Severe Thunderstorm Warning 129, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
237 PM MDT Fri Aug 1 2025

.KEY MESSAGES...

 - Daily chances for showers and thunderstorms will continue
   through Tuesday.

 - There will be heavy rainfall at times this afternoon through
   Saturday across North-central and Central Montana.

 - Temperatures will remain slightly below seasonal averages
   through early next week.

&&

.DISCUSSION...

 - Meteorological Overview:

This afternoon the upper-level ridge remains over North-central,
Central, and Southwestern Montana. This afternoon there will be
southwest flow aloft with an upper-level shortwave trough that will
move through the upper-level flow. This will bring multiple rounds
of showers and thunderstorms to the area this afternoon and evening
with the strongest thunderstorms in the afternoon and evening. This
afternoon through this evening there will be anomalously high
precipitable water (PWAT) in excess of 1 inch over North-central and
Central Montana. As result of this combined with slow storm motions
some of the thunderstorms will produce heavy rainfall this afternoon
and this evening which will lead to flash flooding. The greatest
risk for flash flooding is in urban areas and burn scars. On
Saturday the upper-level ridge remains over the area but most of
the upper-level moisture will move out of the area. However there
will still be rounds of showers and thunderstorms through most of
the day on Saturday. Over the Hi-Line there will remain
anomalously high PWAT in excess of one inch on Saturday. This will
lead to some thunderstorms producing heavy rain along the Hi-Line
on Saturday. The greatest risk for showers and thunderstorms on
Saturday is over the Hi-Line.

On Sunday the upper-level ridge remains over North-central, Central,
and Southwestern Montana. An upper-level shortwave will move through
the upper-level flow which will bring chances for showers and
thunderstorms to the area on Sunday. On Monday and Tuesday there
will be an upper-level shortwave trough that will move through
the upper-level flow. This will bring showers and thunderstorms
to North-central Montana, Central, and Southwestern Montana. On
Wednesday there will be upper-level zonal flow over North-central,
Central, and Southwestern Montana which will dry out the area. On
Thursday an upper-level ridge builds back in over North-central,
Central, and Southwestern Montana before an upper-level trough
begins to move towards Montana. This will bring showers and
thunderstorms to the area on Thursday.

 - Forecast Confidence & Scenarios:

The Storm Prediction Center has issued a Marginal Risk (5% chance
for wind gusts in excess of 58 mph and/or hail with a diameter of an
inch or greater within 25 miles of a point) for locations east of a
Inverness to Great Falls to Belgrade line. This afternoon and
evening there is a 45 - 70% chance for thunderstorms across
Central and North-central Montana. This afternoon and evening
there is a 30 - 50% chance for thunderstorms across Southwestern
Montana. On Saturday there is a 40 - 65% chance for thunderstorms
across North-central, Central, and Southwestern Montana. On
Sunday and Monday there is a 25 - 50% chance for thunderstorms
across North-central, Central, and Southwestern Montana. On
Tuesday there is a 15 - 40% chance for thunderstorms across North-
central, Central, and Southwestern Montana.

On Friday there is a 20 - 30% chance for a quarter inch of rain or
greater across North-central Montana. Due to this, combined with
saturated soils and the convective nature of the rainfall a flash
flood watch remains in effect for much of North-central and Central
Montana from now until 2 AM. On Saturday there is a 40 - 60% chance
for a quarter inch of rain or greater and a 20 - 40% chance for a
half inch of rain or greater across Northern Blaine County. On
Saturday across the rest of North-central Montana there is a 20 -
40% chance for a quarter inch of rain or greater. See the Hydrology
section for more information.

At the end of this forecast period there is high uncertainty in
what the upper-level pattern will be. There are three possible
scenarios. There could be an upper-level ridge over North-central,
Central, and Southwestern Montana, which would bring a round of
showers and thunderstorms to the area. There could be an upper-level
trough over North-central, Central, and Southwestern Montana which
would bring cool temperatures and rain showers to the area. The
third scenario is that the upper-level flow is zonal. This would
bring warm temperatures with mostly dry weather. -IG

&&

.AVIATION...
01/18Z TAF Period

Rain, with embedded thunderstorms, will continue to pivot around a
MCV over Northern Hill and Blaine Counties through 18-22z this
afternoon, with generally broken VFR conditions expected at the
KHVR terminal. Additional showers and thunderstorms are then
expected to develop, initially over the higher terrain of the
Continental Divide and Southwest Montana between 18-20z this
afternoon, with these showers and thunderstorms then lifting to
the northeast at 15-25kts. While VFR conditions will largely
prevail throughout the 0181/0218 TAF period heavy rain beneath
shower and thunderstorm activity, most notably north of a KHLN to
KLWT line, will periodically reduce VIS and/or CIGS to MVFR/IFR
conditions. Widespread mountain obscuration is not expected;
however, localized obscuration is likely near/beneath
precipitation. - Moldan

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

&&

.HYDROLOGY...

...Localized flash flooding potential over portions of Central
and North- central Montana this afternoon and tonight...

Another shortwave moves in from the southwest bringing locally
heavy showers and thunderstorms to North-central and Central Montana
this afternoon and tonight. Since these storms will be scattered
there won't be widespread flash flooding, however areas that are
under these storms may observe rainfall rates up to and over an
inch per hour. This will especially be impactful to the flood
prone areas, including urban centers, burn scars, and low lying
areas. A flood watch remains in effect for much of North-central
and Central Montana north of the US Highway 12 corridor,
excluding the Northern Hi-Plains and Hi-Line locations going
eastward to the Sweet Grass Hills. High moisture and slow storm
motion continue on Saturday, but lower instability may reduce the
coverage of stronger storms. -RCG/IG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  58  80  57  83 /  80  50  20  10
CTB  57  74  56  76 /  70  60  30  20
HLN  58  80  56  86 /  50  40  20  10
BZN  52  83  51  85 /  30  30  20  20
WYS  39  78  39  76 /  10  40  30  30
DLN  48  78  47  80 /  20  30  20  20
HVR  59  83  59  81 /  70  60  50  50
LWT  53  77  53  78 /  80  50  30  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for Bears Paw Mountains and
Southern Blaine-Big Belt, Bridger and Castle Mountains-Cascade
County below 5000ft-East Glacier Park Region-Eastern Pondera and
Eastern Teton-Fergus County below 4500ft-Gates of the Mountains-
Helena Valley-Hill County-Judith Basin County and Judith Gap-
Little Belt and Highwood Mountains-Northern Blaine County-Snowy
and Judith Mountains-Southern High Plains-Southern Rocky
Mountain Front-Upper Blackfoot and MacDonald Pass-Western and
Central Chouteau County.

&&

http://www.weather.gov/greatfalls

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