420
FXUS65 KTFX 172352
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
552 PM MDT Mon Aug 17 2026

Aviation Section Updated.

.KEY MESSAGES...

- Scattered shower and thunderstorm activity continues
  through Tuesday with a few storms producing localized strong,
  gusty winds, brief heavy downpours, and hail in addition to
  cloud to ground lightning.

- Warmer, drier, and seasonably breezy conditions move in for the
  second half of the workweek.

- An approaching Pacific trough will bring increased shower
  and thunderstorm activity, breezy to windy conditions, and
  elevated fire weather concerns this weekend.

&&

.DISCUSSION...
/Issued 1258 PM MDT Mon Aug 17 2026/

 - Meteorological Overview:

A shortwave passage will maintain scattered shower and
thunderstorm activity for the next couple of days. Similar to
yesterday, ML CAPE levels will be running in the 500 to 1,000 J/kg
range with effective shear generally between 25 and 35 kts.
Minimum RHs will continue to dip into the 20s, so brief wind
strong wind gusts and cloud to ground lightning will be the
primary hazards; however, there will be hail and brief heavy
downpours with the stronger cores given the previously mentioned
modestly elevated shear and PWATs around 0.75 inches.

Ridging aloft strengthens over the Great Basin with heights
rising as high as the 588 to 594 dm range over the Northern
Rockies by Friday. This will reduce shower and storm coverage while
diurnal mixing maintains seasonably breezy conditions. Despite the
strength of this ridging, weak surface fronts look to keep
temperatures mostly in the 80s over central and north-central MT.
These features will have a harder time surpassing the central/SW
mountains, so upper 80s and even lower 90s are a safer bet down
there.

Subtle shortwaves will begin to move into western/southwestern MT
as early as Friday evening in advance of a trough moving onto the
Pacific NW coast. This wave will interact with diurnal
instability and initiate isolated shower and thunderstorm
activity, primarily over and near the higher terrain of W/SW MT
with localized gusty winds and lightning being the most logical
hazards.

The Pacific NW trough then approaches the Northern Rockies during
the weekend and brings increased shower and thunderstorm activity
and breezy to windy conditions. The main concern for this weekend
looks to be dry and continued very warm conditions and increasing
fire weather concerns. Southwesterly flow aloft looks to develop
in the wake of the exiting trough early next week with passing
shortwaves maintaining daily scattered shower and thunderstorm
activity. - RCG

 - Forecast Confidence & Scenarios:

Mostly afternoon and evening showers and thunderstorms through
Tuesday night...

Scattered showers and thunderstorms will develop in a similar
fashion as yesterday given the continued moist and modestly sheared
environment. The one exception to this is the lack of a more defined
forward moving surface front over the plains acting as a lifting
mechanism. For this reason, storm coverage may be a bit more
widespread over the southwest, but the more isolated central/north-
central MT storms are being highlighted with the better quality
storm maintenance and upscale growth, especially as they cross the
highway 87 corridor and the Little Belt Mountains. Should one of
these storms gain momentum, then localized severe wind gusts and
lightning will be the primary hazards followed by brief downpours
and hail. Other areas will see similar hazards to monitor, although
with less intensity in most cases given the mostly singular or weak
multiple cellular storm structure. Peak timing for today`s showers
and storms will generally fall between 2 and 9 pm with coverage
diminishing heading into the overnight hours. Tuesday looks to see a
similar set up with scattered storms, mostly of the pulse variety
with few isolated stronger storms capable of localized strong, wind
gusts, brief downpours, and hail in addition to cloud to ground
lightning.

Elevated fire weather concerns this weekend...

A Pacific trough looks to bring breezy to windy conditions to the
Northern Rockies this weekend despite increased shower and
thunderstorm activity. Much of the forecast area has over a 50%
chance for wind gusts exceeding 30 mph while minimum RHs fall to or
near critically low levels. This is also reflected in the Hot Dry &
Windy Index (HDWI) with probabilities for exceeding the 75th
percentile on Saturday running above 50% for most locations. Recent
rainfall has tamed the previously maxed out Energy Release
Component (ERC) values and they are tentatively forecasted to
remain shy of critical levels. If the mid-week warm up exceeds
expectations, then elevated to critical fire weather conditions may
develop for some areas. This situation will continue to be monitored
for the need of fire weather products. - RCG

&&

.AVIATION...
18/00Z TAF Period

Scattered showers and thunderstorms will continue moving through
the region until 18/04Z. Most terminals will remain VFR through
the period however brief MVFR/IFR conditions and mountain
obscuration could be observed near any showers or storms. Hazards
from these storms include localized strong, gusty winds, brief
downpours, and hail within the stronger cores. A 10% chance for
low ceilings and fog could impact the KWYS area from 18/09Z to
13Z.

For KGTF: Winds will be light and variable tonight with some smoke
moving over the area as it is carried by northwesterly winds
aloft. Tomorrow will see a 10% chance for more thunderstorm
activity in the terminal area which could briefly impact crosswind
thresholds due to outflows. - Akins

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  83  53  86 /  20  20  10   0
CTB  48  77  51  82 /  50  20  10   0
HLN  54  83  51  85 /  20  20  10   0
BZN  51  80  49  84 /  30  30  30   0
WYS  39  75  36  79 /  60  30  30   0
DLN  49  80  47  83 /  50  20  20   0
HVR  51  84  54  86 /  30  20  20   0
LWT  50  79  51  82 /  50  30  20   0

&&

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

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