484
FXUS65 KTFX 300245
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
845 PM MDT Tue Jul 29 2025

.KEY MESSAGES...

- Showers and thunderstorms will continue through the rest of the
  evening with strong to severe storms possible in Central and
  Southwest Montana.

- Daily chances for showers and thunderstorms will continue
  through the rest of the week with the potential for localized
  heavy rain through at least Saturday.

&&

.UPDATE...

Heavy rain showers and thunderstorms along the I-90 corridor
appear to be weakening a bit as they move north. Watch for some
localized heavy rain, mainly south of a line from Helena to White
Sulphur Springs and north of I-90 through midnight, when these
showers and storms should dissipate. Ludwig

&&

.DISCUSSION...
/Issued 607 PM MDT Tue Jul 29 2025/

 - Meteorological Overview:

Southwest flow aloft continues to bring an active pattern through
the forecast period. For this afternoon and evening, an upper
level shortwave moving from Idaho through Southwest MT will bring
scattered showers and thunderstorms. Sufficient CAPE of up to
1,500 j/kg and deep layer shear of 35kts will support the threat
for another round of strong to severe thunderstorms. DCAPE of
~1,000 j/kg or greater, deep inverted V profiles, and decent mid
level lapse rates of ~8 degrees C/km support a severe wind and
hail threat. The best chances for thunderstorms will be in
Southwest MT to Central MT, where that shortwave travels through.
Coverage in North-Central MT remains more isolated, as there is
uncertainty in the northern extent of where the shortwave travels
before dissolving. PWATs will be high again today, nearing an inch
across North- Central Montana. This will cause concern for
localized areas of heavy rainfall with storms. Thunderstorm
coverage being more isolated will keep the heavy rainfall more
localized with storms. In Southwest MT, PWATs aren`t as high
(~0.75"), so heavy rainfall will remain more localized down there
as well.

Heading into Wednesday, steering flow aloft decreases which will
slow down storm propagation. A skinnier CAPE profile and high PWATs
again will help aid in the threat for heavier rainfall again. The
primary concern will be for burn scars and urban areas. Since this
threat is mainly driven by convection, it`ll be hard to pinpoint
exactly where the heavy rainfall will set up. As a result, flood
watches aren`t anticipated as of right now. This moisture will
stick around through the end of the week. Thursday looks to be the
day with the highest PWATs (reaching 1"-1.25" across North-
Central MT). Ensembles hint at southwest flow aloft will keep low
end chances for precipitation during the beginning of next week
as well.  -Wilson


 - Forecast Confidence & Scenarios:

Thunderstorms are already underway across the region and are
expected to continue through the rest of the evening. The main
threat will be wind and hail as well as flooding from any slowly
moving or training thunderstorms. View the HYDROLOGY section for
a detailed view of the upcoming hydrologic concerns across the
region.

There will be another chance for isolated to scattered
thunderstorms across the entire region tomorrow, although
currently the threat for severe weather is lower tomorrow than it
is today.  -thor

&&

.AVIATION...
30/00Z TAF Period

VFR conditions will prevail at all terminals during this TAF Period
outside of during any thunderstorms. The primary concern will be
showers and thunderstorms in Southwestern Montana. These showers
and thunderstorms will produce heavy downpours, frequent
lightning, small hail, and gusty, erratic winds. Isolated
thunderstorms in Southwestern Montana will produce large hail and
wind gusts in excess of 40 kts. At the KCTB terminal there is a
15 - 25% chance for showers and thunderstorms between 29/22Z and
30/06Z. At the KHVR terminal there is a 25% chance for showers and
thunderstorms between 29/21Z and 30/00Z. There will be periods of
mountain obscuration during thunderstorms. More showers and
thunderstorms are possible tomorrow afternoon. Higher chances for
development are across Southwest Montana once again. There is a
30-50% chance of gusts over 30kts tomorrow, with some indications
of gusts in excess of 40 kts.  -Keyes

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

&&

.HYDROLOGY...

Over the past month, our region has been the tale of opposites.
Large portions of southwestern Montana have been running below 50%
of normal with some locations running below 25% of normal for July.
On the other hand, locations across central and north- central
Montana are currently running between 100-200% of normal for the
month of July with portions of Glacier, Hill, and Fergus counties
running 200-300%+ above normal.

Additional rainfall, heavy at times, is expected to impact much of
the region through the rest of the week with a marginal risk of
excessive rainfall Thursday through Saturday. Excessive rainfall
due to slow-moving and/or training showers and thunderstorms will
be the main item to watch over the next few days as the worst of
the impacts are expected to be more localized as opposed to a more
widespread/regional flood event. Regarding the severity of impacts,
the obvious main concern will be locations across central and
north-central Montana where reports of flash flooding have
already been noted in the previous days. Urban areas and anywhere
with poor drainage may see significant impacts at times depending
on how this event materializes.

While current QPF amounts through Saturday are not overly
impressive by themselves, PWAT values in the model soundings
suggest the potential for much higher amounts. Even if the lower
amounts were to materialize this has become a situation where any
amount of rain, especially if it falls within a short period, will
have the potential to cause flash flooding.

Significant impacts are not expected along large rivers. However,
minor flash flooding is possible along creeks and small rivers
immediately following heavy rain. Remember, if you encounter
flooding along the road turn around, don`t drown! Flooding can be
especially difficult to spot at night.

Headline-wise, a hydrologic outlook has been issued for the time
being across central and north-central Montana. At this time,
there is not enough confidence to warrant a flash flood watch,
however, if guidance continues to trend in the direction of
heavier rainfall across saturated zones, then additional hydro
products will need to be considered in the future.   -thor

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  83  58  85 /  10  20  10  10
CTB  54  78  55  79 /  20  10  10  10
HLN  57  84  58  87 /  30  30  10  40
BZN  53  83  53  88 /  50  30  20  40
WYS  42  81  41  81 /  30  50  20  60
DLN  49  81  49  84 /  50  40  20  40
HVR  55  84  57  85 /  10  10   0  10
LWT  52  74  54  79 /  40  40  30  20

&&

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

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