828
FXUS65 KTFX 122302
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
502 PM MDT Wed Aug 12 2026

Aviation Section Updated.

.KEY MESSAGES...

- Below normal temperatures through Saturday, with highs only
  rising into the mid-50s to low 70s on Thursday and Friday.

- Showers and thunderstorms will become more widespread tonight
  into Thursday, few areas possibly getting over a half inch of
  rainfall.

- Lightning over the next few days could start new wildfires.

&&

.DISCUSSION...
/Issued 141 PM MDT Wed Aug 12 2026/

 - Meteorological Overview:

Shower and thunderstorm activity will continue to increase through
the afternoon and evening hours, most along the Rocky Mountain Front
and Central Montana mountains, with these showers and storms then
moving east to northeast over the adjacent plains and valleys into
the morning hours on Thursday. A few storms could produce strong
wind gusts and/or heavy rainfall this afternoon/evening, with the
best opportunity for strong wind gusts occurring across Southwest
Montana where the sub-cloud layer is still relatively dry leading to
inverted-V soundings. Additionally, breezy and gusty north to
northeast winds will develop over the plains of Central and North
Central Montana from the late afternoon hours and persist through
the evening hours tonight as the H850 flow increases in conjuction
with surface pressure rises on the order of 3-6mb/6hr as surface
high pressure continues to nose south from Canada. - Moldan

The chance for thunderstorms will increase Thu into Friday morning
as precipitable water increases. Thus the chance for isolated flash
flooding is possible from heavier showers. Additionally, a few
storms could produce strong wind gusts on Thursday/Friday,
especially over Southwest MT where it will be a bit more unstable.

The summertime upper level ridge of high pressure will try to
rebuild over the northern Rockies starting this weekend. This will
result in warmer, but more unstable air moving back into the CWA
this weekend. Expect daily chances for showers/thunderstorms from
Saturday through Tuesday. Again, the main concern from storms over
the weekend will be lightning and strong wind gusts. - Brusda


- Forecast Confidence & Scenarios:

Heavy Rainfall through Friday night...

Ensemble model guidance continues to support numerous opportunities
for showers and thunderstorms through Friday night; with climate
anomaly indicators suggesting that some of the showers and
thunderstorms could produce periods of heavy rainfall anomalous to
August climatology, most notably along the Rocky Mountain Front and
across portions of Central and Southwest Montana (generally between
the I-90 and Montana Hwy 200 corridors). Included below is a table
listing the NBM5.0 probabilities for 0.25", 0.5" and 1" for select
locations across North Central, Central, and Southwest Montana

                   Rainfall Probabilities thru 6am Saturday
LOCATION               > 0.25" |   > 0.50"   | > 1.00"
Browning                 100%  |      90%    |    65%
Cut Bank                  95%  |      90%    |    50%
Havre                     95%  |      85%    |    35%
Great Falls               90%  |      60%    |    20%
Lewistown                100%  |      85%    |    55%
Helena                    95%  |      60%    |    25%
White Sulphur Springs    100%  |      90%    |    60%
Bozeman                   95%  |      80%    |    35%
Dillon                    90%  |      65%    |    25%
Ennis                     95%  |      80%    |    55%
West Yellowstone          95%  |      80%    |    35%

While rainfall amounts exceeding 2" can not be ruled out by Saturday
morning, most notably across the Glacier National Park region, most
areas have a 10% or less chance of seeing this occur. - Moldan&&

.AVIATION...
12/18Z TAF Period

VFR conditions will generally prevail throughout this TAF period,
with the exception of periods of MVFR conditions due to passing
thunderstorms. Thunderstorm activity in Southwest MT will begin
around 22Z, moving northeast. Thunderstorm activity will also
begin near KCTB around 23Z, moving east and southeast. Light rain
will begin in North-Central/Central MT following the thunderstorm
activity. Mountain obscuration is expected throughout this TAF
period, as is a decrease in slant range visibility due to smoke
from regional wildfires.

