Severe Thunderstorm Warning 103, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
218 PM MDT Mon Jul 28 2025

.KEY MESSAGES...

 - Increased risk for strong to severe thunderstorms this afternoon
   and evening across Central Montana and vicinity.

 - Daily opportunities for afternoon and evening showers and thunderstorms
   will be around through the remainder of the week and weekend.

 - Slower moving thunderstorms later in the week will be capable
   of producing heavy rain, increasing the risk for localized
   flash flooding.

&&

.DISCUSSION...

 - Meteorological Overview:

A persistent southwesterly flow aloft will continue across the
Northern Rockies through the end of the week and into the weekend.
Meanwhile at the surface, easterly winds will continue to usher in
increasing moisture, especially over the plains. The threat this
week will slowly transition from severe weather to excessive
rainfall due to slow moving and potent precipitation producing
thunderstorms.

As for today: Subtle H5 speed max will continue its approach to the
region. 18Z RAOB at TFX demonstrates around 30 kts effective shear,
which should increase further as the subtle speed max arrives later
this afternoon. This increased shear, combined with observed
instability in excess of 1,000 m2/s2 should foster an increased risk
of severe thunderstorms, including supercells through the remainder
of the day across portions of Central MT and vicinity. The initial
concern is for large hail given the straight hodographs and lower
observed DCAPE, but a threat for wind should develop in far eastern
areas (Fergus county and vicinity) as storms begin to interact and
congeal later this afternoon and early evening. There appears at
least a low-end threat for another round of showers and
thunderstorms late tonight across portions of the Hi-Line, though
this is low-confidence at this time.

Looking ahead to Tuesday: Better moisture begins to work into
Southwest Montana. As such, better instability in these areas
develops tomorrow afternoon. Shear does look to decrease Tuesday,
but looks to stay just high enough to support a risk of strong to
severe wind gusts from decaying thunderstorms. Given the forecast
instability in excess of 1,500 m2/s2, brief instances of large hail
cannot be rule out either, even if shear is lower. Overall storm
motions appear to slow down Tuesday, which raises concerns for
excessive rainfall. Given there is only a marginal risk for
excessive rainfall across much of the forecast area, we are electing
to hold off from a Flood Watch at this time.

Wednesday onward: Although southwesterly flow aloft continues later
in the week, the magnitude weakens considerably. Given ample
moisture that is forecast to remain in place, and sufficient
instability, the slower storm motions that develop as a result of
the weakened flow will raise concerns for excessive rainfall
resulting in flash flooding. Areas most prone to flash flooding
include burn scars and urban areas. Given there is lower confidence
in the specifics such as when and where, we will hold off from any
flood watches at this time.

Overall, trends will be monitored for the need of a flood watch
across portions of the region through the remainder of the week. -AM

 - Forecast Confidence & Scenarios:

The greatest uncertainty this forecast period is related to the risk
for flash flooding Wednesday onward across the region. Confidence in
persistent thunderstorms is largely related to mesoscale features
(Such as stalled outflow boundaries, mountain anchoring, etc), which
are likely not going to be resolved until the day or of night prior.
Hence, no flood watches are being issued at this time, but will
continue to be considered. -AM

&&

.AVIATION...
28/18Z TAF Period

Low clouds are expected to persist along the Rocky Mountain Front
including KCTB through the next couple hours before lifting ahead
of the expected thunderstorm development this afternoon/evening.

A period of VFR conditions is expected before afternoon
thunderstorms start to develop and track northeast across the
region through this evening. A few storms later this afternoon
could produce damaging winds, large hail and localized heavy
rainfall with brief local local terrain obstruction and reduced
visibility.  Hoenisch/thor

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  82  59  82 /  50  20  30  30
CTB  52  76  54  78 /  30  20  30  20
HLN  56  82  58  82 /  60  40  40  40
BZN  51  84  54  82 /  40  50  40  40
WYS  40  81  43  80 /  10  30  30  50
DLN  47  84  49  81 /  20  50  30  40
HVR  55  82  56  84 /  40  30  10  10
LWT  53  76  53  76 /  60  50  50  50

&&

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

http://www.weather.gov/greatfalls

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