-- Discussion containing changed information from previous version are highlighted. --
151
FXUS65 KTFX 261140
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
540 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.

&&

.DISCUSSION...

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

&&

.AVIATION...
-- Changed Discussion --
26/12Z TAF Period

VFR conditions are expected to continue through at least 27/12Z
across Southwest (KWYS, KEKS, KBZN), Central (KHLN, KGTF, KLWT), and
North Central (KCTB, KHVR).

An increasingly moist and unstable southwesterly flow aloft will
spread high- and mid-level cloudiness northeast over the area, as
well as cause some mountain wave turbulence. Breezy southerly winds
will also blow through the valleys of Southwest and Central Montana,
while more easterly winds are expected over the plains. Warm
temperatures will also cause density altitude issues mainly between
18Z and 02Z. A disturbance in the flow will is then forecast to
bring isolated thunderstorms to Southwest Montana between 21Z and
02Z and to Central and North Central Montana mainly between 00Z and
06Z. The main potential impact from these storms will be strong
erratic wind gusts, but brief periods of MVFR conditions in heavy
rain and hail are possible. Another chance for showers and weak
thunderstorms moves into Central and North Central Montana after
08Z. -Coulston

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
-- End Changed Discussion --

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  86  57  72  50 /  10  40  80  20
CTB  81  55  69  46 /  10  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 /  40  30  50  10
DLN  83  52  74  40 /  50  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