Special Weather Statement, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
137 PM MST Mon Jan 18 2021

.DISCUSSION...A light northerly flow has promoted the
development of widespread stratus clouds across western Montana
and central Idaho. Patchy areas of valley fog are possible tonight
into Tuesday as high pressure amplifies across the Northern
Rockies. Valley fog and stratus will burn off Tuesday afternoon
and high temperatures will reach into the mid 30s.

An upper level trough will move across British Columbia and
Alberta Tuesday night into Wednesday morning. This disturbance
will lack moisture, therefore, only isolated snow showers are
expected for areas along the Montana and Canadian border. The
primary impact will be an increase of winds across higher
elevation mountain locations, where winds could gust over 40 mph.

Model ensembles continue to display a wide array of potential
forecast scenarios Thursday through the upcoming weekend. A
positively tilted omega block will amplify across the Gulf of
Alaska Thursday. As this ridge builds, a broad area of low
pressure will dig southward along the British Columbia coast.
Model ensembles then begin to diverge as this area of low pressure
moves into the western US. A couple of scenarios are possible at
this time:

* Light snow and cold (8 in 10 chance): the upper level low moves
  off the west coast and closes off over California. The first
  push of Canadian air would move across the divide Thursday, with
  a secondary push of cold air arriving Friday. In this scenario,
  a period of light to moderate snowfall is possible across the
  northern Rockies, particularly for the I-90 corridor southward
  Friday. Cold temperatures would follow into the weekend with
  highs into the 20s and lows into the single digits and teens.

* Higher impact snow and cold (2 in 10 chance): the upper level
  low moves across the Pacific Northwest and closes off near the
  ID/OR/NV border. In this case, a surface trough will develop across
  central Idaho, with a strong reinforcement of arctic air moving
  over the divide Friday. A prolonged period of moderate snowfall
  could occur across the Northern Rockies in this scenario, with
  several inches of snow being possible. Low temperatures below 0
  would be likely Saturday and Sunday morning across western
  Montana, particularly along the Continental Divide.

&&

.AVIATION...A building ridge and mid-level dry air will help clear
out some of the mid-level clouds over the region, but stable
conditions will keep ceilings in most terminals. With moisture
trapped in the low levels and increasing stability, fog and
stratus are concerns for terminals KMSO and KGPI tonight. Model
soundings favor stratus over fog right now, but either is
possible. Current TAFs also mention some potential for light snow
and mist at KGPI. This would be a very light snow falling out of
the stratus and isn't expected to accumulate quickly.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

Special Weather Statement
National Weather Service Missoula MT
814 PM MST Mon Jan 18 2021

MTZ007-043-190400-
Butte/Blackfoot Region MT-Potomac/Seeley Lake Region MT-
814 PM MST Mon Jan 18 2021

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN MISSOULA...
NORTHEASTERN DEER LODGE...NORTHEASTERN GRANITE...NORTH CENTRAL SILVER
BOW AND SOUTHWESTERN POWELL COUNTIES UNTIL 900 PM MST...

At 813 PM MST, Doppler radar was tracking intense snow showers along
a line extending from near Ovando to near Silver Bow. Movement was
south at 25 mph.  This line of snow showers will create a quick
burst of moderate snowfall along Interstate 90 from Drummond to
Butte.

Visibility one-half mile or less will be possible.

Locations impacted include...
Deer Lodge, Drummond, Silver Bow, Warm Springs, Galen, Gregson Hot
Springs, Ovando, Garrison, Jens, New Chicago, Crackerville,
Racetrack, Garnet, Lost Creek, Opportunity, Goldcreek, Ramsay, Hall
and Greenough.

LAT...LON 4693 11345 4581 11273 4613 11259 4703 11315
TIME...MOT...LOC 0313Z 006DEG 23KT 4704 11318 4607 11264