Heavy Snow in Montana, January 18, 2021

Area Forecast Discussion
National Weather Service Great Falls MT
532 AM MST Mon Jan 18 2021

.SYNOPSIS...

Light snow will gradually decrease over the plains of North
Central and Central Montana through the morning hours today, with
moderate to at times heavy snow continuing over the mountains and
in the northerly upslope regions of Central and Southwest Montana
through this afternoon. Winds will be on the increase during the
day on Tuesday, becoming strong and gusty in wind prone areas of
the Rocky Mountain Front and across Central Montana Tuesday
afternoon through Wednesday morning.

&&

.Early Morning Update...

Have upgraded Cascade and Meagher Counties from a Winter Weather
Advisory to a Winter Storm Warning. Areas near Kings Hill Pass
have picked up nearly 4" of snow in the past 1-1.5hrs, with the
threat for snowfall rates to continue at a 2-3"/hr clip for the
next several hours. Storm total snow over and near Kings Hill Pass
will likely be within the 22-28" range by the late afternoon
hours; however, I would not be surprised if some isolated
locations are higher given the strength of the upslope flow. -
Moldan

&&

.AVIATION...
355 AM MST Mon Jan 18 2021 (18/12Z TAF period)

Moist northwest flow aloft combined with subtle disturbances
sliding southeast within the flow is leading to areas of light snow
across a large portion of North Central and Central Montana this
morning, with this activity expected to gradually decrease in
coverage and intensity by the noon hour. The one exception will be
in the northerly upslope favored regions near the KGTF and KLWT
terminals where snow or vicinity snow will linger to more of the
late afternoon/early evening hours. The same story can be said for
the KEKS and KBZN terminals where light snow is expected over or
within the vicinity of the terminals for much of the day today.
Mountains will remain obscured through much of the overnight hours
tonight. MVFR/IFR/LIFR conditons will continue through the afternoon
hours at most terminals, most notably at KHVR, KLWT, KBZN, and KEKS.
- Moldan

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

&&

.HYDROLOGY...
300 PM MST Sun Jan 17 2021

The flood advisory continues for the Jefferson River near Three
Forks until further notice.

The river gauge reading there continues to hover around minor
flood stage, with little variation over the last 24 hours.

Although no reports of flooding have been received so far, the
current forecast indicates that the river level should remain near
flood stage for the time being. Any impacts from this ice jam
should be minor and along the river bank.

&&

.PREV DISCUSSION...
/ISSUED 355 AM MST Mon Jan 18 2021/

Today through tonight...primary concern throughout the period is the
on-going snow across region, especially across Central Montana and
eastern portions of Southwest Montana.

Northwest flow aloft will dominate the period, with multiple subtle
shortwaves embedded within the flow dropping southeast and across
North Central through Southwest Montana through the evening hours
hours tonight. The main shortwave and core of H500 height falls has
already moved over Wyoming as of 09z this morning, but additional
subtle waves at H500 combined with a re-enforcing surface cold front
pushing south from Canada has led to an area of light snow
developing over the plains of North Central and Central Montana. Any
snow over the plains will be on the light side, generally less than
1/2", and come to an end prior to the noon hour today. Across
Central Montana in the mountains and northerly upslope regions the
snow has been moderate to heavy at times for most of the night, with
areas of moderate to at times heavy snow expected to continue into
the afternoon hours today before decreasing in intensity and areal
coverage this evening. Froude numbers in excess of 1 through the day
today continue to indicate that most if not all of the snow will be
confined to the Central Montana mountains and Montana Hwy 200
corridor between Belt and Lewistown, with areas like Great Falls
only seeing some light snow early this morning with the passage of
the re-enforcing cold front and upper level wave. Additional
snowfall accumulations in the upslope and mountainous regions of
Central Montana of 2 to 5 inches above 4000 feet are expected, with
5 to 10 inches above 6000 feet. Progressing further south into
eastern Southwest Montana (i.e. Madison, Gallatin, and Bridger
Rangers and northerly upslope regions) the snow has been slow to
develop so far tonight, but is expected to increase in areal
coverage and intensity as surface and upper level flow becomes
increasingly northerly. Here additional snow accumulations of up to
3 inches at lower elevations are possible, with 4 to 8 inches in the
mountains.

