841
FXUS65 KTFX 160237
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
837 PM MDT Sun Sep 15 2024

.SYNOPSIS...

Seasonably mild temperatures and mostly dry conditions continue
through Monday before the next fall weather system approaches and
brings an initial wave of scattered showers and thunderstorms Monday
and Tuesday. Periods of widespread rain and cool, raw conditions are
then expected Tuesday night through early Thursday.

&&

.Update...

Evening update has been published, with the main adjustment being
to linger low chance (i.e. 15% to 20%) PoPs longer through the
evening hours for areas along and west of a Bozeman, to Great
Falls, to Havre line. Isolated to scattered showers and
thunderstorms from late this afternoon and through the evening
hours (so far) have been producing occasional wind gusts of
between 40 to 50 mph, mainly across Southwest Montana. The
probability for wind gusts in excess of 40 mph with any
additional showers or storms here and points further through the
remainder of the evening and overnight hours is approximately
10%, so can`t completely rule out additional instances of gusty
and erratic winds. Otherwise the remainder of the forecast remains
on track and no additional changes were made to the on-going
forecast. - Moldan

&&

.AVIATION...
16/00Z TAF Period

Isolated to scattered showers, and even a few thunderstorms, will
continue to move over predominately Southwest and into Central
Montana through this evening. VFR conditions will prevail even
beneath these showers; however, VIS may briefly fall to 6sm amongst
gusty and erratic winds. While an isolated shower can`t be ruled out
further north over the plains of Central and North Central Montana
over this same timeframe, most if not all of precipitation falling
out of these high based showers is not expected to reach the ground.
Beyond 06z Monday shower coverage will shift to predominately
Southwest Montana, mainly south of the I-90 corridor, and remain
here through 12z Monday before expanding further north through the
remainder of the day. Mountain peaks may be obscured at times over
the next 24 hours, mainly beneath/near shower and thunderstorm
activity. - Moldan

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

&&

.PREV DISCUSSION...
/ISSUED 515 PM MDT Sun Sep 15 2024/

Southwesterly flow ahead of a Pacific NW trough will continue to
bring variable cloudiness and a few lighter end showers to the
northwestern two thirds of the CWA. Lapse rate up to around 8C/km
may also initiate a few isolated storms over the higher terrain of
the southwest this evening. Later tonight into Monday the Pacific
trough dives southeastward toward the Great Basin and pushes
additional moisture and instability into the Northern Rockies. An
initial wave of showers and storms look to develop over the
southwest late tonight with additional rounds of convective
activity spreading northward Monday and Monday night.

Ensembles are now favoring the Pacific trough to move south of
the MT/ID border before trekking northward through eastern MT and
eventually into the Canadian prairies on Wednesday. Scattered
showers and thunderstorms will continue during the day on Tuesday
with the latest cams looking more favorable for strong to severe
thunderstorms Tuesday afternoon, particularly for plains locations
east of I15. MLCAPE values in these areas may exceed 1,500 J/kg
while deep moisture streams in from the southeast and bulk shear
increases to the 25 to 35 kt range. Strong gusty winds, heavy
downpours, and hail will be valid concerns if these initial cam
models are correct and the expected pattern holds together.

The most widespread period of general rains is likely to occur
Tuesday night into Wednesday, though precipitation may linger a
little longer for central/north-central location east of I15. 72
hour exceedance probabilities of 1 inch or more of rainfall for
the the period ending 6 am Thursday have increased to the 60 to 80
percent range for most of Central/North-central MT with
probabilities for 2 inches or more now running above 50 percent
for the Golden Triangle area. Southwestern location south of the
highway 12 corridor don`t look to fare as well, but still look to
see storm totals mostly between a quarter and half inch. There
remains a risk for locally excessive precipitation over sensitive
burn scar areas and additional areas may also be impacted should
the potential for higher rainfall totals increase further. Snow
levels look to fall as low as around 7,000 ft, lowest over the
southwest late Tuesday and early Wednesday. Measurable snow should
mostly be limited to the higher mountain peaks.

Winds will also be a concern over the plains on Wednesday when
H700/H500 40 to 50 kt winds wrap around the departing trough. Wind
exceedence Probabilities support sustained winds over 25 mph (80
+ percent chance), along with similar probabilities for gusts over
40 mph. This in combination with rain, summer foliage, and a
slightly more northerly directional component may increase impacts
compared to most run of the mill wind events of similar
magnitude. No wind highlights are planned as of right now, but
this situation will be closely monitored over next few days. - RCG

Larger scale troughing remains across the western US in the wake
of the exiting low later this week with troughing shifting
eastward next weekend. There is still considerable
spread/uncertainty with features moving through the trough by next
weekend with some potential for a cooler system in NW flow moving
across the area. - Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  72  49  71 /  20  20  40  80
CTB  43  70  45  69 /  20  20  30  80
HLN  51  74  51  71 /  30  40  60  90
BZN  46  77  46  68 /  20  70  70  80
WYS  39  69  36  55 /  30  80  90  90
DLN  48  75  45  60 /  50  60  80  80
HVR  49  77  52  78 /  20  30  40  70
LWT  46  74  47  73 /  10  30  40  80

&&

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

$$

http://www.weather.gov/greatfalls