-- Discussion containing changed information from previous version are highlighted. --
380
FXUS65 KTFX 222320
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
520 PM MDT Wed May 22 2024

..Aviation Section Updated...

.SYNOPSIS...

Scattered showers and thunderstorms will transition to areas of
rain and mostly mountain snow tonight into Thursday. Snow impacts
will be most significant over the central and southwest mountain
passes, but some southwest valleys will see at least a period wet
snow as well. The unsettled pattern will continue into the
weekend, but temperatures will warm to near seasonal averages.

&&

.DISCUSSION...

Remainder of this afternoon and tonight...A closed Pacific low
continues to slide southeastward into ID this afternoon, leaving
diffluent flow aloft over the Northern Rockies for increasing
instances of showers and isolated thunderstorms. This activity
will become more numerous through the afternoon hours before
gradually transitioning to more of a stratiform precipitation
event overnight tonight into Thursday. Snow levels will fall
tonight as the low pressure system moves into S ID. Model guidance
continues to place the core of heaviest precipitation over
central and southwestern areas south of the MT highway 200
corridor. The colder air with this system will be a little farther
south as well, impacting southwestern mountains along and just
east of the Continental Divide with the heaviest snowfall. This
includes MacDonald and Homestake Passes and the stretch of I15
between Helena and Butte. Lower elevation impacts will be mostly
reserved for the southwest valleys, especially the Dillon area
where localized snowfall amounts may approach and exceed 6 inches.


Given the uncertainty of lower elevation snowfall amounts and
relatively short opportunity for impacts , about 6 to 9 hours, I
decided not to upgrade any of the Winter Weather Advisories;
however, at least a short period of slushy and slippery roads can
be expected in these areas despite lower snow amounts than the
mountain passes. As far as rain goes, forecast probabilities
support widespread 0.50 to 1 inch rainfall amounts along and south
of the MT highway 200 corridor, with higher amounts along the
north facing mid-slopes of the central ranges. Flooding is
generally not expected for most locations, but a Flood Watch goes
in effect tonight for the Little Belt Mountains. Please see the
hydrology section for more information.

Thursday through Sunday...The Pacific low pressure system begins
to move east of the region on Thursday, leaving a chilly northerly
flow aloft in its wake. Rain and snow will diminish in a
northwest to southeast fashion and impacts from snow will wane
with diurnal heating and precipitation slowing down. No changes
were made to the expiration times for the winter highlights, but
some products may be lifted a little ahead of schedule,
particularly the Winter Weather Advisories for lower elevations.
Continued cold air advection aloft will make it a struggle
afternoon highs to surpass the 40s and 50s.

Ensembles favor unsettled conditions remaining in place through
the weekend. Expect lower end isolated scattered shower/thunder
activity on Friday before weak troughing and diffluent flow aloft
interacts with moisture and modest instability for more widespread
activity on Saturday into early Sunday. Despite a return to
wetter conditions, temperatures still trend closer to seasonal
averages.

Monday through next Wednesday...Ridging aloft moves in early next
week, though there are discrepancies among model suites in
regards to the timing of peak peak amplification over the Northern
Rockies. Overall, well above normal temperatures are expected
amid dry conditions. There are signs of some troughing returning
for the second half of the week, but there are significant
differences on timing and depth at this point. - RCG

&&

.AVIATION...
-- Changed Discussion --
23/00Z TAF Period

A spring storm will bring IFR/MVFR/low-VFR conditions to all
terminals through the 2300/2400 TAF period. Shower and thunderstorm
activity across the Northern Rockies will gradually wane through 03-
09z Thursday, with the precipitation becoming predominately
stratiform in nature. A changeover from rain to a rain/snow mix or
all snow is expected during this convective to stratiform
transition, mainly along and south of a KLWT to KGTF line and most
notably at the KBZN, KEKS, and KWYS terminals. Main threat from any
thunderstorms this evening will be gusty and erratic winds, in
addition to cloud to ground lightning and small hail. Mountains will
be obscured over the next 24 hours. - Moldan

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
-- End Changed Discussion --

&&

.HYDROLOGY...

A Flood Watch remains in effect for areas near creeks and streams
over in the Little Belt Little mountains tonight through Friday
morning. Current probabilistic forecasts support one to two inches
of rainfall/liquid equivalent precipitation through Friday. Snow
levels will fluctuate with this weather system, with mostly snow
expected for areas above 6,000 feet. However, there are concerns
for flooding on area creeks and streams given recent
precipitation/saturated soils and at least a portion of the
upcoming precipitation falling as rain at all elevations. Those
near water should be vigilante and ready to seek higher ground if
flooding occurs.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  37  53  36  64 /  90  70  30  30
CTB  34  54  35  62 /  90  40  50  40
HLN  37  55  36  67 / 100  90  10  20
BZN  32  49  28  63 / 100 100  10  20
WYS  25  43  24  54 /  90  90  10  20
DLN  30  47  26  60 /  90 100   0  10
HVR  37  55  36  65 /  70  30  30  50
LWT  34  47  31  60 /  90  90  10  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Thursday for Elkhorn and
Boulder Mountains-Northwest Beaverhead County-Ruby Mountains and
Southern Beaverhead Mountains-Southern Rocky Mountain Front-
Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from midnight tonight to 6 PM MDT
Thursday for Beaverhead and Western Madison below 6000ft-
Gallatin Valley-Gates of the Mountains-Madison River Valley-
Missouri Headwaters.

Flood Watch through Friday morning for Little Belt and Highwood
Mountains.

Winter Storm Warning until 6 PM MDT Thursday for Big Belt,
Bridger and Castle Mountains-Gallatin and Madison County
Mountains and Centennial Mountains-Little Belt and Highwood
Mountains.

Winter Storm Warning from 6 AM to 6 PM MDT Thursday for Snowy
and Judith Mountains.

&&

$$

http://www.weather.gov/greatfalls