Tuesday through Wednesday night...transient ridging over the
Northern Rockies Tuesday morning will be flattened out by a potent
H500 shortwave moving over Southern and Central Canada from Tuesday
afternoon through Wednesday afternoon. Lee-side surface trough will
begin to sharpen by the late morning hours on Tuesday as a strong
area of low pressure moves across northern British Columbia and into
northern Alberta. Surface pressure gradient along the Rocky Mountain
Front will strengthen rapidly Tuesday afternoon, peaking at between
200-250 microbars/km by the late afternoon/early evening hours on
Tuesday. This strong of a pressure gradient alone leads me to
believe that surface winds/gusts will approach 75 mph in the East
Glacier and Browning areas. Further strengthening the idea that high
winds will materialize not only at East Glacier and Browning, but
also Cut Bank, is the presence of a stable layer at or slightly
above ridgetops with 50-70kt winds located below this stable layer.
Have therefore upgraded the High Wind Watch for the Rocky Mountain
Front and Eastern Glacier County to a High Wind Warning in
anticipation of high winds materializing here. I have also issued a
High Wind Watch for Toole County as model guidance indicates that
mountain wave activity could move as far east at the I-15 corridor
here by Tuesday evening/night. Finally, I have also issued a High
Wind Watch for Cascade and Judith Basin Counties as BUFKIT soundings
here also support the potential for mountian wave activity and
strong/gusty winds from a strengthening surface pressure gradient
along the northern periphery of the Little Belt Mountains.

Thursday through next Monday...temperatures throughout the period
continue to trend near to below normal as northerly flow transitions
to broad troughing over the Western CONUS. Snow is possible over the
mountains and in northerly upslope regions from Thursday night
through Friday night, but accumulations at this time appear to be on
the light side. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  40  20  41  32 /  30  10   0   0
CTB  38  19  39  29 /  20   0   0   0
HLN  37  18  37  24 /  30  30   0   0
BZN  34  12  33  16 /  40  20   0   0
WYS  23  -7  22   0 /  30  20   0   0
DLN  34  10  33  15 /  10  10   0   0
HVR  39  20  41  29 /  20   0   0  20
LWT  31  16  38  26 / 100  30   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from noon Tuesday to 9 AM MST Wednesday
Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

Winter Storm Warning until 9 PM MST this evening above 4000 feet
for Cascade...Meagher.

Winter Weather Advisory until noon MST today Gallatin.

Winter Weather Advisory until 9 PM MST this evening above 4000
feet for Fergus...Judith Basin.

High Wind Watch from Tuesday evening through Wednesday morning
Cascade...Judith Basin.

High Wind Watch from Tuesday afternoon through Wednesday morning
Toole.

Flood Advisory until further notice due to an ice jam on the
Jefferson River near Three Forks.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
408 AM MST MON JAN 18 2021

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

0402 AM     HEAVY SNOW       7 SSE NEIHART           46.84N 110.70W
01/18/2021  M14.0 INCH       MEAGHER            MT   TRAINED SPOTTER

            SNOW BEGAN SUNDAY AFTERNOON.

&&

MOLDAN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
810 AM MST MON JAN 18 2021

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

0808 AM     SNOW             5 NE HILGER             47.31N 109.28W
01/18/2021  M5.0 INCH        FERGUS             MT   TRAINED SPOTTER

            5 INCHES OF SNOWFALL OVERNIGHT. SNOW ENDED
            BY 6AM.

&&

BH

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
812 AM MST MON JAN 18 2021

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

0810 AM     SNOW             1 NNW LEWISTOWN         47.07N 109.43W
01/18/2021  M4.0 INCH        FERGUS             MT   TRAINED SPOTTER

            4 INCHES OF SNOW SINCE SUNDAY AFTERNOON.

&&

BH