620
FXUS65 KTFX 131717
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1117 AM MDT Sun Jul 13 2025

Aviation Section Updated.

.KEY MESSAGES...

 - Dry and warm today.

 - Transition day Monday, with a cold front bringing gusty winds and
   an opportunity for thunderstorms from north to south through the
   afternoon and evening.

 - Much cooler and wetter Monday night into early Wednesday.

 - Trending warmer and drier late week.

&&

.UPDATE...
/Issued 937 AM MDT Sun Jul 13 2025/
Updated forecast is out. Just trended forecast towards current
trends. Main concern today will be the very warm temps this
afternoon and then the chance for an isolated thunderstorm over
the very far northern/southern portions of our CWA. Summer storm
still on track to arrive Monday and then continue through Tue.
Brusda

&&

.DISCUSSION...
/Issued 937 AM MDT Sun Jul 13 2025/

 - Meteorological Overview:

An initially anti-cyclonic northwesterly flow aloft will be in place
today. The result will be for another warm day, though by the
evening low-end chances for showers and thunderstorms will develop
as the flow aloft begins its transition to be more cyclonic in
nature. Areas with the best chance to see a shower or thunderstorm
late this afternoon into the overnight is along the Hi-Line, as
well as south of the I-90 corridor in SW MT.

A rather sharp trough diving southeastward within the northwesterly
flow aloft Monday will begin the transition to a cooler and wetter
pattern across the region. A cold front will dive southward from
Canada Monday afternoon and evening, bringing a gusty northerly wind
shift to the region, especially as the front pushes through narrow
SW MT valleys. Ahead of the front temperatures will remain quite
warm through the day, especially in Southwest Montana. There appears
a conditional risk for stronger thunderstorms Monday afternoon and
evening near the cold front, largely associated with the timing of
the front which remains uncertain at this time.

Although cooler air will push in behind the front, surface and
near-surface moisture will quickly rise. PWATs ahead of the front
largely look to be less than 0.75" or so, but quickly rise above
1" behind the front. This moisture, forcing associated with upper
level troughing, and any terrain enhancements will all contribute
to a period of rain across the region. Areas on and adjacent to
the plains are favored for the most rain, while areas near the
Southwest MT and ID border look to be much drier overall. Although
it is July, this system does feature cool enough air to support
mountain peak snow in and around Glacier NP. Further discussion of
snow levels is in the confidence and scenario section.

As mentioned prior, this system will usher in cooler air to the
region. Areas over the plains look coolest, with daytime highs
Tuesday  struggling to reach the lower 60s. By Wednesday
temperatures do look to begin trending warmer, but another well
below average temperature day is forecast for the plains. Further
south into the Southwest MT valleys the coolest air will struggle a
bit to make inroads, Hence, although these areas do cool for Tuesday
and Wednesday, they do not look to cool nearly as much as areas
further north on the plains.

This system departs during the day and into the evening Wednesday,
with another period of northwesterly flow in its wake. Deterministic
guidance and ensembles are not yet confident as to whether this
flow aloft will be more cyclonic or anti-cyclonic in nature, but
they do agree that temperatures will at least slowly trend back
toward normal by the end of the week, with much lower chances for
precipitation compared to early to mid-week. -AM

 - Forecast Confidence & Scenarios:

This afternoon into the overnight:

A small subset of guidance features a stronger thunderstorm across
portions of the Hi-Line tonight. If this were to play out, it looks
like the stronger thunderstorms would initially develop in
Alberta and move southeastward, crossing the border this
evening/tonight. The confidence that this solution is realized is
low at this time, however.

Monday:

There is still timing uncertainty with the front, with quicker
guidance moving it through as early as Monday morning, while less
aggressive guidance holds it off til Monday afternoon and
evening. The timing of the front will play a large role in
when/where/if a stronger thunderstorm forms.

Another potential concern will be for the cold front pressing
through Central and Southwest Montana valleys. Typically wind prone
areas from northerly wind pushes such as between Helena and Three
Forks and between Whitehall and Twin Bridges have a roughly 20%
chance for a 50 mph gust along and behind the front.

Precip amounts and snow levels Monday through early Wednesday:

Previously bi-modal ensembles (One much drier and a few on the
wetter side) appear to be converging on the wetter solution for the
region. The chance for event total precipitation over a half of an
inch is roughly 50% or so over the plains. Further, the chance for
an inch of precipitation is around 20% or so.

Snow levels with this system look to fall to their lowest Tuesday
morning across the Northern Rocky Mountain Front, including eastern
Glacier NP. NBM 10th percentile snow levels (90% chance for snow
levels higher than this) are briefly around 7,000-7,500 ft or so
Tuesday morning. NBM 50th percentile snow levels (Middle of the road
snow levels) Tuesday morning are in the 8,000-8,500 ft range. -AM

&&

.AVIATION...
13/18Z TAF Period

VFR conditions prevail through the TAF period. There will be
isolated showers and thunderstorms across the MT/Canadian border
and the ID/MT border this afternoon and evening. However,
confidence remains low for precipitation to affect terminals
(KCTB/KHVR/KWYS). There is some uncertainty in the timing of the
cold front tomorrow. Some models have the cold front arrive
earlier in the morning tomorrow, which will cause a northerly wind
shift to the northern part of the state. Some hold it off a few
hours later. -Wilson

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  94  59  90  52 /   0  10  20  70
CTB  87  55  73  47 /  10  20  50  80
HLN  94  61  92  53 /   0  10  10  60
BZN  94  56  95  52 /   0  10  10  40
WYS  85  45  84  44 /   0  10  20  30
DLN  89  53  90  50 /   0  10  10  20
HVR  93  58  83  51 /   0  30  30  70
LWT  88  57  88  48 /   0  10  20  70

&&

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

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