Area Forecast Discussion
Issued by NWS Great Falls, MT

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939
FXUS65 KTFX 151013
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
413 AM MDT Mon Jul 15 2024

.SYNOPSIS...

Above normal temperatures with generally dry conditions will
continue through this week across Southwest, Central, and North
Central Montana, as a high pressure ridge gradually builds into
the region. However, a few thunderstorms cannot be ruled out each
day, mainly over the mountains of Southwest and Central Montana,
but also along the Hi-Line today.

&&

.DISCUSSION...

Today through Tuesday... A lingering northwesterly flow aloft will
keep temperatures 5 to 10 degrees above normal for the next couple
of days across North Central, Central, and Southwest Montana. A
disturbance in the flow will keep the threat for mainly afternoon
and evening thunderstorms along the Hi-Line and over the mountains
of Southwest Montana. Relatively weak instability, lift, and
upper level dynamics will likely hinder any strong thunderstorm
development, but the warm and dry air at the surface will make
erratic wind gusts from any storms the main concern. However, with
precipitable water values around 1 inch over the plains (a bit
above normal), storms on the Hi-Line may also produce a brief
heavy shower. The weak afternoon instability may be enough to
mix some of the breezy and gusty west to northwest winds down to
the surface today. However, model probabilities of gusts exceeding
30 mph remain below 40 percent. The upper level high pressure
ridge over the Western United States will gradually build into the
Rocky Mountain region tonight through Tuesday, which will help
decrease winds aloft and limit the risk for mixing down of gusty
winds during the day on Tuesday, as well as the risk for
thunderstorm development.

Wednesday through next Monday... Forecast model ensembles continue
to predict a very warm and dry period into early next week. They
indicate that the high pressure ridge will remain fairly strong
for this time of year as it lingers over the Four Corners region,
extending north into Montana. This shift of the ridge axis to just
east of the region sets up the potential that at least some
monsoonal moisture could be drawn into the area. As of now,
guidance keeps the chance for thunderstorms at less than 40
percent and focused over the mountains of Southwest and Central
Montana, but we`ll monitor the situation for potentially higher
chances. Temperature-wise, current NBM guidance keeps high
temperatures well into the 90s through this period at lower
elevations (10 to 15 degrees above normal), with some ensemble
guidance hinting at some locations potentially exceeding 100
degrees over the weekend into early next week. This could threaten
daily record highs for some locations. Also, despite the
relatively dry air in place, this kind of a prolonged hot period
may limit how much overnight cooling will occur. This may require
excessive heat highlights if models start to forecast warmer
temperatures, but highlight-worthy temperatures are not being
forecast at this time.
-Coulston

&&

.AVIATION...
15/06Z TAF Period

VFR conditions are expected to continue through at least 16/06Z
across North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN),
and Southwest (KBZN, KEKS, KWYS) Montana.

A west-northwest flow aloft will keep scattered to broken mid-
and high-level cloudiness over the area. An exiting disturbance
will take the lingering showers and thunderstorms north and east
of KHVR east out of the area through 12Z. However, the band of
cloudiness associated with it still has a chance to sag south
over the Hi-Line (including KCTB and KHVR) through around 18Z,
possibly bringing periods of lower VFR ceilings. Otherwise,
another disturbance in the flow will bring a chance for isolated
showers and thunderstorms to the Hi-Line and Southwest Montana
between 18Z and 03Z as the airmass becomes weakly unstable, but
the terminal with the best chance to see a thunderstorm or two
will be KWYS, so it`s been addressed with a PROB30 group. Breezy
west to northwest winds will also become gusty at times between
18Z and 03Z.

Warmer than normal temperatures will likely cause density
altitude issues, and smoke from wildfires will cause reduced
slantwise visibility from KHLN south through Southwest Montana.
-Coulston

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

&&

.FIRE WEATHER...

High temperatures today and Tuesday will remain between 5 and 10
degrees above normal with afternoon humidity in the 20 to 30
percent range over the plains and in the 10 to 20 percent range in
the valleys. Gusty west to northwest winds are also forecast to
mix down to the surface today, but model guidance keeps the chance
for gusts of 30 mph or higher below 40 percent. As high pressure
builds into the area on Tuesday, the threat for additional gusty
winds will decrease.

The forecast trend continues to show that the high pressure ridge
will gradually move east and settle over the Rockies through at
least this weekend, which should warm temperatures to between 10
and 15 degrees above normal through the period, with some guidance
hinting at slightly higher temperatures (highs possibly exceeding
100 degrees this weekend). This is forecast to keep afternoon
humidity at lower elevations in the 10 to 20 percent range with a
daily threat of at least isolated thunderstorms over the
mountains of Southwest and Central Montana from monsoonal
moisture. This will likely result in at least an increase in fire
weather concerns.
-Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  90  57  90  61 /   0   0   0   0
CTB  90  55  88  57 /  10  10  10   0
HLN  93  58  95  60 /   0   0   0   0
BZN  91  53  92  55 /   0   0   0   0
WYS  80  42  82  42 /  30  20  20  10
DLN  84  50  88  52 /  10  10   0   0
HVR  89  59  88  60 /  10  10   0   0
LWT  84  54  85  56 /  10   0  10   0

&&

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

$$

http://www.weather.gov/greatfalls