274
FXUS65 KTFX 230636
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1236 AM MDT Thu May 23 2024

...Aviation Section Update...

.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.

&&

.UPDATE...

No significant adjustments needed to the overnight period of the
forecast, just a few tweaks.

Showers will persist over Southwest Montana overnight, but the
threat for additional thunderstorms there has decreased
significantly. Farther north, widespread showers have mostly
stayed north and east of Great Falls, along and north of a quasi-
stationary frontal boundary. However, an upper level jet max of
northeasterly winds will move south over the plains overnight,
which should push this boundary, as well as the widespread
precipitation, into the Great Falls area by midnight. This jet max
will also bring gusty northerly winds to much of the area
overnight into Thursday morning. Otherwise, the forecast appears
to be on track with precipitation initially decreasing on the Hi-
Line after midnight as the low pressure system moves east into
northwest Wyoming, then over Central and Southwest Montana during
the day on Thursday as the system continues its trek east.
However, a shortwave trough in the northwest flow aloft left in
the wake of this system will bring an increased chance for showers
to North Central Montana again during the afternoon and evening.
-Coulston

&&

.AVIATION...
23/06Z TAF Period

Note: The NWS Great Falls Weather Forecast Office manages the
following TAFs: KCTB, KHVR, KGTF, KLWT, KHLN, KBZN, KEKS and KWYS.

A wintry spring storm impacts our region, today, causing conditions
to reside in the MVFR/IFR/LIFR range for most of this 23/06Z TAF
period. Periods of heavy snowfall is forecast to occur over the
mountainous areas, with rain/snow at lower elevations. Mountain
obscuration should be expected. Today, generally, northerly winds
gust up to 35KT along The Northern Rocky Mountain Front and portions
of central Montana, while winds gust to 25KT across southwestern
Montana. During tonight`s evening hours, winds begin to settle,
becoming light and variable.

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

&&

.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.

&&

.PREV DISCUSSION...
/ISSUED 520 PM MDT Wed May 22 2024/

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

&&

.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