088
FXUS65 KTFX 261732
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1132 AM MDT Wed Jun 26 2024

...Updated Aviation Section...

.SYNOPSIS...

An exiting high pressure ridge will help warm temperatures up to
15 degrees above normal today, but an increasingly moist and
unstable southwesterly flow aloft will bring scattered showers
and thunderstorms for this afternoon and evening. A Pacific
weather system will then bring a chance of isolated to scattered
strong to possibly severe thunderstorms for Thursday. Cooler
temperatures with more showers than thunderstorms are expected in
the wake of the system on Friday, leading into a drier and warmer
Saturday.

&&

.UPDATE...

Today there will be showers and thunderstorms primarily across
Southwestern Montana this afternoon and evening. Isolated
thunderstorms will produce strong to severe wind gusts primarily
south of I-90. The Storm Prediction Center issued a Marginal Risk
(5% chance within 25 miles of point) for severe wind gusts for
that area.

For the update I increased PoPs across Southwestern Montana and
Southern Central Montana based on current radar imagery and hi-res
model guidance. I made a minor update to QPF and Sky grids based
on current observations and trends. The rest of the forecast is on
track. -IG

&&

.AVIATION...
26/18Z TAF Period

VFR conditions will prevail across all terminals during this TAF
period. Wednesday across all terminals it will be warm and so there
will be density altitude issues from the beginning of the TAF period
through Wednesday evening. Across the Southwestern Montana terminals
(KBZN, KWYS, and KEKS) there will be isolated showers and
thunderstorms Wednesday afternoon through Wednesday evening. Due to
the isolated nature of these showers and thunderstorms only VCSH was
used in the TAFs. Some of the thunderstorms will produce strong to
severe wind gusts. For the KHLN terminal Wednesday evening there is
a 40% chance for a rain shower or two and so a PROB30 group was
included for it. There is a 20% chance for these showers to affect
the North-central Montana Terminals (KGTF, KHVR, KCTB, and KLWT)
Wednesday evening through early Thursday morning. Thursday morning
across all terminals there will be isolated showers and
thunderstorms. For the KHVR, KLWT, and KGTF terminals these
thunderstorms will be strong to severe with hail and wind gusts as
the primary threats. Some of the thunderstorms Thursday morning will
produce heavy rainfall and there is a low chance of intermittent
reductions in visibility to MVFR levels because of it. There will be
isolated instances of low-level wind shear across the Southwestern
Montana terminals (KBZN, KWYS, and KEKS) Thursday morning. -IG

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

&&

.PREV DISCUSSION...
/ISSUED 540 AM MDT Wed Jun 26 2024/

Today... A low pressure trough off the Pacific Northwest coast
will put a warm and increasingly moist southwesterly flow aloft
over Southwest, Central, and North Central Montana today. This
will warm temperatures up to 15 degrees above normal and put much
of the area under a Minor heat risk with a few areas of Moderate;
this will cause difficulty for those sensitive to the heat,
especially for those outdoors for a long period of time or for
those without air conditioning. This warmup will also help
increase afternoon and evening instability, resulting in
scattered showers and thunderstorms spreading northeast over the
area with the passage of a disturbance in the flow aloft. While
severe storms are not generally expected today, weak directional
wind shear ahead of a cold front over the Northern Rockies and
warm/dry low levels of the atmosphere may combine to produce a
strong to weakly severe thunderstorm with strong wind gusts.
Showers and storms should decrease overnight.

Thursday... The trough is forecast to move into the Pacific
Northwest on Thursday, which will tap into a deeper feed of
moisture for the further increasing southwest flow aloft over the
area. Although instability will decrease somewhat overnight, this
increase in moisture will combine with the passage of a Pacific
cold front to increase the instability enough late in the morning
to increase coverage of showers and thunderstorms before noon
across the area. The 0-3km wind shear associated with the flow
aloft mostly increases to mostly between 30 and 40 kt for the area
east of a line from Bozeman to Helena to Havre, which puts that
area under a Marginal risk for isolated severe thunderstorms
(gusts of 58+ mph and/or hail 1+ inch in diameter). However there
is a core of 40 to 60 kt wind shear over portions of Meagher,
Judith Basin, and Fergus County, where the Storm Prediction Center
(SPC) has issued a Slight risk for isolated to scattered severe
thunderstorms. In fact, it has a portion of Fergus County under a
10 percent probability of damaging winds (75+ mph gusts) and very
large hail (2+ inches in diameter), and a less than 5 percent
probability of a tornado. The favored period for the strong storms
overall seems to be between 10 am and 4 pm. Broad scale westerly
winds will also increase behind the cold front, bringing
widespread 15 to 25 mph winds with gusts up to 40 mph; there is
also a 60+ percent probability of 50+ mph gusts along the Rocky
Mountain Front, but High Wind criteria is unlikely to be reached.
Thunderstorm activity should decrease during the late afternoon
and evening hours, as the trough starts to move through the area.
Overall, a brief cooling trend begins on Thursday, as temperatures
should struggle to reach seasonal averages.

Friday... The cooling continues as the low pressure trough forms
more of a closed off low pressure area and slows down its progress
over Northeast Montana. Moisture wrapping around this system will
combine with the passage of a Canadian cold front to shift the
breezy winds more northerly to focus another round of showers
(with fewer and weaker thunderstorms) more so over North Central
and Central Montana. Temperatures will mostly remain up to 10
degrees below normal, which will lower snow levels to mostly
between 7800 and 8500 feet, limiting light measurable snowfall to
the mountain peaks well above pass level. The low pressure area is
then forecast to exit the area Friday night, taking the showers
with it, as overnight lows fall into the lower to mid 40s at lower
elevations and into the 30s in the mountains.

Saturday through next Wednesday... A broad high pressure ridge
between systems will bring dry conditions for Saturday and near
normal temperatures for Saturday into Sunday. However, a broad low
pressure trough will bring an increasing chance for showers and
thunderstorms again for Sunday through Monday, as temperatures
cool back to slightly below normal. As of now, the long-term model
guidance is forecast another broad high pressure ridge to dry out
the area and warm temperatures back slightly above normal for
Tuesday into Wednesday.
-Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  86  57  72  50 /  10  40  80  20
CTB  81  55  69  46 /  20  30  50  40
HLN  89  60  75  52 /  20  40  80  10
BZN  86  55  77  44 /  40  40  80  20
WYS  78  48  72  37 /  50  30  50  10
DLN  83  52  74  40 /  40  40  50   0
HVR  84  58  74  51 /   0  30  90  30
LWT  81  54  72  44 /  10  30  90  20

&&

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

$$

http://www.weather.gov/greatfalls