     Mountains.

     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Friday Orofino/Grangeville Region.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
510 PM MST THU FEB 4 2021

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

0509 PM     SNOW             1 NE NINEMILE           47.03N 114.39W
02/04/2021  M1.3 INCH        MISSOULA           MT   TRAINED SPOTTER

            2.5 INCHES ON THE GROUND.

&&

AL