643
FXUS65 KTFX 131748
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1148 AM MDT Sat Jul 13 2024

...Aviation Section Updated...

.SYNOPSIS...

Hot temperatures will continue today over Southwest Montana, and
they will return to the plains of North Central Montana. Breezy
westerly winds and very dry conditions will make for critical
fire weather conditions across much of Southwest and Central
Montana, while a disturbance is forecast to bring scattered
thunderstorms to the Hi-Line. Winds and storms will decrease
overnight, but another hot and dry day is forecast on Sunday, but
with less wind.

&&

.UPDATE...

Some slight adjustments were made to the precip chances this
afternoon/evening east of a line from Havre to Lewistown based on
updated model guidance. The potential for severe weather this
afternoon is still prevalent along portions of Blaine and Fergus
county, but the main event still remains further to the east
towards Phillips and Petroleum counties. That being said,
thunderstorms are likely to initiate in Blaine and Fergus county
with that initial convection potentially producing some larger
hail early on. The window of opportunity for severe weather this
afternoon will be from 2 PM to 7 PM after which the threat will
continue off towards northeastern Montana.

Otherwise, the primary threat is still the critical fire weather
conditions for this afternoon. Wind gusts came up just a little
bit with this update while minimum RH came down a couple
percentages in the zones already covered by the Red Flag Warning.
So for now, the warning is good in its current form and does not
require any changes. There is some marginal concern about near-
critical fire weather conditions continuing into tomorrow as the
front moves through but looking through the latest models the
window of opportunity for critical conditions looks much smaller
Sunday than it does today. As such, no products will be added with
this update but this situation will be monitored closely for the
next forecast package.

-thor

&&

.AVIATION...
13/18Z TAF Period

VFR conditions will prevail throughout the 1318/1418 TAF period;
however, monsoonal moisture lifting northeast from the Great Basin
and towards the Northern Rockies this evening will overspread
Southwest Montana by Sunday morning and bring increasing mid- to
upper level cloudiness to the KWYS, KEKS, and KBZN terminals. Smoke
from wildfires burning across the Western US will also lead to hazy
skies across much of Southwest Montana (i.e. KWYS, KEKS, and KBZN
terminals) through the early evening hours tonight, mainly reducing
slantwise visibility on approach. At the KHLN terminal, west to
northwest winds will keep most of the smoke and haze from the Horse
Gulch fire west of the terminal through this evening, but decreasing
winds overnight may allow for reductions in slantwise visibility
through the morning hours on Sunday at the terminal. Finally,
temperatures in excess of 90 degrees across all terminals this
afternoon will lead to density altitude concerns.

A weak disturbance moving over Southeast Alberta/Southwest
Saskatchewan this afternoon and through the early evening hours will
help to initiate isolated to scattered thunderstorms near and east
of the KHVR terminal in North Central Montana. The best chance,
approximately 30-40%, for any thunderstorms impacting the KHVR here
appears to be between 19-23z this afternoon; but may linger as late
as 04z this evening. Main threat from any thunderstorm, especially
east of the KHVR terminal over Blaine County, would be a period of
heavy rain, hail, and strong outflow winds.

An isolated thunderstorm or two can`t be ruled out across Southwest
Montana near the KEKS and KWYS terminals this afternoon and evening,
mainly between 18z and 00z, but the probability for said activity is
less than a 20% chance. Should any storm develop the main threat to
terminal operations would be gusty and erratic outflow winds. -
Moldan/Coulston

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

&&

.FIRE WEATHER...

The Red Flag Warning remains in effect from 2 pm today through
midnight tonight for Fire Weather Zones 117 and 118 due to hot
temperatures, low humidity, and gusty winds.

The forecast remains on track with keeping the main focus for
critical fire weather conditions today over portions of Southwest
and Central Montana (including Fire Weather Zones 117 and 118).
Significantly dry fuels will combine with increasing westerly
winds (gusting mostly to 35 mph at times) and continued low
afternoon humidity (as low as 10 percent). The area of most
concern in Zone 117 is Meagher County, as fuels in areas farther
north in the Zone (including Cascade and Judith Basin Counties)
have not dried out as much.

