Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
218 PM MDT Thu Jul 31 2025

.KEY MESSAGES...

 - Below normal temperatures will continue through the weekend
   thanks to an active and unsettled pattern over the Northern
   Rockies.

 - Daily chances for showers and thunderstorms exists across most
   locations through the weekend, with the potential for locally
   heavy rainfall beneath slower moving thunderstorms through
   Saturday.

 - A few thunderstorms through the evening hours tonight could
   produce damaging wind gusts and large hail, mainly along and
   south of the Montana Hwy 200 corridor.

&&

.DISCUSSION...

 - Meteorological Overview:

Convective initiation was underway early this afternoon over the
higher terrain of Southwest Montana, Northeast Idaho, and
Northwest Wyoming thanks to differential heating and the approach
of a upper level wave embedded within predominately southerly flow
aloft. This wave, which was moving over Idaho, will lift north
and over Southwest Montana through the remainder of the afternoon
and North Central Montana through Friday morning. Areal coverage
of showers and thunderstorms will continue to expand through the
remainder of the afternoon hours along and south of the US Hwy 12
corridor, with a rather continuous line of shower and
thunderstorms then lifting north into Central and North Central
Montana, generally along and west of a Judith Gap to Shelby line,
through the evening hours tonight. Additional shower and
thunderstorm development is then likely during the late evening
hours tonight/early morning hours on Friday east of the Judith Gap
to Shelby line, with these showers and thunderstorms lingering
through sunrise on Friday thanks to support from the nocturnal low
level jet and subsequent orographic lift over the island ranges.
- Moldan

Daily chances for showers and thunderstorms will help regulate
temperatures throughout the weekend and beginning of next week to
near to slightly below normal. This limbo pattern of southwest flow
aloft looks to continue well into next week, bringing daily chances
for precipitation. Though, moisture does decrease a bit after
Saturday, decreasing the heavy rainfall threat. Models hint towards
the second half of next week, southwest flow aloft weakens to more
of a zonal flow aloft pattern. However, it is uncertain if there is
finally a lull in precipitation or if the active period continues. -
Wilson

 - Forecast Confidence & Scenarios:

Severe Thunderstorms Today :

A marginal risk for severe thunderstorms exists along and southwest
of a Harlowton, to Neihart, to Great Falls, to Dutton, to Heart
Butte line; with a 5-15% chance for damaging winds and large hail
(albeit across a smaller portion of Southwest and Central Montana).
DCAPE values ranging from 1000 to nearly 1400J/kg, especially along
and south of the I-90 corridor, will help to support isolated
instances of damaging wind gusts in excess of 58 mph, especially
beneath initially collapsing cores. Mid-level lapse rates between 7-
8 degrees C/km and CAPE ~1,500 J/kg will also help to support hail
with thunderstorms; however, a lingering concern with respect to
overall hail size remains to be the higher freezing levels around
10000ft, which would help to lead to some melting. - Moldan/Wilson

Heavy Rainfall through Saturday :

Hi-Res models continue to show the best coverage of showers and
thunderstorms lifting north from Southwest and into portions of
North Central Montana through the early evening hours tonight, with
slow overall storm motions of between 15-25 mph. These slow storm
motions combined with increase PWATS to in excess of 0.75" north
of the I-90 corridor will help to set the stage for moderate to
locally heavy rainfall, which could lead to localized flooding in
poor drainage or urban areas and over recent burn scars. Primary
concern with respect to flooding continues to be the Horse Gulch
burn scar, where scattered convection in the region and lower
overall flash flood guidance with respect to surrounding terrain
will pose a threat for flash flooding and debris flow. For this
reason a Flash Flood Watch remains in effect until midnight
tonight. - Moldan/Wilson

Friday, the bulk of thunderstorms look to be across the Continental
Divide and North-Central MT plains. Models are struggling on the
placement of the heavier precipitation amounts.
Therefore I've held off on flood products for now. However,
scattered convection and high PWATs will pose another threat for
localized heavy rainfall. If the coverage of heavy rainfall
increases, then a flood watch may be needed in the future.
- Wilson

&&

.AVIATION...
31/18Z TAF Period

VFR conditions will prevail at all terminals during this TAF Period,
outside of during any thunderstorms. The primarily concern will be
showers and thunderstorms across North-central, Central, and
Southwestern Montana this afternoon and evening. These showers
and thunderstorms will produce heavy downpours, frequent
lightning, small hail, and gusty, erratic winds. At the KCTB and
KHVR terminals there is a 10 - 25% chance for showers and
thunderstorms between 01/02Z and the end of the TAF Period. There
will be periods of mountain obscuration during thunderstorms. -IG

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  86  58  85  58 /  20  40  30  50
CTB  80  56  78  57 /  10  20  40  80
HLN  85  58  82  58 /  40  70  60  40
BZN  86  50  86  51 /  40  20  50  20
WYS  81  40  78  39 /  30  20  20  20
DLN  84  48  81  48 /  30  20  60  20
HVR  85  59  84  60 /  10  20  20  60
LWT  78  54  80  53 /  20  40  40  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flash Flood Watch until midnight MDT tonight for Big Belt,
Bridger and Castle Mountains.

&&

http://www.weather.gov/greatfalls

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