333
FXUS65 KTFX 171040
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
440 AM MDT Sat May 17 2025

.KEY MESSAGES...

 - Scattered showers and isolated thunderstorms develop this
   afternoon and evening.

 - Widespread rain develops tonight with higher elevation snow near
   the continental divide.

 - Rain becomes more widespread across central and eastern
   portions of north-central Montana on Sunday and lingers into
   Monday for eastern portions of north-central Montana.

 - Temperatures near seasonal averages today cool to below average
   Sunday and Monday before moderating next week.

&&

.DISCUSSION...

 - Meteorological Overview:

A few lingering light showers early this morning in association with
a subtle disturbance in moist westerly flow will exit to the east
with mainly dry and seasonable conditions through early this
afternoon. Flow aloft turns more southerly later today through
tonight as a negatively tilted upper level trough shifts inland.
Models continue to come into better agreement with the progression
of this trough across the region as the initial wave moves into the
region tonight before a secondary wave drops southeast from BC into
the larger scale trough and closed low that remains centered
south/southeast of the area later Sunday through Monday.

Showers and thunderstorms initially developing across SW MT early
this evening should transition to an area of widespread rain that
lifts north through central/north-central MT overnight as an
embedded mid-level disturbance lifts north along a deformation axis
between two previously mentioned waves. Focus for moderate to locally
heavy precipitation with this initial round will be areas near the
continental divide, particularly along the Rocky Mountain front
Sunday morning as flow turns southeasterly.

A second round of widespread precipitation develops Sunday through
Sunday night across eastern portions of central/north-central MT in
association with a TROWAL extending north from the larger scale
upper low moving by to the southeast. There is still some
uncertainty with the exact location of this feature and the
western extent of moderate to locally heavy precipitation and its
duration with some potential for precipitation to linger through
Monday night across eastern areas (through with decreasing
precipitation intensity).

Snow levels initially 8000ft or higher will fall to around 6000 ft
later tonight along the Continental Divide while remaining higher
across areas to the east, where snow levels will lower to around
6000 ft Sunday night. While the larger scale trough and widespread
precipitation move out of the area by Tuesday, additional
disturbances moving through westerly flow will keep some chances for
showers in the forecast through much of next week as temperatures
moderate back to near seasonal averages.

 - Forecast Confidence & Scenarios:

Probabilities for precipitation amounts in excess of 1.00" are as high
as 50-60% along the immediate east slopes of the Rocky Mountain
Front with the initial wave of moisture tonight through Sunday
morning. Probabilities for an inch or more precipitation amounts with
the second wave of moisture across eastern portions of north-central
MT remain around 30-40%.

Passes along the continental divide will likely see some snow later
tonight into Sunday morning with Big Hole and Chief Joseph passes in
Beaverhead county having the highest risk (30-50%) of accumulations
of 2" or more. Some snow with minor accumulation is also possible
over passes in Jefferson and Lewis and Clark counties. Hoenisch

&&

.AVIATION...
17/06Z TAF Period

At all terminals except for the KWYS and KBZN terminals (at the very
end of the TAF Period for KBZN) VFR conditions will prevail during
this TAF Period outside of any thunderstorm. At all terminals except
for the KGTF terminal there is a 20 - 40% chance for rain showers
between now and 17/12Z. At all terminals except for the KCTB
terminal after 17/18Z there is a 20 - 50% chance for rain showers
and a 15 - 30% chance for thunderstorms. Any thunderstorm will
produce gusty, erratic winds, lightning, and small hail. At the
KBZN, KEKS, and KWYS terminal after 18/00Z rain showers will
transition to steady rain and continue through the end of the TAF
Period. At the KWYS, KBZN, KEKS, and KHVR terminals there is a 10 -
30% chance for fog to form between now and 18/14Z. There will be
mountain obscuration in Southwestern Montana during the beginning
and end of this TAF Period. -IG

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

&&

.HYDROLOGY...

A few rivers are forecast crest near bankfull towards the end of
the event. This includes the Gallatin River at Logan and the
Jefferson River near Three Forks. With lowering snow levels and
generally slow rain rates, flooding is generally not expected;
however, those near waterways or areas normally susceptible to
flooding should remain situationally aware of their surroundings
in case localized heavier rainfall brings unexpected water rises.
- RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  66  47  54  41 /  10  90 100  60
CTB  64  42  51  38 /  10  70  90  60
HLN  67  46  53  39 /  20  80  90  50
BZN  65  43  51  34 /  30  60 100  60
WYS  59  36  46  27 /  40  80 100  80
DLN  62  40  49  30 /  40  80  90  40
HVR  69  44  56  40 /  30  40  90  80
LWT  62  42  56  37 /  40  60 100  90

&&

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

$$
http://www.weather.gov/greatfalls