813
FXUS65 KTFX 101132
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
432 AM MST Fri Jan 10 2025

Aviation Section Updated.

.KEY MESSAGES...

 - Strong winds are expected along the Rocky Mountain Front today.

 - Snow develops this evening over the mountains and spreads into
   the lower elevations by Saturday morning with widespread
   accumulating snow expected through early Sunday morning.

 - Additional accumulating snowfall is expected across portions of
   central Montana through early Monday morning.

&&

.DISCUSSION...
/Issued 328 AM MST Fri Jan 10 2025/

 - Meteorological Overview:

The main event for this weekend will center around a large trough
moving through the area. Ahead of the trough, winds will pick up
this morning/afternoon allowing for high winds across the northern
Rocky Mountain Front. As the trough arrives, snow will develop
first in the mountains this evening and spread to the lower
elevations of central and southwest Montana by Saturday morning. The
heaviest snow is expected early Saturday morning with snowfall
rates up to 1 inch per hour possible for the Little Belts,
Highwoods, Snowies, and Bears Paw. Near-continuous light to
moderate snow will continue across much of the region through
Sunday morning, letting up as the day progresses on Sunday.
Scattered light snow showers will linger through early Monday
morning before the trough fully departs the region, allowing for an
upper level ridge to develop over the western CONUS. This will
bring generally drier and slightly warmer conditions to Montana
through the middle of next week.


 - Forecast Confidence & Scenarios:

Wind: Strong winds have already started to develop along the
higher elevations of the northern Rockies. As the morning
progresses, the stronger winds are expected to mix down towards
the Cut Bank area. Confidence remains similar to the previous
forecast with a greater than 80% confidence in portions of the
northern Rocky Mountain Front seeing gusts over 58 mph through
this afternoon.

Looking at the rest of north-central Montana, gusty winds are
expected through the rest of today and into the weekend. But
confidence in high winds appearing anywhere else is below 50%. The
main concern with the winds will be blowing snow causing periods
of reduced visibility as well as drifting along the road.


Snow: Confidence continues to increase in the potential severity
of this event along the mountains of central Montana.

For the Little Belts, there is a greater than 80% chance of 12
inches or more through the duration of the event with a 60 to 80%
chance of 24 inches or more and a 10 to 30% chance of 36 inches
or more.

For the Highwoods, there is a greater than 70% chance of 12
inches or more through the event with a 30 to 50% chance of 18
inches or more and a less than 20% chance of 24 inches or more.

For the Snowies, there is a greater than 80% chance of 12 inches
or more through the event with a 40 to 60% chance of 18 inches or
more and a 10 to 30% chance of 24 inches or more.

For the Bears Paw, there is a greater than 80% chance for 9 inches
or more through the event with a 20 to 40% chance of 12 inches or
more.

When looking at the probabilities and impacts at lower elevations,
the area most likely to see 2 or more inches of snow is covered by
the current array of winter weather advisories. That being
said, as with any winter event in Montana, there are wild cards at
play that will need to be considered.

One of wild cards for central Montana will be Lewistown.
Currently there is a greater than 80% chance of at least 4 inches
through the event. But there is a 40 to 60% chance of greater than
6 inches and 20 to 40% chance of greater than 8 inches. The
potential is very much present for winter storm criteria being met
should some of the stronger snow showers set up at lower
elevations along the foothills. At this point in time, however,
confidence was not high enough to warrant adding it to the current
winter storm warning. But this will be something to watch as the
event unfolds.

Another wild card is the US-87/200 corridor from Belt to Lewistown
which hold similar probabilities to Lewistown and, once again,
depending on how the stronger snow showers set up there could be
spots that reach the higher amounts that would trigger a warning.
But similar to the issue facing Lewistown, confidence in this
scenario playing out is not high enough to add it to the current
warning.

Finally, the wild card in southwest Montana is Bozeman Pass. Currently
there is a greater than 80% chance of 4 inches or more at pass
level with a 40 to 60% chance of 6 inches or more and a 20 to 40%
chance of 8 inches or more. As it stands, the advisory is
sufficient based on probabilities. However, the potential for
impacts along the pass is what ends up being the most concerning
with this event. If there ends up being a shift in the
probabilities towards higher amounts or a shift towards greater
impacts, an upgrade would not be unreasonable. Similar to the
other wild cards in this event, this will be monitored closely for
any changes.


