Severe Thunderstorm Warning 121, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
550 PM MDT Sun Sep 6 2026

Aviation Section Updated.

.KEY MESSAGES...

- Scattered to widespread showers and thunderstorms develop this
  evening along and north of Highway 200, with isolated large
  hail possible.

- Periods of widespread rainfall Monday and Tuesday, with periods
  of moderate to heavy rainfall across Rocky Mountain Front and
  Sweet Grass Hills.

- Drier conditions Wednesday and Thursday.

- Another round of precipitation for next weekend.

&&

.DISCUSSION...
/Issued 218 PM MDT Sun Sep 6 2026/

 - Meteorological Overview:

A passing shortwave will develop scattered to widespread showers
and thunderstorms mainly north of the MT Highway 200 corridor
late this afternoon through the evening. The incoming trough
brings a good synoptic environment for the threat of a few severe
thunderstorms across North-Central MT, with hail as the primary
threat. High PWATs nearing an inch this afternoon will bring an
isolated flash flood threat to where soils have been saturated
from recent precipitation. Behind this system, a vertically
stacked low pressure system will move into the area Monday morning
through Tuesday morning, bringing periods of widespread rain for
mainly North-Central MT. Based on the current low track, periods
of moderate to heavy rainfall will be possible for the Rocky
Mountain Front and US Highway 2 corridor. This system will also
bring in cooler fall-like temperatures Monday and Tuesday. Monday
night into Tuesday morning, low to mid level winds increases
around the closed low. This results in breezy to windy conditions
across North-Central MT.

After the system exits during the day Tuesday, drier conditions
move in for Wednesday and most of Thursday. 700mb winds increasing
along the Rocky Mountain Front Thursday will also increase
surface winds, though confidence is low for high winds.

Heading into late Thursday night into Friday, a cold front will
re-introduce low end chances for precipitation. Though, better
chances for precipitation arrived with the next incoming trough
Saturday and Sunday. Ensembles hint at a cooler system incoming
next weekend with snow levels falling to at least the higher
mountain peaks. Though details and confidence in this next system
is low.

 - Forecast Confidence & Scenarios:

The biggest question mark of the day today was cloud cover and
instability limiting severe thunderstorm potential today. The
synoptics providing both directional and speed shear (40-50kts)
bring a favorable environment for thunderstorms to become severe
across North-Central MT. The morning cloud cover and fog could've
limited instability development, but there looks to be a few
breaks in the clouds early this afternoon to keep an eye on
isolated severe storms. Since all the instability will be
elevated, the storms will be elevated and therefore a primary hail
threat, with a low end severe wind gust threat. The best window
for severe thunderstorms to develop will be between 6-10pm with
the best spatial corridor along and north from the Helena to
Lewistown line. A limiting factor late tonight as storms get
closer to the Hi-line is the drop off of instability and increase
in saturated profiles. Therefore, it seems like those initial
storms in the early to mid evening will be the storms to watch.

Models have been consistently showing additional 1-3" rain
accumulations along the Rocky Mountain Front and the Sweet Grass
Hills. How far east along US Highway 2 does this extend remains
uncertain, but this northwest shift in the low track favors the
Rocky Mountain Front for the corridor of heaviest precipitation.
With thunderstorms today and tomorrow, some high-res guidance show
up to isolated 4" if that area gets good convective rain rates.
Rainfall amounts drop off significantly south of the MT Highway
200 corridor to less than a quarter of an inch. -Wilson

&&

.AVIATION...
07/00Z TAF Period

Main concern this afternoon is strong thunderstorms moving across
the region from the Southwest to the Northeast with some being
severe and capable of producing large hail and wind gusts of 50
KT. These storms will primarily impact KGTF, KHLN, and KLWT
however KHVR could be impacted if convection persists into the
night. MVFR conditions for Northern and Central MT will continue
with chances for IFR overnight as the storm mode transitions to a
more widespread moderate to heavy rain system.

For KGTF: Severe thunderstorm potential will be from 01z to 02z
this evening. The primary hazards expected will be strong gusts
and a potential for large hail. MVFR ceilings will return
overnight into tomorrow morning along with restrictions to
visibilty due to moderate rain and mist until late tomorrow
morning. - Akins

&&

.HYDROLOGY...

Areas along the Rocky Mountain Front have received 0.75" to 1.5"
of rainfall over the past day. There hasn't been much of a
response in creeks/streams yet due to the drier grounding soaking
all the rain up. The main concern is another additional
widespread 1 to 3 inches and isolated 4 inches through Tuesday
morning on a more saturated ground. Areas that get the higher end
3 to 4 inch totals may see minor overflow in creeks/streams along
with muddied rural roads. The wet conditions can also cause
rock/debris slides in the higher terrain. Therefore, a flood watch
was issued from this afternoon through Tuesday. Farther east
across North-Central MT will see wetting rain, but confidence for
those higher end totals remain low. One thing to watch for is any
eastern shift of these amounts farther east along US Highway 2
towards Hill County. -Wilson

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  64  47  66 /  90 100  40  50
CTB  46  56  44  60 /  90 100 100  80
HLN  51  66  44  70 /  80 100  20  10
BZN  48  69  40  71 /  30  80  10   0
WYS  41  60  30  64 /  90  40  10   0
DLN  46  66  36  71 /  90  50   0   0
HVR  51  68  45  64 /  90 100  90  90
LWT  50  68  44  64 /  90  90  20  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday morning for East Glacier Park Region-
Eastern Glacier, Western Toole, and Central Pondera-Eastern
Pondera and Eastern Teton-Eastern Toole and Liberty-Northern
High Plains-Southern High Plains-Southern Rocky Mountain Front.

&&

http://www.weather.gov/greatfalls

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