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661
FXUS65 KTFX 170354
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
954 PM MDT Mon Sep 16 2024

.SYNOPSIS...

An impactful period of weather will overspread the region through
Thursday morning including burn scar flooding concerns, strong to
severe thunderstorms at times, widespread rain with areal flooding
concerns, and gusty winds. Concerns diminish heading into Thursday,
though unsettled conditions remain heading into the weekend.&&

&&

.UPDATE...

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Main update to overnight portion of the forecast was to keep
scattered shower/thunderstorm mention in through midnight for much
of Southwest and Central Montana to better reflect current
conditions. Activity has been weakening over the last hour or so,
but some cells could still produce periods of brief heavy rain and
wind gusts to 45 mph. Patchy fog may also develop overnight with
partial clearing of skies, but mainly in the valleys of Southwest
and Central Montana. -Coulston
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&&

.AVIATION...
17/00Z TAF Period

VFR conditions with relatively light winds are generally expected to
persist through at least 18/00Z across Southwest (KWYS, KEKS, KBZN),
Central (KHLN, KGTF, KLWT), and North Central (KHVR, KCTB) Montana.

Through 15Z, a low pressure system will move slowly east across the
Great Basin, and the resulting southerly flow aloft will continue to
move high- and mid-level cloudiness over the forecast area. However,
instability over Southwest and Central Montana will keep scattered
showers and thunderstorms over that area through around 04Z. The
main threats will be erratic wind gusts mostly up to 45 kt and
occasional lightning strikes, but a few storms may produce brief
periods of MVFR conditions in heavier rainfall.

After 15Z, the low will move northeast into northwest Wyoming,
putting the forecast area under more of an easterly upslope flow
aloft, which will cause showers and thunderstorms to become more
widespread from southwest to northeast. Increasing instability will
again bring a threat for strong erratic wind gusts and lightning,
but some storms could become strong with periods of heavy rain and
possibly some hail, which could cause periods of MVFR conditions.
-Coulston

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

&&

.HYDROLOGY...

Several rounds of precipitation will move across the region
over the next few days. Rounds of showers and thunderstorms will be
the initial concern this afternoon and tomorrow afternoon, with
primary concern for recent burn scars. The 25km neighborhood
probability for a half inch of precipitation over an hour in the
vicinity of the Horse Gulch burn scar Tuesday afternoon is 75%.
A Flood Watch has been issued for this burn scar related to
potential for flash flooding.

Precipitation becomes more widespread going into Wednesday and
Thursday, with the focus shifting northward onto the plains. In the
Golden Triangle area there is a greater than 50% chance for 2 inches
of rain (Peaks around 70-80% in and around Judith Basin and Chouteau
counties). In fact there is nearly a 50% chance in these two
counties for 3 inches of rain at lower elevations. Local flooding
concerns may arise in flood prone areas and areas near terrain where
heavier rain is most persistent. -AM

&&

.PREV DISCUSSION...
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/ISSUED 630 PM MDT Mon Sep 16 2024/

Through Tuesday evening... A closed low across CA/NV is within a
broad upper trough across the western CONUS drifting slowly eastward
this afternoon. The initial upper level low will then lift
northeastward tonight into Tuesday toward the northern high plains
as another upper level low dives southward along the Pacific coast
toward CA. The initial closed upper low will dig out a surface low
across Wyoming and eventually eastern Montana that eventually
meanders west (Becomes stacked with the upper low) toward eastern
portions of the plains beginning late Tuesday (But mostly
Wednesday).

The result of all of this will be for multiple rounds of showers and
thunderstorms through the evening Tuesday. The initial push of
showers and thunderstorms arrives this afternoon across Southwest
Montana initially, with activity moving northward through the
evening into Central Montana. Forecast soundings do feature inverted-
v profiles, but aren`t overly deep. Hence gusty winds will be a
concern, but the threat for severe gusts (58+ mph) is low today.
These showers and thunderstorms diminish somewhat late evening before
another round of showers and a perhaps few thunderstorms moves in
from the south as better forcing arrives from the upper level low
late tonight.

Showers will be ongoing across southern portions of the region
heading into Tuesday as the upper level disturbance continues its
approach. This should help to keep instability on the low side
across Southwest Montana Tuesday. Relatively warm, relatively moist
air being advected in form the east across the eastern plains
Tuesday will lead to a complicated severe thunderstorm setup.
Guidance ranges from dew points in the lower 50`s all the way up
into the lower 60`s by late Tuesday afternoon across eastern areas,
with members that advect in better moisture also featuring better
instability. To further complicate things, there looks to be some
degree of a capping inversion across these areas, bringing into
question how many thunderstorms will even be able to form in a
theoretically more favorable environment otherwise. All that said,
if thunderstorms do manage to form in a favorable environment, hail
and gusty winds will both be a concern late tomorrow across eastern
areas. The Storm Prediction Center highlights this risk with a
slight risk for severe weather along and east of a line from Havre
to Lewistown.

Localized impacts to burn scars will be possible with thunderstorms.
See the hydrology section for more information.

Wednesday into early Thursday... The closed upper low becomes
stacked with the surface low Wednesday (Indicative of a mature
storm) as the two stall briefly across the region. Strong forcing
around the western and southwestern side of this system will
result in a period of heavy rain, across the majority of Central
and North- central Montana Wednesday into Wednesday night. The
chance for 2 inches of rain east of I-15 is at or above 50% with
this system. See the hydrology section for flooding concerns.

Another concern Wednesday will be a risk for strong winds across the
plains on the west and southwest side of the stacked system.
Forecast soundings show winds aloft (2,000ft to 5,000ft AGL on the
plains) on the order of 45-60kts at times. There is concern that
these winds will get mixed to the surface across the plains where
precipitation is heaviest as well as due to terrain influence. At
this point there is a greater than 50% chance for sustained winds
of 40 mph Wednesday across the lower elevations of Judith Basin
county and eastern Fergus county. The chance for a 58 mph gust is
largely the same chance over these same areas. With this in mind,
a High Wind Watch was issued where confidence was highest in
stronger winds. Additional watches may be needed as confidence in
the situation grows.

Snow levels look to drop to around 8,000 ft, perhaps briefly lower,
in portions of Southwest Montana Tuesday night through Wednesday
evening, though overall amounts look minimal outside of the highest
peaks. -AM

Thursday afternoon into early next week...As this trough/closed low
system move out of the region Thursday, northwest flow aloft moves
in. Embedded upper air disturbances along this northwest flow aloft
will keep low chances for precipitation through the weekend.
However, there is uncertainty regarding to the timing of this trough
exiting, which can affect precipitation chances for the later half
of the week. However, the northwest flow aloft will keep
temperatures below average through the forecast period. -Wilson
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&&

.PRELIMINARY POINT TEMPS/POPS...
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GTF  47  71  52  59 /  20  70  90  90
CTB  45  71  52  60 /  10  70  90  90
HLN  50  72  51  60 /  40  80  90  80
BZN  45  68  45  56 /  50  90  90  80
WYS  35  55  34  50 /  50  80 100  80
DLN  45  62  44  59 /  50  80  80  40
HVR  50  80  55  66 /  10  60  90 100
LWT  47  75  47  56 /  30  70  90  90
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&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Wednesday morning through Wednesday evening
for Fergus County below 4500ft-Judith Basin County and Judith
Gap-Snowy and Judith Mountains.

Flash Flood Watch from Tuesday afternoon through Wednesday
afternoon for Big Belt, Bridger and Castle Mountains.

&&

$$

http://www.weather.gov/greatfalls