For KGTF: While small, there is a non-zero chance for cross winds
in excess of 15 kts. However, sustained winds and wind gusts will
be out of the northeast, with the exception of passing convection
which could cause erratic winds. The latest HREF is continuing to
show a thunderstorm in the Great Falls area around 13/02Z. Light
rain is expected to follow this convective activity. -Dzomba


&&

.AVIATION...
13/00Z TAF Period

Initial concern will be for continued wildfire smoke resulting in
some visibility reductions in addition to scattered showers and
thunderstorms across SW MT and vicinity early this evening. Gusty
and erratic winds and lightning are the main concern with these
thunderstorms.

Additional shower activity moves northeastward over the plains late
evening, coinciding with lowering ceilings. Confidence in exact
placement of showers is on the lower side, hence there is
relatively weak mention of precipitation in TAFs at plains sites.
Low ceilings continue through the night and into the day Thursday
over the plains. Additional shower and thunderstorm activity
develops across SW MT and vicinity early Thursday afternoon and
spreads northeastward into the evening.

For KGTF: Brisk northeast winds will continue through the evening,
though this wind direction offers little in terms of cross-wind
concerns. Lower clouds moving in this evening largely look to remain
in an environment above 0C, so icing concerns remain low through the
overnight. I cant quite rule out some outflow late evening, mainly
after 02Z. The magnitude of this outflow is very questionable and
will depend on how convective trends unfold over the next few
hours further south. Showers will become more numerous later
tonight into early Thursday, though confidence in specifics with
respect to timing remains low. -AM


&&

.FIRE WEATHER...

No fire weather statements will be issued at this time, but the
main concern through this weekend will be lightning and
thunderstorms that produce strong outflow wind gusts. Initially
the thunderstorms will be on the wet side today through Friday,
with drier thunderstorms possible over the weekend. Note any
lightning that does occur away from the center of the storm will
have a higher probability of starting a wildfire. Brusda

&&

.HYDROLOGY...

A Flash Flood Watch remains in effect for the Horse Gulch Burn scar
within the Big Belt Mountains from late this afternoon through early
Thursday morning as the first round of showers and thunderstorms
capable of producing heavy rainfall lifts northeast and over the
area. Probabilities for heavy rainfall increase further (from that
of today/tonight) over the burn scar on Thursday evening as PWATs
increase across the Northern Rockies, and for this reason another
Flash Flood Watch has been issued for the Horse Gulch Burn scar from
6PM Thursday through 12AM Friday. This second round of showers and
thunderstorms, while shorter in temporal scale, is likely to produce
the heaviest rainfall amounts of the work week, with a reasonable
high end to rainfall amounts nearing 0.5" over this short timeframe.
Additional opportunities for heavy rainfall will exist on Friday over
the aforementioned burn scar, but for now we will hold off on
issuing any additional Flash Flood Watches given uncertainty of
where convection will ultimately track on Friday.

Showers and thunderstorms producing heavy rainfall are also possible
over the on-going Bobcat Lakes and Sand Creek Wildfires on Thursday
and Friday, with latest HREF guidance suggesting a 20% chance that
rainfall amounts could exceed 0.25/hr over the freshly burned
terrain within the Pioneer Mountains from Thursday afternoon and
through the evening hours. These higher rainfall rates falling on
the newly scarred terrain will have the potential to lead to debris
flows, especially where the ground has become hydrophobic due to
high intensity burning. For now we will holdoff on issuing any Flash
Flood Watches for the Pioneer Mountains given the marginal
probabilities, but should these probabilities or forecast confidence
in heavy rainfall increase further then Flash Flood Watches are
likely to be needed given the risk to property and personnel working
on the fires. - Moldan


&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  66  49  63 /  80  80  70  60
CTB  45  56  43  59 /  80 100  70  30
HLN  53  69  52  67 /  60  40  80  80
BZN  50  74  49  70 /  40  40  80  90
WYS  40  68  38  65 /  40  60  80  90
DLN  48  73  48  70 /  20  70  70  90
HVR  48  63  47  67 /  90  90  60  20
LWT  48  64  47  64 /  90  80  80  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 4 AM MDT Thursday for Big Belt, Bridger
and Castle Mountains.

Flash Flood Watch Thursday evening for Big Belt, Bridger and
Castle Mountains.

Flood Watch Thursday afternoon for Big Belt, Bridger and Castle
Mountains.

Flash Flood Watch Thursday afternoon for Northwest Beaverhead
County.

&&

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