974
FXUS65 KTFX 162355
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
555 PM MDT Fri May 16 2025

Aviation Section Updated.

.KEY MESSAGES...

- Daily rounds of scattered showers and thunderstorms are
  expected through Saturday.

- Widespread rain and mountain snow returns Saturday night through
  Monday morning, with winter impacts mostly over and above pass
  level late Sunday through Monday morning.

- The unsettled spring weather pattern continues for much of
  next week, with daily rounds of scattered showers and
  thunderstorms.

- Near to slightly below average temperatures are generally
  expected for the next seven to ten days.

&&

.DISCUSSION...
/Issued 259 PM MDT Fri May 16 2025/

 - Meteorological Overview:

A moist and modestly unstable westerly flow aloft becomes more
southerly on Saturday in advance of an approaching Pacific trough.
Scattered, mostly afternoon, showers and thunderstorms are
expected today and Saturday, with a few stronger storms over the
southwest Saturday afternoon/evening before the activity gradually
transitions to stratiform rain while moving northward into North-
central MT Saturday night. Periods of rain and mountain snow will
then continue through Monday morning. Ensembles are in agreement
with the primary mid- level low moving southeastward from the
Pacific NW through ID and eventually into the Great Basin. Much of
the precipitation will be associated with a developing H700 that
will move through the southern half of the state.

This set up will encourage widespread rainfall with totals
generally ranging between a third to one inch of moisture for most
locations Saturday night through Monday afternoon. Of course
there are some variations among deterministic models highlighting
different localized areas with rainfall amounts in excess of one
inch. Snow levels will begin to fall from 8,000 feet Sunday
morning to as low as 5,000 feet Sunday night into Monday morning.
Most of the winter impacts will be observed during this time,
especially where the areas of heaviest precipitation temporarily
drops snow levels even further.

Most of the precipitation ends in a east to west fashion on
Monday, but the overall unsettled weather pattern will continue
for the remainder of the week. A progressive westerly flow will
send shortwaves through the Northern Rockies for periods of
scattered showers and thunderstorms and breezy conditions, though
there are uncertainties with the timing and positioning of each
system. Temperatures will continue to roughly remain near seasonal
averages. - RCG

 - Forecast Confidence & Scenarios:

Widespread rain and snow Saturday night through Monday morning...

Confidence continues to increase in regards to widespread rain
and mountain snow during this period. Although ensembles are
largely in agreement with the positioning of the trough, there are
differences among their members with the precise locations of
heavier rainfall. Overall, probabilistic guidance supports
widespread liquid equivalent precipitation amounts between a half
and one inch Saturday night through Monday afternoon. Nearly the
entire forecast area has a 60% + of seeing a half inch or more of
moisture, but only areas over and near the Southern Rocky Mountain
Front and the Central MT island ranges have a greater than 40%
chance for an inch or more. Impacts are expected to be limited,
but the situation will be monitored for potential hydrologic
impacts, including unexpected water rises. Please see the
hydrology section for more information.

Colder air on the backside of the low pressure system will begin
to lower snow levels Sunday morning, falling to around 5,000 feet
by Monday morning. Areas receiving heavier precipitation may see
their snow levels briefly drop an additional 500 to 1,000 feet.
Current probabilistic guidance keeps the greatest chances (70% +)
for 2 inches of snow or more above 7,000 feet. While this will
hold true for much of the event, it does not reflect the lowering
snow levels form heavier/convective precipitation or H700
temperatures bottoming out as low as -8C over the southwest Sunday
night and Monday morning. Yes, the most impactful snows will be
above pass level, but brief winter road impacts down to the valley
floors over Central and Southwest Montana are certainly not out
of the question. - RCG

&&

.AVIATION...
17/00Z TAF Period

Predominately low-VFR/VFR conditions are expected throughout the
1700/1800 TAF period across the North Central and Central Montana
terminals (i.e. KCTB, KHVR, KGTF, KLWT, and KHLN), with scattered
rain showers moving across/near the KLWT, KHVR, and KCTB through
03-07z Saturday. Further south across the terminals of Southwest
Montana (i.e. KBZN, KEKS, and KWYS) rain showers and/or low CIGS
will bring MVFR/low-VFR conditons through generally 07-13z
Saturday, with most shower activity ending prior to 09z Saturday.
Clearing skies through sunrise on Saturday will then aide in
patchy fog developing across Southwest through North Central
Montana; however, confidence in fog reducing VIS/CIGS at any given
terminal remains too low for mentioning in this TAF issuance.
Mountain obscuration will gradually decrease through Saturday
morning, especially over Central and North Central Montana. -
Moldan

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

&&

.HYDROLOGY...

A Pacific weather system will bring periods of widespread rain
and mountain snow Saturday through Monday morning. The
precipitation will begin in the form of scattered showers and
thunderstorms over Southwest Montana Saturday afternoon before
gradually transitioning to areas light to moderate rainfall as it
expands northward into the plains Saturday evening and night.
Periods of rain and mountain snow will then continue for all areas
through Monday morning.

Total liquid precipitation amounts will generally fall between a
half and one inch with localized higher amounts. Snow levels will
gradually lower beginning on Sunday, falling to as low as 5,000
feet by Monday morning. 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  37  66  47  54 /  20  20  60 100
CTB  36  64  42  50 /  30  10  50  90
HLN  38  66  46  54 /  20  30  80 100
BZN  39  65  42  53 /  60  30  70 100
WYS  32  59  35  46 /  80  30  90 100
DLN  36  61  40  49 /  50  30  90  90
HVR  41  68  43  54 /  50  50  40  80
LWT  37  62  42  54 /  50  40  60  90

&&

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

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