736
FXUS65 KTFX 182313
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
513 PM MDT Fri Sep 18 2026

Aviation Section Updated.

.KEY MESSAGES...

 - Thunderstorms, a few of which look to be on the stronger side, develop
   through the day today, mostly across Central and Southwest
   Montana.

 - Areas of precipitation continue into Saturday, resulting in
   increased concerns for localized flooding.

 - A drier and warmer stretch looks to ensue late weekend through
   much of next week.

&&

.DISCUSSION...

 - Meteorological Overview:

A somewhat elongated upper level low across the Pacific NW and
vicinity will slowly shift eastward through the day today. Although
forcing from this approaching upper system will result in showers
and thunderstorms developing rather early in the convective cycle,
cool temperatures aloft will still yield sufficiently steep lapse
rates for a few stronger thunderstorms. Areas where shear ends up
being maximized will have a threat for marginally large hail.
Otherwise the main threat will be for slow moving, heavy rain
producing thunderstorms through the day today.

Difluent upper level flow on the northeast side of the upper level
system look to combine with lingering instability and forcing from
the approaching trough to result in an extended stretch of rain,
mostly near and south of a line from Helena to Lewistown southward
to a line from Dillon to Big Sky tonight through the day Saturday.
This prolonged stretch of rain will result in concern for excessive
rainfall, particularly over burn scars, but for urban areas as well.

Precipitation slowly exits eastward late Saturday night and Sunday
morning as a messy/weak upper level ridge builds in Sunday into
Monday. Much of next week looks dry, though weak disturbances cannot
be ruled out, which would be capable of producing spotty light
precipitation at times. -AM

 - Forecast Confidence & Scenarios:

Rain through Saturday:

Confidence is increasing in a concentrated area of moderate
rainfall, mainly along and south of a Helena to Lewistown line
southward through a Dillon to Big Sky line. Over 48 hours ending
Sunday morning the corridor along I-90 east of Butte through Bozeman
has around a 90% probability to receive 1 inch of rain. These
probabilities slowly decrease as you go further north and south of
this corridor.

This rainfall looks to be partially convective driven and partially
stratiform in nature. The risk for excessive rainfall leading to
flash flooding will be highest while convective processes are
dominant, which will be primarily in the afternoon and evening. That
said, burn scars will be susceptible to flash flooding at any time
given the significantly lower precipitation amount thresholds. -AM

&&

.AVIATION...
19/00Z TAF Period

VFR conditons will prevail at the KWYS, KGTF, KCTB, and KHVR
terminals during the duration of this TAF Period outside of during
any thunderstorms. During the duration of this TAF Period showers
and thunderstorms will affect the KBZN, KEKS, and KHLN terminals.
Showers and thunderstorms will affect the KLWT terminal from 19/06Z
through the end of the TAF Period. During any thunderstorms there
will be a brief period of mountain obscuration, heavy rainfall,
small hail, frequent lightning, and gusty erratic winds. At the
KBZN, KLWT, KHLN, and KEKS terminals there will be at least a period
of MVFR-level ceilings during this TAF Period. At the KHLN terminal
between 19/12Z and 19/16Z rain showers will reduce visibility to at
least MVFR-levels with around a 20% chance for visibility to drop to
IFR-levels. At the KGTF terminal during the majority of this TAF
period there is at least a 20% chance for rain showers. -IG



&&

.HYDROLOGY...

A Flash Flood Watch continues for parts of central and southwest
Montana through Saturday, targeting areas near recent (Summer 2026)
and vulnerable older (2024 Horse Gulch) burn scars. As noted
previously, the peak threat for heavy rainfall rates exceeding
0.50"/hr and associated potential runoff and debris flow is expected
early this evening. Beyond burn scars, a wider area of moderate to
locally heavy rain developing tonight and persisting into Saturday
could trigger minor urban and rural drainage concerns. High-
resolution ensemble models place the highest risk along and north of
the I-90 corridor up to US-12, showing a 6080% chance of 24-hour
rainfall totals reaching or exceeding 2" across parts of Jefferson,
Broadwater, and Meagher counties. Hoenisch

Additionally, the Helena Valley, Canyon Ferry Area, Meagher County
Valleys, and Gallatin Valley were added to a Flood Watch from this
evening through Saturday afternoon, with the primary concern being
the flooding of low-lying and flood-prone locations due to excessive
runoff from heavy rain falling across urban areas. Hourly rainfall
rates of between 1/4" and 1/2", especially from the mid-evening
hours tonight through early morning hours on Saturday, are possible
across these areas which could lead to rapid runoff due to surfaces
being hydrophobic which would then lead to flooding of low-lying
areas. - Moldan


&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  66  48  73 /  40  30  10   0
CTB  47  67  45  73 /   0   0   0   0
HLN  52  61  48  73 / 100 100  30   0
BZN  48  59  45  67 /  90  90  80  20
WYS  39  54  36  62 /  80  80  50  20
DLN  47  58  44  67 / 100  90  40   0
HVR  49  70  48  74 /   0   0   0   0
LWT  50  58  48  65 /  70  70  80  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flash Flood Watch from 6 PM MDT this evening through Saturday
evening for Big Belt, Bridger and Castle Mountains-Elkhorn and
Boulder Mountains-Little Belt and Highwood Mountains.

Flood Watch from 6 PM MDT this evening through Saturday
afternoon for Canyon Ferry Area-Gallatin Valley-Helena Valley-
Meagher County Valleys.

Flash Flood Watch from 3 PM MDT this afternoon through Saturday
afternoon for Northwest Beaverhead County.

&&

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