213
FXUS65 KTFX 212347
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
447 PM MST Thu Nov 21 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Snow and freezing drizzle around in spots across Central and North-
   central Montana through Friday evening, posing impacts to
   travel.

 - A strong cold front will advance south on Saturday and usher in much
   colder temperatures through the start of the upcoming work
   week, especially to the plains of Central and North Central
   Montana.

 - Accumulating snow will accompany the cold front, with snow
   lingering through the overnight hours on Saturday.

 - Low temperatures by Monday morning are likely to fall below
   zero across much of the plains of Central and North Central
   Montana, with the single digits above zero in Southwest
   Montana.

&&

.DISCUSSION...
/Issued 133 PM MST Thu Nov 21 2024/

 - Meteorological Overview:

A Southwesterly flow aloft will be in place across the region
through Friday evening as a robust upper trough sits off-shore of
the Pacific Northwest and a broad upper level ridge sits south and
southeast of the Northern Rockies. At the surface a stationary front
will remain draped from west to east across Central Montana, with
a cooler surface airmass to the north of it, and a slightly
warmer surface airmass to the south. This will set the stage for
an overrunning precipitation event that will come in waves through
the weekend.

The initial wave passes across the region this afternoon and
evening along the Northern Rocky Mountain Front and adjacent
plains. A period of snow will develop along the Rocky Mountain
Front and adjacent plains, with a low- end, but potentially high
impact, chance for light freezing drizzle across portions of the
plains.

A brief break occurs late tonight before another push of moisture
arrives early tomorrow morning from southwest to northeast across
the region. Forecast soundings show a warm nose nudging just above
freezing at times across Central and North-central Montana during
the day Friday, which will result in an opportunity for freezing
drizzle and slick roads. Where precip type remains snow, mostly
across the Hi-Line, a period of light accumulating snow will be the
primary concern. Another brief break in precipitation occurs late
Friday night before another round of moisture moves in for Saturday.
-AM

Saturday through Monday...An emerging surface low from Western
Montana will track east and across Central Montana through Saturday
afternoon and towards Eastern Wyoming by Saturday evening.
Temperatures on the south-side of the surface low track will warm
into the mid-30s to upper 40s on Saturday while areas north,
especially to the northwest, of the low stay in the 20s. As the
surface low begins to move to the southeast and towards Eastern
Wyoming Saturday afternoon colder air dammed up against the Canadian
Rockies in Alberta will begin to spill southward into North Central
through Southwest Montana. Strong pressure rises behind the
departing low will lead to increasing northwest to north winds
across all areas, with the strongest winds occurring across the
plains of Central and North Central Montana where H850 winds of 20-
35 kts will mix down. A band of light to at times moderate snow is
expected to occur on the west to northwest side of the low through
the daylight hours on Saturday, with this band of snow moving
southeastward through the evening hours. Snow will gradually taper
through the overnight hours on Saturday and into Sunday morning;
however, lingering snow showers may persist over the mountains and
in northerly upslope areas through the day on Sunday. High
temperatures on Sunday will be well below normal, with highs ranging
from the teens across the plains of Central and North Central
Montana to the 20s to low 30s in the valleys of Southwest and
Central Montana. The coldest night of the upcoming week is then
expected Sunday night as skies clear and surface high pressure moves
south along the Canadian Rockies, with a slight moderation in
temperatures through the day on Monday outside of the Milk River
Valleys where the shallow cold air will be stubborn to mixout. -
Moldan

 - Forecast Confidence & Scenarios:

Snow and low-end chance for freezing drizzle through the evening:
Additional snow along the Northern Rocky Mountain Front should
largely be confined to the high terrain. The chance for an
additional 3 inches of snow at Marias Pass is around 20%. The chance
for any freezing drizzle is less than 15% across the plains through
tonight.

Freezing drizzle and snow Friday: The best chance for freezing
drizzle will be across central and portions of North-central
Montana, where there is largely a 40-50% chance for a light glaze of
ice during the day Friday, highest between Sieben and Helena (Up
to a 70% chance). Further north, snow will be the main impact.
Between Friday morning and late Friday evening, the chance for 2
inches of snow is largely at and above 50% along the Hi-Line. One
piece of uncertainty that is sneaking in this afternoon is that
surface temperatures across Central Montana have warmed much above
previous thinking. These warmer surfaces may hold on to enough
heat to mitigate freezing drizzle or rain that does fall tomorrow.
No changes have been made to the ongoing Winter Weather
Advisories at this time, but they may see significant changes as
trends become more clear tonight into Friday. -AM

Snow Accumulations (Saturday through Sunday) - Probabilities for 6"
or more of snow continue to increase along the Rocky Mountain Front,
along and north of the of the US Hwy 2 corridor from Marias Pass
through Fort Belknap, and in the Bears Paw Mountains as a band of
snow pivots around the north side of a surface low positioned to the
south across Central Montana. Lower elevations across the
aforementioned areas and in the Bears Paw Mountains now have between
a 30-50% chance for 6" or more of snow, with the Continental Divide
north of Rogers Pass through Marias Pass and across Glacier National
Park having a 40-80% chance for 9" or more of snow. Across the
remainder of the Central and North Central Montana plains, generally
northwest of the US Hwy 87 corridor from Great Falls to Havre, the
25-75th percentile range is generally between 1 to 5 inches, with
pockets of higher amounts. Given these probabilities, additional
Winter Storm Watches or Advisories have been issued for portions of
Central and North Central Montana, with an increasing likelihood
that Winter Weather Advisories will need to be issued for the
remaining mountains of Southwest and Central Montana on Saturday.

