522
FXUS65 KTFX 212021
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
221 PM MDT Tue Jul 21 2026

.KEY MESSAGES...

 - Showers and thunderstorms are expected each afternoon through
   Saturday.

 - Isolated thunderstorms in Southwestern Montana have the
   potential to produce localized flash flooding Wednesday
   afternoon/evening.

 - Temperatures will gradually warm up through Saturday and stay
   warm through early next week.

&&

.DISCUSSION...

 - Meteorological Overview:

An upper-level ridge is over North-central, Central, and
Southwestern Montana this afternoon. This will allow temperatures
to warm up this afternoon. Monsoonal moisture is beginning to work
its way in over the area this afternoon. This will bring a few
isolated showers and thunderstorms to North-central, Central, and
Southwestern Montana this afternoon. On Wednesday the upper-level
ridge remains over North-central, Central, and Southwestern
Montana. An upper-level shortwave will move through the upper-
level flow and bring showers and thunderstorms primarily south of
I-90 Wednesday afternoon and evening. Due to an increase in
monsoonal moisture the thunderstorms will produce heavy rainfall.
There is a lack of steering flow so the thunderstorms will also be
slow moving. This has the potential to lead to some localized
flooding. See the Hydrology section for details. On Wednesday
isolated showers and thunderstorms have the potential to make it
further north but stay south of an Augusta to Great Falls to
Lewistown line.

On Thursday the upper-level ridge remains over North-central,
Central, and Southwestern Montana. Another upper-level shortwave
move through the flow. This will bring another day of hot weather
with isolated showers and thunderstorms. On Friday the upper-level
ridge remains over North-central, Central, and Southwestern Montana.
This will increase temperatures to well-above seasonal averages.
Winds will increase due to low-level and mid-level winds increasing.
This will dry out the area and bring elevated fire weather
conditions to North-central Montana. On Saturday the upper-level
ridge remains over North-central, Central, and Southwestern
Montana. It will be the hottest out of this forecast period across
the area on Saturday with well-above seasonal average temperatures.
On Sunday a weak upper-level trough moves through which will
slightly cool down temperatures and bring some windy conditions to
the area. On Monday the upper-level ridge remains over North-
central, Central, and Southwestern. This will allow warm and dry
conditions to persist. -IG

 - Forecast Confidence & Scenarios:

On Wednesday there is a 40 - 60% chance for thunderstorms across
Southwestern Montana. On Wednesday there is a 15 - 35% chance for
thunderstorms across most of North-central and Central Montana. On
Thursday there is a 20 - 40% chance for thunderstorms across North-
central, Central, and Southwestern Montana. On Friday there is a 25 -
 30% chance for thunderstorms across Southwestern Montana. On
Saturday there is a 25 - 40% chance for thunderstorms across
Southwestern Montana.

On Friday there is a 40 - 80% chance for high temperatures of 100
degrees or greater across most of North-central Montana. On Saturday
there is greater than a 60% chance for high temperatures of 100
degrees or greater across North-central Montana. On Saturday
there is a 30 - 50% chance for high temperatures of 100 degrees or
greater from the Helena Valley through the Canyon Ferry area to
Townsend to the Three Forks to Bozeman area. -IG

&&

.AVIATION...
21/18Z TAF Period

VFR conditions will prevail across all terminals during this TAF
Period outside of during any thunderstorms. At the very end of this
TAF Period there is a 30% chance for showers and thunderstorms at
the KBZN terminal. At the KWYS terminal between 21/21Z and 22/03Z
there is a 15 - 20% chance for showers and thunderstorms. Any
thunderstorm will produce heavy rainfall, small hail, frequent
lightning, and gusty, erratic winds. Across Southwestern Montana
during the beginning of this TAF period smoke from wildfires will
reduce slantwise visibility. -IG

&&

.HYDROLOGY...

On Wednesday the Weather Prediction Center (WPC) has issued a
Slight Risk (15% chance for flash flooding within 25 miles of a
point) for flash flooding across most of Southwestern Montana.
Between the I-90 corridor and a line between Augusta to Great
Falls to Lewistown WPC has issued a Marginal Risk (5% chance for
flash flooding within 25 miles of a point)for flash flooding on
Wednesday. In addition across the mountains of Southwestern
Montana there is a 50 - 70% chance for rainfall rates in excess of
one half inch per hour on Wednesday. In addition there is around
a 50% chance for rainfall rates in excess of three quarter of an
inch per hour across the mountains of Southwestern Montana on
Wednesday. There is a 20 - 40% chance for an inch of rain or
greater across the mountains of Southwestern Montana on Wednesday.
As a result a Flash Flood Watch was issued for the mountains
south of I-90 from Noon to Midnight on Wednesday. -IG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  60  90  62  95 /  10  20  20  10
CTB  55  87  58  93 /  10   0  10   0
HLN  62  90  63  92 /  10  30  40  20
BZN  58  90  58  90 /  20  60  70  30
WYS  51  82  51  81 /  30  80  70  50
DLN  57  85  55  86 /  40  60  70  20
HVR  57  93  60 100 /  10   0   0  10
LWT  55  86  57  89 /  10  20  20  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Wednesday afternoon through Wednesday evening
for Gallatin and Madison County Mountains and Centennial
Mountains-Northwest Beaverhead County-Ruby Mountains and
Southern Beaverhead Mountains.

&&

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