380
FXUS65 KTFX 181738
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1138 AM MDT Sun Aug 18 2024

...Updated Aviation Section...

.SYNOPSIS...

A southwesterly flow aloft will bring a chance for scattered
showers and thunderstorms to Southwest and North Central Montana
through Monday, mainly during the afternoon and evening hours.
Some of these storms could become strong, with gusty winds and
hail the main hazards. Temperatures will remain 5 to 10 degrees
above normal through Monday, but they will cool back closer to
normal for the mid to late week period, with a continued chance
for showers and thunderstorms each day.

&&

.Update...

On-going forecast is performing well this morning and no update is
needed. High temperatures will warm into the mid-80s to mid-90s
this afternoon across Southwest through North Central Montana,
with isolated to scattered showers and thunderstorms developing
during the mid- to late afternoon across Northeast Idaho and
Southwest/West Montana. These showers and thunderstorms will then
lift northeast across the remainder of Central and North Central
Montana through the midnight hour; however, an isolated shower or
storm may linger along the Hi-Line and mountains along the Idaho
border even beyond this hour. A few of the thunderstorms this
afternoon and evening could become severe, with the primary severe
weather threat being damaging winds (less than a 15% chance of
occurring). - Moldan

&&

.AVIATION...
18/18Z TAF Period

VFR conditions will prevail during this TAF period except for during
any thunderstorm when visibilities will be briefly reduced to MVFR
levels. At all terminals excluding the KHVR and KCTB terminals there
is a 20 - 30% chance for showers and thunderstorms moving in from
the southwest in Idaho beginning between 18/20 and 18/21Z and continuing
through at least 19/00Z. At all terminals there is 20 - 30% chance for
showers and thunderstorms between 19/00z and 19/07Z. At the KWYS terminal
a 20% chance for thunderstorms continues through the end of the TAF
period. Any thunderstorm will produce gusty erratic winds, frequent
lightning, and brief heavy downpours. An isolated thunderstorm will
produce strong to severe wind gusts. From 19/02 - 19/09Z there will be low-
level wind shear around the North-central Montana terminals (KCTB,
KGTF, KLWT, and KHVR). From 19/09Z to 19/13Z there will be low-level wind
shear around the Southwestern Montana terminals (KEKS, KBZN, and
KEKS). During the beginning of this TAF period winds at the KEKS
terminal will gust up to 18kts. -IG

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

&&

.PREV DISCUSSION...
/ISSUED 524 AM MDT Sun Aug 18 2024/

Today through Monday... This will likely be the most active period
of the next week or so. A southwesterly flow aloft will remain
over the Northern Rockies to begin this week, between a deep low
pressure trough along the Pacific Northwest coast and a high
pressure ridge over the Great Plains. This pattern will keep
temperatures 5 to 10 degrees above normal across North Central and
Southwest Montana, which will help create mainly afternoon and
evening instability and bring a chance of scattered showers and
thunderstorms. A nearly stationary north-south oriented frontal
boundary will also provide a good focusing mechanism for
thunderstorm development. Dry sub-cloud layers with afternoon
relative humidity in the 20 to 30 percent range at lower
elevations will make strong and erratic wind gusts the main
hazard, prompting the Storm Prediction Center (SPC) to put much of
the forecast area under a Marginal Risk for severe thunderstorms
each day (5+ percent chance of 58+ mph gusts). The tap into
Pacific and monsoonal moisture in the flow will also keep
precipitable water values between 0.75 and 1.00 inches, which is
slightly above normal, maintaining the potential for isolated
brief heavy showers.

The differences between today and Monday will be the strengths of
passing disturbances overhead and their associated wind shear.
Today`s relatively weak disturbance will keep bulk shear values in
the 20 to 30 kt range, while a stronger disturbance on Monday
will bring 30 to 40 kt of shear. As a result, the potential for
large hail (1+ inch diameter) is higher on Monday, as included in
the SPC Marginal Risk (5+ percent probability).

Tuesday... A shortwave trough will eject from the main Pacific
trough and move over the forecast area. This will help shift the
frontal boundary east more so over Central and Eastern Montana,
cooling temperatures off a few degrees, and putting the focus for
additional shower and thunderstorm activity there. However, the
passage of the shortwave will also increase the resulting west-
southwesterly flow aloft, which will translate down to the surface
in the wake of the frontal boundary. While strong winds are not
expected, it will be a somewhat breezy day; NBM guidance indicates
a 30 to 50 percent probability of at least 15 mph sustained winds
and 30 mph gusts Tuesday afternoon on the Rocky Mountain Front
and over Southwest Montana south of Interstate 90.

Wednesday through next Sunday... Ensemble model cluster guidance
is slowly coming into better agreement with at least the early
portion of this period. Most (85 percent) of the clusters start
to bring the low pressure trough onshore into the Western United
States through Friday, while 15 percent of them keep it along the
coast. As a result, the NBM cools temperatures back closer to
normal, while keeping a low chance for additional showers and
thunderstorms. Over the weekend, there is slightly less agreement,
but again the majority (70 percent) of the clusters keep at least
a weak low pressure trough over Montana, prompting the NBM to cool
temperatures even more with a slightly increased chance for
showers and thunderstorms. -Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  92  58  92  57 /  20  30  30  30
CTB  86  55  87  55 /  10  20  20  30
HLN  93  59  90  56 /  30  30  50  30
BZN  91  55  88  51 /  20  20  40  20
WYS  79  45  77  43 /  20  30  60  20
DLN  85  53  84  48 /  30  20  40  10
HVR  95  63  93  61 /   0  30  10  40
LWT  91  58  90  55 /  20  20  40  30

&&

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

$$

http://www.weather.gov/greatfalls