Extreme Cold in Montana, December 31, 2021

Area Forecast Discussion
National Weather Service Great Falls MT
445 PM MST Fri Dec 31 2021

...Updated aviation section...

.SYNOPSIS...
An Arctic airmass with bitterly cold wind chills will remain over
the region through Saturday morning. Southwest winds increase and
bring warming temperatures this weekend, but blowing and drifting
snow will be possible as temperatures remain near and below
freezing. More Arctic air and additional chances for snow is
forecast to return early next week.

&&

.DISCUSSION...

Tonight through Saturday...

Final shortwave disturbance rotating through the upper level trough
across the region will move off to the SE this evening before the
upper trough axis shifts east of the area overnight. At the surface,
cold high pressure and Arctic airmass centered across southern AB/SK
today slides southeast across eastern MT into the Dakotas by
Saturday morning. Just a few lingering light snow showers/flurries
through the rest of this afternoon across far SW MT with clouds
decreasing from NW to SE across the area through the rest of this
afternoon and evening. Main weather impacts through tonight will be
related to the cold with temperatures already in the single digits
above/below zero across the entire area falling to single digits and
teens below zero this evening and as low as 20 below to 30 below
zero across much of north-central MT overnight. Wind Chill
advisories have been expanded across SW MT where northerly winds
of 10-15 mph through valleys and passes will create wind chills
around 20 below zero through early this evening before winds
diminish tonight. Further north, lighter winds but colder
temps/wind chills are on track for tonight with coldest readings
expected along the Hi-line. As the upper trough and surface high
move off to the east later tonight, lee-side surface troughing
develops along the east slopes of the Rockies and deepens though
the day Saturday. Southwest winds along the east slopes will bring
some warming to these areas late tonight with these winds
spreading east across the plains through the day Saturday and
warming temperatures into the teens (above zero) by Saturday
afternoon for most areas. Gusty winds along the east slopes and
adjacent plains areas increase through the day Saturday with
potential for blowing/drifting snow impacts and a winter weather
advisory has been issued to highlight this for areas from Glacier
county south along the Rocky Mtn Front and east across Cascade and
Judith Basin counties. These winds could also bring some blowing
and drifting snow to southern portions of Lewis and Clark county
including Sieben Flats and MacDonald Pass, though little wind is
expected in the Helena Valley. Snow depth across the plains is
generally less than 3 inches so there is some uncertainty with the
extent and duration of any blowing/drifting snow impacts. Hoenisch

Saturday night through Monday...High temperatures across the
region will steadily warm throughout the period, with the warmest
days of Sunday and Monday peaking in the upper 20s to mid-30s at
most locations. Colder air trapped in the 'wind protected'
southwest valleys may result in highs running slightly cooler than
guidance for these locations, especially for Saturday and Sunday.
However, kept the NBM numbers due to overall lower confidence
with stronger winds aloft. Increased surface winds still appear to
be the biggest impact of the period for north-central and
portions of southwest MT. Both probabilistic and deterministic
models  indicate the potential for strong winds over the Rocky
Mountain Front and eastern Glacier county Saturday night into
Sunday. Given the 90 percent chance for 48 kt + wind gusts at Cut
Bank and 60 percent chance for 64 kt + gusts along the Rocky
Mountain Front late Saturday through Sunday, a High Wind Watch was
issued for these zones. Windy conditions persist into Monday but
look to remain well below criteria at this time.

Monday night trough next Friday...A cold front brings an initial
push of Arctic air to the plains and areawide light snow Monday
night. The cold air will likely become well established over the
entire region by mid-week, with slow moderation thereafter. This
will likely result in more wind chill highlights and possibly
winter weather highlights due to periods of accumulating snow.
-RCG

&&

.AVIATION...
445 PM MST Fri Dec 31 2021 (00/00Z TAF period)

VFR conditions should prevail overnight tonight as high pressure
slides through the area. A few patchy MVFR clouds will be possible
early this evening across the southwest, but should dissipate rather
quickly after sunset. Winds will begin to increase across the area
tomorrow morning, especially on the north and east slopes of
mountains on the plains, which will result in some areas of blowing
snow. Ludwig

Refer to weather.gov/zlc for more detailed regional aviation weather
and hazard information.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF -20  18  12  32 /   0   0   0   0
CTB -23  17   9  30 /   0   0   0   0
HLN -18  14   6  27 /   0   0   0   0
BZN -20  12   0  27 /   0   0   0   0
WYS -21   7  -8  16 /  10   0   0   0
DLN -12  14   3  23 /   0   0   0   0
HVR -27  12   1  30 /   0   0   0   0
LWT -21  17   7  35 /   0   0   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM Saturday to 6 AM MST Sunday
Cascade...Central and Southern Lewis and Clark...Eastern
Glacier...Judith Basin...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.

Wind Chill Advisory until 11 AM MST Saturday Beaverhead...
Broadwater...Central and Southern Lewis and Clark...Gallatin...
Jefferson...Madison...Meagher.

Wind Chill Warning until 11 AM MST Saturday Blaine...Eastern
Glacier...Hill...Liberty...Toole.

Wind Chill Advisory until 5 PM MST this afternoon Blaine...
Eastern Glacier...Hill...Liberty...Toole.

Wind Chill Advisory until 11 AM MST Saturday Cascade...
Chouteau...Eastern Pondera...Eastern Teton...Fergus...Judith
Basin...Northern Rocky Mountain Front...Southern Rocky Mountain
Front.

High Wind Watch from Saturday afternoon through Sunday afternoon
Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

http://www.weather.gov/greatfalls