Winter Storm Warning 5, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1117 AM MST Thu Feb 23 2023

...Late Morning Update...

.SYNOPSIS...

Light snow and gusty winds gradually subside today, with dangerously
cold conditions continuing into Friday before temperatures moderate
heading into the weekend. Westerly winds increase late Friday into
Saturday with blowing and drifting snow expected for some areas
until temperatures rise above freezing. The most significant impacts
will be along the Rocky Mountain Front and the adjoining plains.
More passing disturbances bring more gusty winds and mostly mountain
snow Sunday into early next week as temperatures trend closer to
average.

&&

.UPDATE...

We have allowed the Wind Chill Warnings that were in effect to
expire, for now. We have also lowered temperatures for the next
few periods, but especially for today and tonight across the
entire forecast region to reflect observed trends and the lower
one-third to one-half of the NBM probabilistic range.

Given the trend to colder forecast temperatures overnight into
Friday, it is likely Wind Chill Warnings will need to be re-issued
for some areas with the afternoon package.

&&

.AVIATION...
1050 AM MST Thu Feb 23 2023 (23/18Z TAF Period)

Arctic air has settled into the region, bringing some lowered
ceilings and some very light snow/haze area wide. Expect a mix of
VFR/MVFR/IFR conditions, mainly due to low ceilings as visibility
should generally remain above 5SM. Widespread mountain obscuration
is expected. Ludwig

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

&&

.PREV DISCUSSION...
/ISSUED 325 AM MST Thu Feb 23 2023/

Today through Saturday night...

The Northern Rockies will remain under the influence of a broad,
cold mid-level trough through Friday. H700 temperatures are
running near an impressive -28C or about 3 standard deviations
below climatology this morning. This has resulted in lower
elevation temperatures falling in the 5 to 20 below range, with
some mountain areas approaching 30 below despite an abundance of
cloud cover. Wind chills have been consistently approaching and
even exceeded 40 below for many areas along the Divide, so a Wind
Chill Warning was issued for most locations along the Divide and
some of the neighboring plains zone. It will run through 8 am MST
this morning, when chills less than -40 become far more isolated.
No other changes were made to the inherited Wind Chill Advisories.
Mid- level moisture and subtle shortwave energy over the lower
level northeasterly flow will keep ongoing light snow/mist going
through this evening, though accumulations will remain under one
inch for most locations.

Temperatures for today may be a few degrees warmer than yesterday
over the southwest with the coldest H700 temps slowly retreating
northward, but highs will remain near or just below zero for other
locations. Partial clearing tonight could result in widespread
readings in the 15 to 25 below range, but it ultimately depends on
how much cloud cover is able to stick around and inhibit strong
radiational cooling. Lighter winds may take some of the bite out
of these extreme temperatures, but still these are dangerously
cold conditions for anyone spending time outdoors, even short
periods outdoors.

As the broad trough responsible for this Arctic outbreak loosen
it's grip on Friday, westerly winds will begin to increase along
the Rocky Mountain Front and the plains. The winds could become
strong Friday night through Saturday before temperatures rise
above freezing. A Winter Storm Watch was issued for the Rocky
Mountain Front zones and their adjacent plains from noon Friday
through 5 pm MST Saturday. Even though the strongest winds aren't
really expected until Friday evening, lower grade winds may be
sufficient to move snow and potentially block roads. The Cut Bank
area was not included in the watch, but this area along with the
highway 200 corridor farther southeast could end up seeing
advisory level blowing snow impacts. Probabilities for 55 + mph
gusts are only running above 50% for the Rocky Mountain, though
the plains will see gusts in the 30 to 40 mph range. Temperatures
do trend to near and even above average this weekend, though the
Milk River Valley and some southwest valleys will likely see a
delayed warm up. Little to no precipitation is expected for the
first part of the weekend aside from some light mountain snow.

Sunday and Monday...The next chance for precipitation arrives
Sunday as southwesterly flow aloft develops ahead of a shortwave
progged to moved into the Northern Rockies Sunday night into
Monday. Snow will initially set up along the Divide by Sunday
afternoon and then expand southeastward into Central and Southwest
Montana Sunday evening through early Monday. Snow accumulations
will favor mountain areas where there is about a 40 to 60% chance
for snowfall amounts reaching or exceeding 1 inch. Lower
elevations may see a dusting of snow, but will likely mix with
rain during the onset. Downsloping westerly winds will generally
keep plains locations on the dry side. There will be gusty winds
with this system as well, with a high probability (> 60%) of 39
mph + winds for most locations, but a less than 50% chance for
gusts over 55 mph for all areas except the Rocky Mountain Front.

Tuesday through next Thursday...After a transient ridging quickly
passes the region Monday night, a deeper and colder Pacific
trough approaches from the west. Many ensembles suggest that at
least some of the energy with this system will split off into the
southwest, leaving more of a slow moving open wave to pass through
our area. This will result in periods of mostly light snow each
day, though 48 hr snowfall probabilities for 2 + inches of snow
for the period ending 5 am Thursday are less than 50% for lower
elevation areas. A series of cold fronts with this trough may
bring brief periods when temperatures fall below normal. As far as
winds go, stronger westerly winds look to hold off until
Thursday, but southerly winds may become gusty in Southwest
Montana. -RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF -11 -23  12   5 /  40  20   0   0
CTB -13 -24   4  -4 /  70  10   0   0
HLN  -4 -13  23   7 /  30  10   0   0
BZN  -6 -26  19  -3 /  20  10   0   0
WYS  14 -12  21   1 /  60  50  20  20
DLN  -4 -16  22   0 /  20  10   0   0
HVR  -8 -26  -5 -18 /  50   0   0   0
LWT  -8 -23  17   8 /  40  20  20   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until noon MST Friday for Big Belt, Bridger
and Castle Mountains-Cascade County below 5000ft-East Glacier
Park Region-Eastern Glacier, Western Toole, and Central Pondera-
Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-
Fergus County below 4500ft-Gates of the Mountains-Helena Valley-
Judith Basin County and Judith Gap-Little Belt and Highwood
Mountains-Meagher County Valleys-Northern High Plains-Snowy and
Judith Mountains-Southern High Plains-Southern Rocky Mountain
Front-Upper Blackfoot and MacDonald Pass-Western and Central
Chouteau County.

Winter Storm Watch from Friday afternoon through Saturday
afternoon for East Glacier Park Region-Northern High Plains-
Southern High Plains-Southern Rocky Mountain Front.

Wind Chill Advisory until 5 AM MST Saturday for Bears Paw
Mountains and Southern Blaine-Hill County-Northern Blaine County.

Wind Chill Advisory until 5 AM MST Friday for Beaverhead and
Western Madison below 6000ft-Canyon Ferry Area-Elkhorn and
Boulder Mountains-Gallatin Valley-Gallatin and Madison County
Mountains and Centennial Mountains-Madison River Valley-Missouri
Headwaters-Northwest Beaverhead County-Ruby Mountains and
Southern Beaverhead Mountains.

&&

http://www.weather.gov/greatfalls