Similarly hot and dry conditions are forecast for Sunday, but
winds are not forecast to be quite as strong. After a brief and
minor cooldown on Monday, hot and dry conditions are forecast to
return for at least the next week after that.
-Coulston

&&

.PREV DISCUSSION...
/ISSUED 550 AM MDT Sat Jul 13 2024/

Today and Tonight... The moderate westerly flow aloft will
gradually mix down to the surface across much of North Central,
Central, and Southwest Montana through this afternoon with
daytime heating. This will help warm the plains back to around 10
degrees above normal with afternoon humidity in the 15 to 25
percent range, similar to the remainder of the area. These winds
will be strongest from the Big and Little Belt Mountains south
through Southwest Montana, where humidity may fall as low as 10
percent. For that area, there is a 60 to 90 percent probability
of at least 30 mph wind gusts at times and a 20 to 30 percent
chance of at least 40 mph gusts (based upon the HREF short-range
high- resolution model). Therefore, a Red Flag Warning remains in
effect for that area (please see FIRE WEATHER section for
details). Weak energy moving off the mountains of Central Idaho
may also create a few high-based thunderstorms across far
Southwest Montana, with the main threat from these being erratic
wind gusts.

Farther north, a disturbance is forecast to move over a frontal
boundary and bring a few strong thunderstorms to Hill and Blaine
Counties on the Hi-Line this afternoon. Precipitable water values
of between 1 and 2 standard deviations above normal, combined with
moderate wind shear and a favorable region of the passing jet max
for enhanced lift, may cause some thunderstorms there to become
severe with large hail, damaging winds, and locally heavy
downpours. In fact, the Storm Prediction Center (SPC) has that
area under a Slight Risk for severe thunderstorms, with at least
a 10 percent probability that significantly large hail (2 inch
diameter or greater) could occur. However, the latest run (06Z)
of one model (NAM) is picking up on a cluster of thunderstorms
moving east across southern Alberta and Saskatchewan, and seems
to be downplaying the potential for afternoon thunderstorms on
the Hi-Line. Past experience indicates that intermediate model
forecast runs at 06Z and 18Z may not always be as reliable as
those at 00Z and 12Z, as their data set may not be as robust and
does not include upper air soundings, so will maintain focus on
the potential for strong afternoon thunderstorms for now.

Winds and thunderstorms are forecast to decrease after 00Z as the
disturbance exits the area.

Sunday through Monday... A high pressure ridge is forecast to
build back into the forecast area on Sunday, maintaining the hot
and dry conditions. However, it will also help decrease winds
aloft, which will in turn keep winds from becoming quite as strong
as those today. Weak energy in the resulting flow aloft may bring
a few weak thunderstorms to the Hi-Line and to the Southwest
Montana mountains once again during the afternoon. A shortwave
trough is then forecast to move southeast on the leading edge of
the ridge across southern Saskatchewan Sunday night through
Monday, which will swing a Canadian cold front south over at
least the plains of Montana. An overall lack of moisture will
limit the possibility of thunderstorm activity, but a few storms
cannot be ruled out. The main impact of this frontal passage will
be to cool temperatures back closer to seasonal averages with a
shift to more northerly winds, at least over the plains.

Tuesday through next Saturday... The ridge is forecast to remain
unusually strong over the Western United States through this
period, as indicated by good agreement among model ensemble
clusters. This will likely result in a return to widespread much
above normal temperatures through this period, with most lower
elevations continuing to have high temperatures in the 90s and
possibly over 100 degrees. The chance for afternoon and evening
thunderstorms will mainly be focused over the mountainous area of
Southwest Montana. However, the heat will likely be the greater
threat, so prepare for it now if you need to. -Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  59  95  57  88 /   0   0   0   0
CTB  55  90  52  87 /   0   0   0   0
HLN  62  99  60  94 /   0   0   0  10
BZN  58  94  56  91 /  10  10  10  10
WYS  47  85  48  83 /  20  10  20  30
DLN  56  91  55  89 /  10  10  10  20
HVR  59  94  55  86 /  10  10  10   0
LWT  57  90  53  83 /  10   0   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 2 PM this afternoon to midnight MDT
tonight for Central and Eastern Lewis and Clark National Forest
Areas.

Red Flag Warning from 2 PM this afternoon to midnight MDT
tonight for Helena and Townsend Ranger Districts of the Helena
National Forest.

&&

$$

http://www.weather.gov/greatfalls