Changes to Warnings/Advisories: The main change with this forecast
was to extend the winter weather advisories and winter storm
warnings out by 12 hours to cover the heavier snow that is
expected to last through the rest of Saturday into Sunday morning.
Winter weather advisories were then added to address the threat
posed by the additional snowfall expected late Sunday morning
through early Monday morning. In general, these snow amounts are
expected to stay below warning criteria. However, there is the
potential that the Little Belts and Snowies may need an upgrade
later on should confidence increase

Additionally, at this time, the decision was made to leave
Chouteau county out of both rounds of advisories. The main reason
is confidence was low that advisory criteria would be met and, in
particular, impacts would be felt along the US-89 corridor. It is
worth mentioning that the probabilities of 2 inches or more
continue to creep closer to Fort Benton with each model update.
And should this trend continue it is very likely that Chouteau
county will need to be added at a later point in time.


Beyond Monday, cluster guidance is in strong agreement for the
development of a ridge over the western CONUS. This will bring
generally calmer conditions to the region while also allowing for
a slight warm up over the plains through the middle of the week.
The main uncertainty is what will happen towards the end of next
week. Ensemble guidance is picking up on the potential for colder
temperatures and possibly a bit of unsettled weather to return to
the region. But details remain unclear at this time.  -thor


&&

.AVIATION...
10/12Z TAF Period

Forecast confidence at the KHLN and KBZN terminals is quite low
within the first 3-6 hours of the 1012/1112 TAF period given the
models handling of low stratus and/or fog within the Helena and
Gallatin Valleys as of late. Hi-Res Ensemble Forecast (HREF)
guidance continues to support a 10-30% chance for VLIFR/LIFR VIS
and/or CIGS through 15z-17z this morning at these two terminals;
however, since mid- to upper level cloud cover began to move over
the terminals around 03z this morning only the KHLN terminal has
been holding onto observed VLIFR conditions. For this reason
decided to prevail VLIFR conditions at the KHLN terminal through
15z, with a TEMPO group covering any brief period of LIFR CIGS
sloshing back in at the KBZN terminal.

Otherwise, the main concern through the remainder of the TAF
period will be strong, mid- to upper level westerly flow through
00-06z this evening across the Northern Rockies which will lead
to strong and gusty surface winds and/or mountain wave turbulence
and/or instances of low level wind shear. The strongest surface
winds through this evening are expected over the plains of
Central and North Central Montana, most notably along the Rocky
Mountain Front. Additionally, precipitation chances will be
increasing through the mid-afternoon to early evening hours across
all terminals as a disturbance moves overhead, with IFR/MVFR
CIGS/VIS expected within/near this precipitation.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  43  27  35  22 /  10  90  60  70
CTB  40  24  33  19 /  10  80  30  50
HLN  41  28  36  22 /  10  80  50  50
BZN  37  23  34  17 /  10  90  70  60
WYS  22   8  25   2 /  50  90  70  40
DLN  36  20  32  12 /  10  50  10   0
HVR  42  27  36  19 /  10  80  50  60
LWT  43  25  31  21 /  10  80  70  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon MST today for Eastern Glacier,
Western Toole, and Central Pondera-Northern High Plains.

Winter Storm Warning from 5 PM this afternoon to 5 AM MST Sunday
for Bears Paw Mountains and Southern Blaine-Little Belt and
Highwood Mountains-Snowy and Judith Mountains.

Winter Weather Advisory from 6 AM Sunday to 5 AM MST Monday for
Bears Paw Mountains and Southern Blaine-Cascade County below
5000ft-Fergus County below 4500ft-Judith Basin County and Judith
Gap-Little Belt and Highwood Mountains-Snowy and Judith
Mountains.

Winter Weather Advisory from midnight tonight to 5 AM MST Sunday
for Cascade County below 5000ft-Fergus County below 4500ft-
Gallatin Valley-Judith Basin County and Judith Gap-Meagher
County Valleys.

Winter Weather Advisory from 5 PM this afternoon to 5 AM MST
Sunday for Big Belt, Bridger and Castle Mountains-Gallatin and
Madison County Mountains and Centennial Mountains.

&&

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