940
FXUS65 KTFX 131847
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1247 PM MDT Thu Aug 13 2026

.KEY MESSAGES...

- Cooler temperatures persist across the region through the end
  of the week with showers and thunderstorms around at times.

- A few thunderstorms will be capable of producing heavy rain,
  which would be impactful if they were to move over a city or
  burn scar.

- Temperatures trend closer to normal by the end of the weekend,
  with continued opportunities for afternoon and evening showers
  and thunderstorms.

&&

.DISCUSSION...

 - Meteorological Overview:

A southwesterly flow aloft will more or less persist across the
Northern Rockies through the weekend into early next week. This flow
is initially more cyclonic in nature, which will foster plenty of
showers and thunderstorms through Friday evening across the region.
Persistent surface winds from the east and northeast will continue
to usher in cooler temperatures. Precipitable water values will tick
a touch higher today, resulting in increased precipitation
efficiency with showers and thunderstorms that develop. The most
robust thunderstorms that develop over the next few days will be
capable of producing at least briefly heavy rainfall. Should one of
these stronger storms move over urban areas or burn scars, some
localized flash flooding cannot be ruled out. Lightning and gusty
winds will also be a concern with stronger thunderstorms.

Heading into the weekend the nature of the southwesterly flow aloft
becomes a bit more anti-cyclonic. That said, moisture working in
from the southwest will combine with temperatures trending closer to
normal to result in scattered afternoon showers and thunderstorms.

Greater uncertainty in the pattern develops toward the middle of
next week. At this point the main takeaway will be for scattered
showers and thunderstorms most days with temperatures near average.
-AM

 - Forecast Confidence & Scenarios:

Heavy Rainfall through Saturday morning...

Hi-Res Ensemble Forecast (HREF) guidance remains, and has remarkably
been so for the past several days, consistent with showers and
thunderstorms developing each day across the Northern Rockies as
monsoon moisture streams northeast and over low level north-to-east
flow in place across Southwest through North Central Montana. This
mornings 12z RAOB from Great Falls measured PWATs at 0.97", which
was accurately prog`d in the NAEFS since Tuesday. This consistent
run-to-run model guidance gives increased confidence in rainfall
amounts and locations through the end of the work week, with the
heaviest precipitation expected to fall along and south of the
Montana Hwy 200 over this timeframe, most notably between the
I-90 and US Hwy 12 corridors.

Included below are tables listing 24 hour rainfall probabilities
from the NBM5.0 for 0.25", 0.5" and 1" for select locations across
North Central, Central, and Southwest Montana. The first table
reflects the 24 hour period from 12PM Thursday through 12PM
Friday, with the second table reflecting the timeframe from 12PM
Friday through 12PM Saturday.

                 24hr Rainfall Probabilities thru 12PM Friday
LOCATION               > 0.25" |   > 0.50"   | > 1.00"
Browning                  65%  |      40%    |    15%
Cut Bank                  50%  |      25%    |     5%
Havre                     65%  |      30%    |     5%
Great Falls               60%  |      35%    |    10%
Lewistown                 55%  |      30%    |    10%
Helena                    60%  |      40%    |    10%
White Sulphur Springs     60%  |      50%    |    15%
Bozeman                   65%  |      45%    |    15%
Dillon                    80%  |      50%    |    10%
Ennis                     65%  |      40%    |    10%
West Yellowstone          70%  |      55%    |    15%

                 24hr Rainfall Probabilities thru 12PM Saturday
LOCATION               > 0.25" |   > 0.50"   | > 1.00"
Browning                  20%  |      10%    |    <5%
Cut Bank                  15%  |      10%    |     5%
Havre                     20%  |      10%    |    <5%
Great Falls               30%  |       5%    |    <5%
Lewistown                 50%  |      30%    |    10%
Helena                    40%  |      15%    |     5%
White Sulphur Springs     70%  |      50%    |    10%
Bozeman                   75%  |      50%    |    15%
Dillon                    75%  |      35%    |    10%
Ennis                     85%  |      50%    |     5%
West Yellowstone          55%  |      30%    |    <5%

Rainfall amounts exceeding 2" can not be ruled through Saturday
morning within the corridor between the I-90 and US Hwy 12
corridors, with probabilities near a 10% chance here. - Moldan

Risk for strong thunderstorms:

While inverted-V soundings will be around across SW MT the next two
days, these inverted-Vs are rather shallow all things considered.
This will help reduce the risk for a strong to severe gust. At this
time the main concern will continue to be lightning, aside from
briefly heavy rainfall. -AM

&&

.AVIATION...
13/18Z TAF Period

Scattered showers will occur across all terminals during this TAF
period, with the terminals west of KHVR seeing periods of
afternoon thunderstorms. These thunderstorms are capable of
producing lightning, brief heavy rainfall, and gusty winds. The
thunderstorms and rain showers will lower ceilings and create
mountain obscuration throughout the TAF period.

For KGTF: Surface winds will continue to blow from the northeast
until around 14/07Z, in which the direction will shift to a north-
northwesterly flow. After this shift, the probability of a cross-
wind in excess of 15 kts, while small, is non-zero (generally less
than 5%). However, passing afternoon thunderstorms are capable of
producing gusty erratic winds and lowering visibilities to MVFR
conditions. Light rain showers will lower ceilings and create
mountain obscuration throughout this TAF period. -Dzomba


&&

.HYDROLOGY...

A Flash Flood Watch is in effect through midnight tonight for the
Sand Creek and Bobcat Lakes Burn Scars in the Pioneer Mountains,
the Horse Gulch Burn Scar in the Big Belt Mountains, and for the
Elkhorn and Boulder Mountains of Jefferson and Broadwater
Counties. Showers and thunderstorms have been very efficient in
producing rainfall thus far this morning across Central and North
Central Montana, with this trend expected to continue points
further south as PWATs remain near 1" across the Northern Rockies.
HREF guidance supports a 20-40% chance that hourly rainfall
amounts will exceed 0.25" across the Flash Flood Watch area, with
these types of rates already being observed this morning over the
plains of Central and North Central Montana. Steep terrain and
hydrophobic soils will be especially susceptible to flash flooding
and debris flows over this timeframe, and those with interest near
these features should be prepared to seek higher ground should
Flash Flood Warnings be issued. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  64  49  77 /  90  60  50  10
CTB  43  59  44  73 /  70  50  40  20
HLN  52  66  51  77 /  80  80  70  10
BZN  50  70  47  76 /  80  90  80  20
WYS  39  65  35  70 /  80  60  70  60
DLN  48  70  45  74 /  90  90  60  40
HVR  48  69  48  76 /  50  40  40  30
LWT  48  64  46  72 /  80  80  80  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flash Flood Watch until midnight MDT tonight for Elkhorn and
Boulder Mountains.

Flash Flood Watch until midnight MDT tonight for the Horse Gulch
Burn Scar in the Big Belt Mountains

Flash Flood Watch until midnight MDT tonight for the burn scars in
the Pioneer Mountains.

&&

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