Severe Thunderstorm Warning 149, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
816 PM MDT Tue Aug 19 2025

.KEY MESSAGES...

 - Showers and thunderstorms, some strong to severe, tonight over
   the plains of North-central Montana.

 - Still warm Wednesday, with widely scattered afternoon and evening
   showers and thunderstorms.

 - Cooler Thursday and Friday.

&&

.UPDATE...

Made some slight adjustments to the precipitation chances for this
evening based on trending observations and the latest model
guidance. Otherwise the going forecast remains on track.  -thor

&&

.DISCUSSION...
/Issued 505 PM MDT Tue Aug 19 2025/

 - Meteorological Overview:

A building upper ridge over the interior west/Central CONUS is
shunting upper level troughing off the BC coast slightly northward
this afternoon. As a result of the building ridge and increasingly
anti-cyclonic southwesterly flow aloft early this afternoon skies
have been largely clear. This, along with increasing H5 heights has
allowed for temperatures warm quite a bit warmer than previous days,
with most lower elevation locations getting into at least the lower
90s, if not hotter.

The remainder of the afternoon is looking dry across the entire
region, though by the early evening a wave traversing the
southwesterly flow aloft will result in convective initiation across
western Montana. These showers and storms cross the Continental
Divide later in the evening and make their way across the plains of
North-central Montana through the remainder of the evening and
overnight. Although surface inversions will be developing tonight, a
deep elevated mixed layer will still promote downdrafts capable of
strong wind gusts at the surface with the strongest thunderstorms.
This activity looks to diminish or exit eastward by daybreak
Wednesday.

Heading into Wednesday the aforementioned upper trough that was
shunted northward off the BC coast begins moving eastward. Heights
begin falling across the region and temperatures respond accordingly
by cooling ever so slightly compared to Tuesday.  Although surface
cooling will ensue, cooler temperatures aloft will still allow for
another round of showers and thunderstorms in the afternoon and
evening. There appears to be enough shear in place for a few more
organized storms, which look to be most favored across the plains
east of I-15. Gusty winds and hail are both a concern with the
strongest thunderstorms that form.

As the troughing slides eastward and eventually southeastward across
southern Canada, a cold front will be propelled southward toward the
region late Wednesday night and Thursday morning. A few showers and
thunderstorms may accompany this front, but the main story will be
for cooler temperatures moving in for the remainder of the week,
coolest near the Canadian border.

As the troughing slides southeastward from Canada into the Great
Lakes region, an upstream ridge begins to build in across the
western CONUS. There is some uncertainty as to the east/west
positioning of this ridge, with confidence much higher that the apex
of the ridge will be well north in Canada. The east/west
positioning of this ridge is critical, as a further east ridge
would allow for monsoonal moisture to make better inroads toward
Southwest Montana this weekend and early next week. This scenario
would result in at least isolated showers and thunderstorms most
afternoons and evenings this weekend into early next week across
Southwest Montana. Should the ridge axis set up further west,
sufficient monsoonal moisture would not make it to Southwest
Montana, thus resulting in much lower chances for precipitation
there this weekend into early next week. Ultimately this positioning
will be determined by the downstream troughing across the east,
which will take some time to resolve as Hurricane Erin slides
northeastward off the east coast later this week. -AM

 - Forecast Confidence & Scenarios:

The greatest uncertainty in the near term is the timing of
convective initiation over western Montana late this afternoon and
evening. The more aggressive guidance develops convection and moves
it across the Continental Divide around 7 PM or so, while the slower
guidance doesnt move much across the Continental divide until closer
to midnight. Ultimately this should not impact chances for severe
weather too much, as the elevated mixed layer will be present either
way, with only a shallow inversion in place below it.

For Wednesday there is a consistent signal for a favorable kinematic
environment for supercells over the plains by the afternoon. Long,
straight hodographs would favor hail, with wind also being a concern
with the strongest thunderstorms. Consistency decreases with
thermodynamics, convective initiation, and overall coverage. The
least aggressive guidance does not develop much over the plains,
while the most aggressive guidance develops at least a few clusters
capable of being on the stronger side. Details will become more
clear on which of these scenarios play out after convection tonight
moves through.

As for the longer term, uncertainty regarding a ridge this weekend
was discussed above. A further east ridge would be more favorable
for thunderstorms across Southwest Montana this weekend into early
next week, while a further west ridge would lean drier. Either way,
the plains of North-central Montana and vicinity look dry over this
timeframe. -AM

&&

.AVIATION...
20/00 TAF Period

VFR conditions will predominately prevail throughout the 2000/2100
TAF period; however, showers and thunderstorms developing along
the Continental Divide and over the terrain of Western Montana
will lift northeast and over portions of Central and North Central
Montana plains and valleys and provide the opportunity for low-VFR
conditions, generally along and northwest of a KHLN, to KGTF, to
KHVR line. Shower and thunderstorm development will be during the
early evening hours tonight, with this activity then persisting
through the remainder of the evening hours and into the early
morning hours on Wednesday. Main concern with any thunderstorm
will be gusty and erratic wind gusts of between 30-40kts.
Additional concerns throughout the TAF period will be reductions
in slantwise visibility upon descent/ascent, most notably at the
KEKS, KBZN, and KWYS terminals due to local wildfires burning in
Southwest Montana. Mountain obscuration through the next 24 hours
will be limited to beneath/near precipitation. - Moldan

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

&&

.FIRE WEATHER...

Fire weather concerns will continue through Tuesday and Wednesday
with highs in the 90s expected across the plains and valleys across
the region. Minimum RH values will range from the single digits to
the mid teens with Tuesday expected to be the warmest and driest day
of the week. Winds are less of a concern these days, however, an
isolated afternoon dry thunderstorm cannot be ruled out which will
keep fire weather concerns elevated until a cold front passes
through late Wednesday into Thursday. -thor

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  60  92  54  81 /  30  20  20   0
CTB  58  86  48  77 /  50  20   0   0
HLN  60  92  56  85 /  20  20  20   0
BZN  54  93  51  86 /   0  20  20  10
WYS  47  85  41  81 /   0  20  20  10
DLN  52  90  48  84 /   0  20  10   0
HVR  62  93  53  79 /  40  20  20   0
LWT  58  88  53  76 /   0  10  40  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
&&

http://www.weather.gov/greatfalls

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