921
FXUS65 KTFX 180211
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
811 PM MDT Thu Sep 17 2026

.KEY MESSAGES...

 - Several rounds of showers/thunderstorms today, mainly impacting
   SW MT.

 - Increased shower/thunderstorm coverage tomorrow, with a few capable
   of producing strong wind gusts, small hail, and heavier
   showers.

 - The potential for periods of heavier precip at times tomorrow and
   Saturday are causing a minor risk for flash flooding.

&&

.UPDATE...

No significant changes were needed with the update this evening.
Slight changes were made to account for latest trends in
observations and guidance. Scattered showers and perhaps a few
thunderstorms will continue to lift north and northeastward from
Idaho through the overnight. -AM

&&

.DISCUSSION...
/Issued 527 PM MDT Thu Sep 17 2026/

 - Meteorological Overview:

The upper level low, which is currently over the PNW, will continue
to bring southerly flow aloft over the rockies today. Due to weaker
embedded waves making their way over the region, there will rounds
of showers/thunderstorms today into this evening. Drier conditions
are expected after 06Z tonight, although an isolated
shower/thunderstorm or two can`t be ruled out.

As the upper level low approaches the region, there will be an
environment that is conducive to developing stronger storms on
Friday. However, the threat for severe weather will be dependent on
whether there is sufficient shear in the area. A concern associated
with a lower shear environment would be the stagnation of the storm
movement, which could result in increased risk of excessive rainfall
and flash flooding.

As the upper-level low moves eastward and out of the state on
Saturday/Saturday night, there will be a shift to drier and warmer
conditions. However, showers may continue to linger on Sunday in
eastern MT. Monday and Tuesday currently look warm and dry. - ESH

 - Forecast Confidence & Scenarios:

Thunderstorms the next few days:

While confidence is high that showers and thunderstorms will develop
over the next few days, confidence in just how much instability
develops remains on the lower side. Relatively strong forcing will
accompany the passage of this system, particularly on Friday. This
forcing may ultimately result in a convective initiation that is a
bit too early to maximize daytime heating, resulting in ample cloud
cover across much of the region by mid-afternoon that limits
thunderstorm strength. -AM


&&

.AVIATION...
18/00Z TAF Period

Showers and thunderstorms will continue to roll into Southwest MT
this evening. Best chances for showers and thunderstorms stay
south of I-90, though a few isolated clusters can make it`s way
up through the Helena Valley to North-Central MT. Isolated strong
gusty winds will be possible with storms. Models hint at showers
continuing overnight in Southwest MT. Better dynamics for
scattered showers and thunderstorms arrive Friday afternoon in
Southwest MT, with better chances for strong winds, hail, and
heavy rainfall. Moderate to heavy showers can cause reductions in
visibility to MVFR. -Wilson

&&

.HYDROLOGY...

Precipitable Water (PWATs) values approaching 1", which are 2 to 3
standard deviations above normal, are forecasted to reside over
portions of Southwest and Central Montana from Friday morning
through Saturday morning as Pacific moisture streams north to
northeast and across the Northern Rockies. ECMWF EFIs largely
ranging between 0.75 and 0.85 south of a Helena to Lewistown line
and north of a Dillon to Bozeman line, with SOTs approaching 1.5
from a Butte to White Sulphur Springs line, support the potential
for a very unusual rain event with respect to mid-September
climatology thanks to these anomalous PWATs. Developing showers and
thunderstorms over this timeframe will have slow storm motions of
generally 15-25 mph, which will allow for increased residence time
over a given area.

Given all of these indicators (i.e. slow storm motions, anomalous
PWATs, and very unusual forecasted precipitation) the need for Flash
Flood Watch for portions of Southwest and Central Montana has
increased; most notably for areas that are hydrophobic. While the
potential for urban flooding can`t be ruled, especially in the
Helena and Gallatin Valleys, current thinking is that recent burn
scars (regardless of size)from the 2026 season and the Horse Gulch
area will be the most susceptible for Flash Flooding and debris
flows. For this reason a Flash Flood Watch has been issued for
Little Belt and Highwood Mountains, Big Belt, Bridger and Castle
Mountains, Elkhorn and Boulder Mountains, and Northwest Beaverhead
County to account for the recent/past burn scars. Given that hunting
season is well underway across the aforementioned areas those
recreating near burn scars should be prepared to seek higher ground.
- Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  73  52  65 /  10  10  40  50
CTB  47  69  47  67 /  10   0   0   0
HLN  49  75  52  63 /  50  50 100  80
BZN  48  71  48  59 /  10  50  90 100
WYS  39  67  38  57 /  50  80  80  80
DLN  47  68  46  60 /  30  60  80  90
HVR  49  77  50  70 /  10   0   0  20
LWT  50  73  49  59 /   0  20  70  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Friday evening through Saturday evening
for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder
Mountains-Gallatin Valley-Little Belt and Highwood Mountains.

Flash Flood Watch from Friday afternoon through Saturday
afternoon for Northwest Beaverhead County.

&&

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