Severe Thunderstorm Warning 138, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
457 PM MDT Mon Aug 4 2025

Aviation Section Updated.

.KEY MESSAGES...

- Showers and thunderstorms will move south to north this
  afternoon through early Tuesday, some may be strong to severe
  with localized strong wind gusts, hail, and heavy downpours.

- Drier conditions are expected for the mid- week period before
  another Pacific trough brings more scattered showers and storms,
  breezy to windy conditions, and cooler temperatures for the
  second half of the work week.

&&

.UPDATE...

Early evening update has been published to include mention of
large hail and damaging winds in the weather grids for those areas
covered by the severe thunderstorm watch that includes Blaine,
Chouteau, Hill, Liberty, Pondera, Teton, Toole, Cascade, Fergus,
Judith Basin, Meagher, Lewis and Clark, Broadwater, and Jefferson
Counties through 11 PM MDT this evening. - Moldan

&&

.DISCUSSION...
/Issued 155 PM MDT Mon Aug 4 2025/

 - Meteorological Overview:

Showers and thunderstorms will continue to develop through the early
afternoon hours as a somewhat, 1 standard deviation, anomalous
shortwave approaches the Northern Rockies from Northeast Oregon.
This shortwave will continue to lift to the northeast and over the
Northern Rockies through the overnight hours tonight, with the best
upper level support overspreading Southwest through North Central
Montana between 00-12z Tuesday. Areal coverage of showers and
thunderstorms will increase through the mid-afternoon hours, with
showers and thunderstorms expected to congeal into a more or less
continuous line across Central Montana, with this line of
precipitation continuing to lift to the northeast and across the
remainder of North Central Montana through the remainder of the
evening hours. Additional, albeit isolated to scattered, showers
and thunderstorms will persist in wake of the main line of
precipitation through the overnight hours as the aforementioned
shortwave moves overhead. Increasing shear aloft and sufficient
instability through the afternoon and evening hours will help to
support an environment capable of producing isolated to scattered
severe thunderstorms, most notably over the plains of Central and
North Central Montana. All of Southwest through North Central
Montana will be capable of seeing large hail and damaging winds,
with the severe weather threat generally spanning between 2-10 PM.
- Moldan

Some shower and thunderstorm activity may linger over the plains
Tuesday, especially in the Milk River Valley area, but drier and
breezy conditions are generally expected for Tuesday and Wednesday
in the wake of the departing trough. Ensembles are coming into
agreement with another Pacific trough moving through the Northern
Rockies Thursday through early Saturday. Breezy to windy conditions,
increased shower and thunderstorm activity, and temperatures cooling
as low as 10 to 20 degrees below average are expected while the
trough moves its way through the region.

Ensembles remain in agreement with a progressive northwesterly flow
aloft settling in for the weekend into early next week, but there
are discrepancies on the depth and timing of the troughs and ridges
moving within it. Overall, there's an expectation for warming and
drying conditions, but deeper troughs would bring interruptions to
this trend in the form of brief cool downs and increased shower and
thunderstorms chances. - RCG

 - Forecast Confidence & Scenarios:

Strong to severe thunderstorm concerns this afternoon through
Tuesday...

Damaging winds and frequent cloud to ground lightning will be the
primary hazards through this evening with the strongest
thunderstorms; however, instances of large hail and heavy
downpours will also be of a concern, especially across Central
and North Central Montana east of the I-15 corridor. Storm motions
of mostly 20 kts should limit the residence time of showers and
storms over a given area, but rainfall rates approaching 1" per
hour are plausible given PWATS of 0.75" to nearly 1" over the
plains of Central and North Central Montana. Greatest flooding
concerns will be over recent burn scars like Horse Gulch, along
with susceptible urban areas like Helena, Great Falls, and Havre
to name a few.

Shower and storm activity will diminish and end by midnight for
most areas. The one exception to this will be along the Hi-line
where peak convection will begin later in the evening and persist
through Tuesday morning. Additionally lingering moisture and
instability will be supportive for another round of activity
Tuesday afternoon, particularly for Hill and Blaine counties. -
Moldan/RCG

Increased shower and thunderstorm activity, breezy to windy
conditions, and cooler temperatures Thursday through Saturday...

A Pacific trough will move through the Northern Rockies Thursday
through early Saturday. Southwesterly flow aloft develops ahead of
the approaching system late Wednesday into Thursday and brings
breezy to windy conditions for most locations. The strongest winds
look to occur along the Rocky Mountain Front and the higher
terrain and north to south oriented valleys valleys of Southwest
Montana Thursday and Thursday night. The latest NBM probabilities
do not exceed 50% for 45 mph gusts or higher, which compliments
H700 winds generally in the 30 to 35 kt range. Shower and
thunderstorm activity looks to be most widespread Thursday and
Friday while afternoon temperatures run as low as 10 to 20 degrees
below average. - RCG

&&

.AVIATION...
05/00Z TAF Period

VFR conditions will predominately prevail throughout the 0500/0600
TAF period; however, temporary periods of MVFR conditions are likely
beneath showers and thunderstorms given higher precipitable water
values leading to heavier bursts of rain and reduced VIS, most
notably at the KGTF, KLWT, and KHVR terminals. Additionally,
thunderstorms through 06z will be capable of producing strong and
erratic wind gusts of 30-50kts and large hail at any given terminal,
with the highest confidence in at least 40kt gusts at the KGTF,
KLWT, and KHVR terminals. Mountain obscuration, most notably for
peaks above 9kft, is likely through much of the next 24 hours. -
Moldan

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  79  51  85 /  80  30   0   0
CTB  54  74  51  78 /  60  50   0   0
HLN  54  81  52  85 /  80  20   0   0
BZN  49  83  47  88 /  40  20   0   0
WYS  36  75  35  81 /  30   0   0   0
DLN  46  78  43  84 /  40   0   0   0
HVR  58  79  55  86 /  90  50  30   0
LWT  51  75  49  81 /  60  30   0   0

&&

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

http://www.weather.gov/greatfalls

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