074
FXUS65 KTFX 161742
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1142 AM MDT Mon Sep 16 2024

...AVIATION SECTION UPDATED...

.SYNOPSIS...

The next few days will have a few rounds of showers and
thunderstorms Monday and Tuesday. Some stronger to severe storms
are possible across North Central Montana Tuesday afternoon and
evening. Areas of widespread rain with snow at the higher mountian
peaks arrive Tuesday night through Thursday morning. Temperatures
remain below averages through the forecast period.

&&

.UPDATE...

Southerly flow on the eastern/northeastern periphery of an upper
level closed low across CA/NV will result in a round of showers
and thunderstorms this afternoon and into the overnight across
mainly Southwest Montana, but into Central Montana as well. Brief
gusty winds and lightning will be the concern with any
thunderstorms that form. The going forecast is in good shape, with
no update necessary this morning. -AM

&&

.AVIATION...
16/18Z TAF Period

A Pacific trough in the Great Basin area will maintain a difluent
southerly flow over the Northern Rockies for variable cloudiness and
periods of widely scattered showers and thunderstorms. Primarily VFR
conditions are expected, though low VFR clouds will become more
common, especially during times of precipitation. Shower and
thunderstorm activity will become more widespread heading towards
the end of the TAF period. - RCG

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

&&

.PREV DISCUSSION...
/ISSUED 424 AM MDT Mon Sep 16 2024/

An upper level wave passing through this morning will continue to
bring scattered showers to mainly Southwest MT. The upper level
trough moving east across the Northwest U.S will bring areas of
divergence aloft today and Tuesday, which will aid in the
development of showers and thunderstorms. This afternoon, isolated
to scattered showers and thunderstorms develop over Southwest MT
and travel north to the Montana Highway 200 corridor during the
evening. Another round of scattered to widespread showers and
thunderstorms arrive Tuesday afternoon/evening for areas mainly
across the plains east of I-15. CAPE increases to 1,500 J/kg and
to 40kts of bulk shear. This increase of instability, moisture,
and wind shear supports some strong to severe storms. The Storm
Prediction Center has put areas east of a Havre to Lewistown line
in a slight risk for severe weather. There`s a 5-15% for wind
gusts >=58mph and >=1" hail with stronger storms. The corridor
for stronger storms will heavily depend on the low track, so the
severe risk areas may continue to slightly shift over the future
forecast periods.

As the sun sets, instability decreases and the precipitation type
will shift into a more widespread stratiform rain event. Tuesday
night moisture increases as it gets wrapped around the low
pressure system. A heavy band of precipitation will form around
the axis of deformation, which is currently forecasted along and
east of I-15. This will bring a prolonged period through early
Thursday morning of steady precipitation. The NBM 50 percentile
precipitation is giving a swath of 1-2" of rain in this region as
the probability of 2" or more of rain is ranging between 40-60%
during the 48hr period Tuesday- Thursday morning. The upper
ranges of max rain amounts from models (75th-90th NBM percentiles)
is hinting at 2-3.5" of rain, with a 20-40% of 3" or more of
rain. This has prompted the Weather Prediction Center to issue a
slight risk (15%) for excessive rainfall for the Golden Triangle
area Wednesday morning through Thursday morning. Other areas
across the CWA (Rocky Mountain Front and Southwest MT) will also
see wetting rain, with a 40-80% for 0.5" of rain during this time
frame. There is still a slight risk for excessive rainfall over
sensitive burn scar areas should heavier convective showers move
over the area Tuesday/Wednesday. Snow levels look to stay above
7,500ft(may go slightly lower with heavier bursts of
precipitation). This should confine snow accumulations to higher
mountian peaks.

Another concern with this weather system is strong winds forming
along the deformation band on Wednesday. Stronger 700mb winds
increase Wednesday afternoon/evening, and will mix down stronger
winds to the surface. Current probabilities for 48kts(~55mph) are
ranging between 40-75% for portions of Central MT and the Northern
Rocky Mountain Front. If models continue to trend towards higher
probabilities, some wind highlights may be needed for future
forecasts.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  72  48  70  51 /  10  20  70  90
CTB  71  45  68  50 /  10  10  70  90
HLN  76  51  71  50 /  20  40  80  90
BZN  77  46  68  45 /  30  40  80  80
WYS  70  35  56  34 /  50  70  90  90
DLN  74  46  62  43 /  20  60  70  80
HVR  78  51  77  54 /  10  20  60  90
LWT  74  47  74  47 /  10  20  70  90

&&

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

$$

http://www.weather.gov/greatfalls