Low temperatures (Sunday night/Monday morning) - Fresh snow, light
winds, clearing skies, and a ~1030 mb to ~1034 mb surface high
settling south along the Canadian Rockies towards Northern Montana
will lead to ideal radiation cooling processes; most notably over
the plains of Central and North Central Montana. Latest NBM
probabilities for temperatures of 0 degrees or colder are in excess
of a 50% chance along and north of a Augusta, to Great Falls, to
Fort Benton, to Forth Belknap line, with a 20% to 60% chance for
values falling below -10 degrees east of the US Hwy 89 corridor from
the Piegan Port of Entry to Great Falls and west of the US Hwy 87
corridor from Great Falls to Havre (Golden Triangle Area). A "worst
case scenario" for low temperatures would be for skies to clear by
early Sunday evening, which would maximize the number of hours for
cooling. NBM Min temperatures for this worst case scenario generally
range from -20 to -25 degrees across the Golden Triangle Area. -
Moldan

&&

.AVIATION...
22/00Z TAF Period

The main concern for the upcoming TAF period will be periods of
snow and freezing drizzle across portions of north-central and
central Montana. Mountain obscuration along the Rocky Mountain
Front will be present through the entire period with increasing
mountain obscuration expected for central Montana mountains after
22/12Z.

North-Central Montana (KCTB, KHVR): MVFR/IFR conditions will
prevail for KCTB through the period starting with low clouds with
a chance of snow through 22/06Z and then another, heavier, period
of snow starting after 22/16Z. The second round of snow after
22/16Z brings a particular concern for LIFR conditions at KCTB
with a smaller but present chance for VLIFR conditions between
22/16Z and 22/22Z.

A mix of snow and freezing drizzle is possible at KHVR over the
next 6 hours with low clouds and MVFR conditions moving in behind
the precipitation and persisting through much of the TAF period.
The concern for IFR ceilings and IFR/LIFR visibilities at KHVR
will be with the second round of snow Friday afternoon.


Central Montana (KGTF, KHLN, KLWT): The main concern for portions
of central Montana will be a mix of snow and freezing drizzle that
is expected after 22/15Z for KHLN and after 22/19Z for KGTF. Aside
from icing concerns, ceilings and visibilities are expected to dip
into MVFR/IFR territory with some periods of IFR possible at KGTF
after 22/15Z.

VFR conditions will persist for KLWT through most of the period
with with a chance for some MVFR ceilings and visibilities Friday
afternoon as a heavier round of snow moves through the area.


Southwest Montana (KBZN, KEKS): VFR conditions will prevail
through the TAF period with gusty south winds expected at KEKS
through the period.

-thor

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  20  27  21  28 /  20  80  60 100
CTB  12  21  14  21 /  60 100  80 100
HLN  28  42  29  42 /  20  70  50 100
BZN  28  50  29  45 /  30  30  40  80
WYS  22  41  19  34 /  60  20  40 100
DLN  28  47  30  42 /  20  10  40  80
HVR  15  28  17  29 /  50 100  70 100
LWT  20  36  20  40 /  10  70  30  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 10 PM MST this evening for East
Glacier Park Region.

Winter Weather Advisory from 5 AM to 11 PM MST Friday for Bears
Paw Mountains and Southern Blaine-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-
Hill County-Judith Basin County and Judith Gap-Little Belt and
Highwood Mountains-Northern Blaine County-Northern High Plains-
Snowy and Judith Mountains-Southern High Plains-Southern Rocky
Mountain Front-Western and Central Chouteau County.

Winter Weather Advisory from 5 AM Saturday to 5 AM MST Sunday
for Cascade County below 5000ft-Eastern Pondera and Eastern
Teton-Western and Central Chouteau County.

Winter Storm Watch from late Friday night through late Saturday
night for East Glacier Park Region-Southern Rocky Mountain Front.

Winter Storm Watch from late Friday night through late Saturday
night for Bears Paw Mountains and Southern Blaine-Eastern
Glacier, Western Toole, and Central Pondera-Eastern Toole and
Liberty-Hill County-Northern Blaine County-Northern High Plains-
Southern High Plains.

Winter Weather Advisory from 5 AM to 11 AM MST Friday for Helena
Valley-Meagher County Valleys-Upper Blackfoot and MacDonald Pass.

&&

$$
http://www.weather.gov/greatfalls