490
FXUS65 KTFX 230618
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1118 PM MST Fri Nov 22 2024

.KEY MESSAGES...

- Snow and or mixed precipitation will continue to impacts to
  Central and North-central Montana through this evening.

- A second wave of snow will affect north-central Montana on
  Saturday with areas of mixed precipitation changing back to all
  snow by Saturday afternoon and evening.

- Saturday afternoon and evening northerly winds behind the cold
  front may gust up to around 40 mph at times and cause short
  periods of localized blizzard conditions along the Northern
  Rocky Mountain Front and the the western portions of the Hi-
  Line.

- The cold front will also bring some brief but intense snow
  showers with gusty winds to Southwest Montana Saturday
  afternoon and evening.

&&

.UPDATE...
/Issued 921 PM MST Fri Nov 22 2024/

Additional update at 1115 pm...Winter Weather Advisories have been
allowed to expire. The widespread precipitation has mostly moved
out of the area, while the precipitation moving into the area is
handled well by additional Winter Warnings and Advisories that
will begin over the next 3 to 6 hours. -Coulston

The primary change with this update was to lower overnight
temperatures a little in areas that received snow today. One of
the main concerns heading into the overnight hours will be the
chance for more freezing rain in the Helena area. Probabilities
are still in the 40 to 60% confidence range for at least another
one hundredth of an inch. But drier air might prevent that from
occurring so for now the winter weather advisory will be allowed
to expire in a couple hours.

Drier air is starting to become a bit more prominent in the
models with the latest run. The current snow forecast may be a
little on the high side if some of the models pan out but there
is still enough support for higher snow amounts that the current
amounts were left in for now.

There is a potential for blowing snow along the Rocky Mountain
Front into the western portion of the hi-line. At this point in
time the going winter storm warning is sufficient as it is
unlikely for locations east of Havre to receive both enough snow
and wind to cause any concerns. That being said, west of Havre
there may be some travel concerns along US-2 Saturday afternoon.

Otherwise, the final element that was looked at was southwest
Montana. There is a bit of lingering uncertainty on whether or
not snow totals are enough to cause the impacts that would
warrant a product being issued. Confidence right now sits at
around 20% for 2 inches or more along Bozeman Pass as well as
portions of I-90 to Homestake Pass. This is not enough confidence
to issue a product but it is enough uncertainty that this will
need to be monitored closely moving forward. -thor

&&

.DISCUSSION...
/Issued 953 PM MST Fri Nov 22 2024/

The primary concerns for this evening and tonight will be
continued light snow and or mixed precipitation for Central and
North- central MT. Any road surfaces that may be wet this
afternoon are expected to refreeze and become slick again once
diurnal heating wanes.

Then attention turns to Saturday and Saturday night when a cold
cold front will bring gusty northerly winds and accumulating snow
to Central/North-central MT, with the worst condtions expected
along the Rocky Mountain Front and the western portions of the
Hi- Line where a potential barrier jet increases the chances for
wind gusts over 40 mph to around 30% Saturday afternoon and
evening. I considered placing this area under a blizzard warning,
but opted to upgrade the winter storm watch to a winter storm
warning due to lack of confidence in longer term blizzard
impacts. Otherwise, The remaining portion of the winter storm
watch was upgraded to a winter weather advisory. Southwest MT
will also be impacted by the cold front, though slightly warmer
temperatures look reduce the overall impacts. No winter weather
products were sent out for this forecast package. These
situations will continue to be monitored going forward.

Please see the still relevant previous discussion for details
about the longer term. - RCG

 - Meteorological Overview:

A deep upper level low remains centered offshore of the Pacific
NW with remnants of the atmospheric river of moisture which has
been directed at the west coast spreading northeast across the
Northern Rockies today. Surface high pressure and a colder
airmass is in place across the plains with widespread
precipitation expected to develop along the continental divide
this morning then spreading east across north-central MT this
afternoon as moisture overruns the cooler airmass across the
plains. Deeper cold air across northern portions of North-
central MT will provide better lift with snow accumulations of
2-3 inches expected today along the Hi-line with higher amounts
near the Rocky Mtn Front and continental divide, including as
much as 4-6 inches near Marias Pass. Precipitation type gets more
complicated across southern portions of north-central MT
including areas from Rogers pass east through Great Falls and
Lewistown, where the cold airmass is more shallow and
precipitation is lighter but could mix with freezing rain or
drizzle.

Another wave of moisture moves across the region Saturday as the
offshore trough nudges inland with a shortwave disturbance
lifting across the Northern Rockies with an associated area of
surface low pressure and upper level frontal feature shifting
across central and southwest MT late Saturday afternoon and
evening. Another round of widespread snow is expected across
north-central MT with areas along the Hi-line again favored for
accumulations slightly higher than todays event with some mixed
precipitation also possible again on Saturday across portions of
central MT. Cooling aloft with the upper level features on
Saturday will create a somewhat unstable environment across
central and southwest MT Saturday afternoon with hi-resolution
models suggesting convective snow showers potentially focused
along the frontal feature shifting across SW MT Saturday
afternoon/evening.

 - Forecast Confidence & Scenarios:

Confidence is high in primarily snow impacts along the H-line
today where a Winter Weather Advisory remains in effect through
this evening. Further south across north-central MT, forecast
temperature profiles do support freezing rain, particularly along
the southern edge of the cooler airmass across southern portions
of Cascade county and northern portions of Lewis and Clark county
today. Probabilities for measurable ice accrual are highest
(>50%) along the MT-200 corridor from Great Falls to Rogers Pass,
though overall QPF in these areas is low enough that the risk for
ice accumulation of 0.10" or more is generally less than 20%.

A winter storm watch remains in place for the Rocky Mtn Front and
Hi- line regions Saturday with a winter weather advisory across
adjacent areas to the south across north-central MT. Snowfall
amounts are a bit higher than today`s event but probabilities for
amounts in excess of 4 inches have lowered some with higher
snowfall amounts focused just north of the Canadian border.
-Hoenisch


&&

.AVIATION...
23/06Z TAF Period

Varying conditions are expected over the next 24 hours as one
system exits the area and the next one begins to push in. While
precipitation has largely ended, a few areas continue to see low
CIGS this evening, primarily KLWT and KHLN. After a lull
overnight, another round of RA and SN will push in from the west,
bringing a round of IFR conditions due to falling and blowing
snow. Ludwig

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  18  30   9  18 /  70  90  70  30
CTB  12  21   3  13 /  70  90  50  30
HLN  27  46  18  33 /  70  80  70  30
BZN  29  48  18  34 /  40  70  80  40
WYS  19  36  16  31 /  20  90  90  70
DLN  31  44  19  34 /  50  70  60  20
HVR  15  30   5  18 /  70  90  50  20
LWT  19  43   9  21 /  50  60  70  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 AM MST Sunday for East Glacier Park
Region.

Winter Weather Advisory from 5 AM Saturday to 5 AM MST Sunday
for Bears Paw Mountains and Southern Blaine-Cascade County below
5000ft-Eastern Pondera and Eastern Teton-Hill County-Northern
Blaine County-Southern High Plains-Western and Central Chouteau
County.

Winter Weather Advisory until 5 AM MST Sunday for Southern Rocky
Mountain Front.

Winter Storm Warning from 5 AM Saturday to 5 AM MST Sunday for
Eastern Glacier, Western Toole, and Central Pondera-Eastern
Toole and Liberty-Northern High Plains.

&&

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