929
FXUS65 KTFX 171828
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1228 PM MDT Tue Sep 17 2024

...Updated Aviation Section...


.SYNOPSIS...

A fall storm system will spread an increasing coverage of showers
and thunderstorms into the region this afternoon and evening with
some strong to severe thunderstorms possible across eastern
portions of north-central Montana. Widespread rain develops
tonight and continues through WEdnesday across north-central MT
with gusty winds also developing across most of north-central and
southwest Montana. Drier weather develops to end the work week.

&&

.UPDATE...

No major changes made to this morning`s update other than to
update POPs and to expand the gusty winds from thunderstorms this
afternoon to line up with the SPC outlook. Ludwig

&&

.AVIATION...
17/18Z TAF Period

Showers and thunderstorms are moving north into Montana from Idaho
and Northwestern Wyoming. These will slowly spread north through the
first 12 hours of this TAF period. By 18/06Z rain showers will
transition to all rain across all terminals. This rain will continue
through the end of the TAF period. During heavier rain showers and
any thunderstorm visibility will be reduced to MVFR levels. At the
KHVR and KLWT terminals thunderstorms will produce strong to severe
wind gusts between 18/00Z and 18/06Z. By the end of this TAF period
at all terminals there will be gusty winds. At the KHVR, KLWT, and
KGTF terminals winds will be strong by the end of this TAF period.
During any heavier rain shower or thunderstorm there will be
mountain obscuration and there will be mountain obscuration during
the last half of this TAF period. At the end of this TAF period
there will be mountain wave turbulence over North-central and
Central Montana. -IG

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

&&

.HYDROLOGY...

A flash flood watch remains in effect for the Horse Gulch fire
burn scar area from this afternoon through Wednesday with initial
concerns for brief higher intensity precipitation with
thunderstorm activity later this afternoon and evening. While the
potential for intense precipitation rates decreases tonight,
steadier moderate precipitation will maintain some threat for
flash flooding and debris flows in the burn scar area.

Local minor flooding concerns may also arise on Wednesday with
persistent moderate rainfall across portions of north-central MT,
particularly where heaviest amounts are expected near the central
MT mountain ranges, where some low lying areas and locations with
poor drainage may begin to see standing water buildup. Hoenisch

&&

.PREV DISCUSSION...
/ISSUED 525 AM MDT Tue Sep 17 2024/

Key Points:

-Scattered severe thunderstorms this afternoon/evening across
eastern portions of N-central MT

-Significant (once in 10-20yr) rainfall amounts for September likely
across portions of N-central MT tonight through Wednesday

-Strong winds developing Wednesday

An early fall storm system is on track to bring several types of
impactful weather to the region over the next several days as a
closed mid-upper level low, currently centered over NE NV lifts
out into WY by this afternoon, then deepens while tracking
northward through eastern MT into SK by Wednesday night.

Today...showers and thunderstorms will increase in coverage this
afternoon as flow aloft turns southeasterly and becomes
increasingly diffluent while importing deeper moisture into the
region. This sets up a favorable environment for strong to severe
thunderstorms this afternoon and evening across eastern portions
of north-central MT, where the the greatest amount of buoyancy
(500-1000 J/kg CAPE) is supported by hi-resolution model
ensembles along with sufficient wind shear for organized
thunderstorm development. Primary severe threat in this area
(JB, Fergus Hill and Blaine counties) will be strong wind gusts
and hail with any thunderstorms today also bringing a risk for
locally heavy rainfall in addition to lightning.

Convection this evening is likely to evolve into an area of
widespread rain overnight as the mid level low and an associated
surface low lift north into eastern MT by Wednesday morning.
Unusually deep moisture is drawn around the the mid level low with
moderate to locally heavy rainfall developing within a
TROWAL/deformation feature wrapping around to the west of the
low. The location of this feature and heaviest precipitation has
shifted slightly east from previous forecasts (near I-15) to an
axis now generally from Hill county south through Judith Basin
and Fergus counties. Much of the Golden Triangle portion of
north-central MT still has an 70% or higher probability for
amounts in excess of one inch with 50% or better probabilities for
amounts in excess of 2 inches now over much of Judith Basin,
Fergus and Chouteau counties. (See hydrologic discussion for
potential impacts from excessive rainfall). Snow levels only fall
to as low 8500 ft, limiting any measurable snow to only the
highest peaks.

Strong low level (850-700MB) winds also wrap around the low as it
slowly tracks north across eastern MT on Wednesday, resulting in
very windy conditions for much of the area with strongest winds
likely across Judith Basin and Fergus counties where
probabilities for gusts in excess of 55 mph are above 60% and a
High Wind Watch remains in effect. Another area of concern for
stronger winds on Wednesday will be across Toole, Liberty and
western Hill counties, where probabilities of 55+ mph winds are
currently around 40%.

The storm exits the area into SK Wednesday night with drier
conditions and lighter winds across the area on Thursday. A more
progressive west to northwest flow aloft pattern is generally
supported by most medium range model ensembles, however
considerable differences exist in the location and timing of
troughing within this pattern, leading to lower than average
confidence in forecast details for this weekend into early next
week. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  72  52  60  48 /  60  90  90  60
CTB  70  51  63  46 /  40  70  60  30
HLN  71  51  63  48 /  80  90  90  50
BZN  67  46  60  43 /  90  80  90  50
WYS  56  34  52  33 /  90  90  80  60
DLN  61  44  58  40 /  70  80  60  30
HVR  79  55  65  50 /  50  80  90  80
LWT  74  48  56  45 /  50 100  90  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Wednesday morning through Wednesday evening
for Fergus County below 4500ft-Judith Basin County and Judith
Gap-Snowy and Judith Mountains.

Flash Flood Watch through Wednesday afternoon for Big Belt,
Bridger and Castle Mountains.

&&

$$

http://www.weather.gov/greatfalls