504
FXUS65 KTFX 180522
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1022 PM MST Mon Nov 17 2025

Aviation Section Updated.

.KEY MESSAGES...

- Patchy fog development is once again possible tonight through
  Tuesday morning across the Hi-Line in North Central Montana and
  in prone valleys of Southwest Montana.

- Passing weather systems will bring spotty areas of light rain
  and mostly mountain snow through Wednesday.

- A more progressive weather pattern brings a return of breezy to
  windy conditions and mountain snow this weekend, followed by a
  shift to more wintry conditions heading into the Thanksgiving
  holiday.

&&

.UPDATE...
/Issued 814 PM MST Mon Nov 17 2025/

Fog remains a bit of a concern this evening, though whether or not
it will actually materialize overnight is a bit tricky. Given the
widespread humidity values already in the 80-90%+ range, I have
opted to increase the coverage of fog, but the presence of clouds
means that uncertainty remains high. Ludwig

&&

.DISCUSSION...
/Issued 814 PM MST Mon Nov 17 2025/

 - Meteorological Overview:

A moist, but weak, southwesterly flow aloft will persist until
Wednesday when a Pacific trough passes through the Northern
Rockies. This will maintain variably cloudy skies and periods of
lighter end rain and snow. Snow levels will be lowest on Wednesday
while the trough axis moves overhead; however, negligible
precipitation amounts and the lack of colder air aloft will offer
little support for accumulating snow. Lingering moisture and light
winds throughout the atmospheric profile will also encourage
areas low stratus and patchy fog to develop over the next couple
of days. This looks to be most prevalent during the overnight and
morning hours near river valleys.

Drier conditions move in for the second half of the workweek with
another Pacific trough diving southeastward into Great Basin and
southwest US. Then the upper level jet moves into the Northern
Rockies this weekend and brings periods mostly mountain snow,
breezy to windy conditions, and cooler temperatures. A series of
troughs may then bring colder temperatures and even periods of
snow for the week of Thanksgiving, though model guidance has been
divergent, especially in terms of trough amplitude and timing.
- RCG

 - Forecast Confidence & Scenarios:

Patchy Dense Fog tonight through Tuesday morning...

Hi-Res Ensemble Forecast (HREF) guidance generally supports
between a 10-30% chance for visibilities to fall below 1/4 mile
along the Hi-Line in North Central Montana, most notably across
Hill and Blaine Counties, and in the West Yellowstone Area. BUFKIT
analysis for these same areas casts some doubt as to whether any
fog will materialize given the presence of low cloud cover, with
the current thinking being that a low stratus deck is more likely
to prevail; however, patchy dense fog is possible in prone areas
if enough clearing can occur during the period. - Moldan

Scattered areas of light rain and snow through Wednesday...

Rain and snow associated with an approaching Pacific weather
system looks to be light and scattered in nature. Temperatures
aloft will not be favorable for accumulating snow until the trough
axis moves overhead on Wednesday. Even then, probabilities for
one inch of snow or more will be around 20% in the mountains and
near zero for lower elevations.

Increasing winds and mountain snow this weekend...

Winds increase heading into the weekend while mountain snow
returns, mostly along the Continental Divide. Recent ensemble runs
have begun to favor the jet stream setting up a little farther
north with shallower troughing. If this trend continues, it would
shift the mountain snow farther north, just clipping the Rocky
Mountain Front. Areas of higher terrain to the south and the
plains/valleys would see little to no precipitation. A more
southerly trajectory would bring more mountain snow, especially
the northwesterly upslope areas, including the central island
ranges and the Madison and Gallatin ranges. The strongest
shortwave and best chance for precipitation looks arrive Saturday
night into Sunday. Westerly surface winds will also be on the
increase with many central and north-central locations seeing
winds in the 30 to 50 mph range.

More wintry pattern shift for the week of Thanksgiving...

Nearly all ensemble members usher in below average temperatures
for the days leading up to Thanksgiving, but only around a third
of them highlight impactful accumulating snow. There are still
significant differences with trough amplitude and timing and there
also has been poor model run to run consistency. Overall,
preparations should be made for the arrival of the coldest
temperatures of the season so far with probabilities for below
freezing highs climbing above 70% for most areas next Tuesday
through Thursday. - RCG

&&

.AVIATION...
18/06Z TAF Period

VFR conditions will prevail across all terminals during this TAF
Period except for the KCTB and KHVR terminals. At the KHVR and KCTB
terminals between 17/08Z and 17/18Z there is a 30 - 40% chance for
fog to form. Between 17/10Z and about 17/18Z at the KHVR and KCTB
terminals there will be IFR-level ceilings with a low chance for
periodic LIFR-level ceilings. During the majority of this TAF Period
there will be periodic mountain obscuration across Southwestern
Montana. -IG

The KWYS TAF will not be issued until airport operations resume
next spring.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  39  56  31  45 /  10  10  10  20
CTB  34  51  31  43 /  20  20  10  10
HLN  36  53  31  45 /  10  10  10  30
BZN  36  54  33  44 /  10  10  10  30
WYS  31  41  28  40 /  40  20  10  40
DLN  34  51  29  47 /  10  10   0  10
HVR  32  48  29  43 /  10  10  20  10
LWT  36  55  28  43 /  10  10  10  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

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