030
FXUS65 KTFX 190519
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1119 PM MDT Wed Jun 18 2025

Aviation Section Updated.

.KEY MESSAGES...

 - Well above normal temperatures linger on Thursday, with showers
   and thunderstorms developing through the afternoon hours. A few
   thunderstorms Thursday afternoon and evening could become
   strong to severe and produce damaging winds and hail.

 - Thunderstorms will once again be possible on Friday, with an
   isolated threat for a severe storm mainly across Central and
   Southwest Montana

 - Temperatures begin to cool on Friday, with well below normal
   temperatures expected across the Northern Rockies from Saturday
   through Tuesday.

 - The threat for accumulating snow continues to grow for
   elevations above 6000 feet, especially across Glacier National
   Park.

&&

.UPDATE...
/Issued 741 PM MDT Wed Jun 18 2025/

Some minor adjustments were made to the winds and haze grids based
on updated model guidance but otherwise no major changes to the
going forecast.  -thor

&&

.DISCUSSION...
/Issued 741 PM MDT Wed Jun 18 2025/

 - Meteorological Overview:

Rest of today through tonight...transient ridging moving over the
Northern Rockies will bring benign

On Thursday...the flow aloft becomes more southwest by the
afternoon, as upper level disturbances approach the CWA from the
Southwest. This will lead to scattered showers/thunderstorms
developing over Southwest and eventually Central MT by the late
afternoon hours. There will be enough instability for an isolated
severe storm.

For Friday...expect the southwest flow aloft to continue as the
main upper level low approaches from the west. There is the
potential for severe storms on Friday across the entire CWA.
However, cloud cover and morning precipitation could lower the
threat for severe storms over the western portions of North
Central MT a bit. Additionally, a cooling trend will begin on
Friday, with afternoon temperatures not quite as warm. The
moisture in the airmass will also increase to over one inch. This
will allow for the potential for some storms to produce heavy
rainfall, with the areas along the Highway 2 corridor from
Browning to Havre the most favored area at this time for heavy
rainfall.

For Saturday and Sunday...it will be officially summer by this
time, so a summer storm that is likely to produce winter
conditions is expected to affect the CWA during this period. Snow
levels will lower Saturday and Saturday night across the CWA, with
Sunday morning likely being the coldest period. Snow levels could
lower to around 5000 feet over the northern portions of the CWA.
Snowfall amounts could be impactful along the Rocky Mountain
Front, especially in Glacier Park. Overall, temperatures will be
well below normal across the entire CWA during this period.

For Monday through Wednesday, expect a nice warming trend with the
chances for precipitation becoming more isolated during this
period. - Brusda

 - Forecast Confidence & Scenarios:

Severe Thunderstorms and Heavy Rainfall on Thursday and Friday :

Some storms could produce strong wind gusts or large hail Thursday
and Friday. However, some of the higher resolution models are not
showing the wind gusts quite as strong as earlier thought.
Additionally, cloud cover on Friday could limit the area that could
be impacted by any severe storms.

Additionally, with precipitable water on the high side Friday, some
storms could produce heavy rainfall, or possible flash flood
conditions. Thus portions of the hi-line are in a marginal risk at
this time. - Brusda

Strong Winds from Friday through Saturday :

Increasing southwest flow during the day on Friday ahead of the
digging longwave trough will overspread Southwest Montana, with H700
wind speeds approaching 45kts along and southeast of a Butte to
Bozeman line during peaking heating/mixing. This flow orientation to
the north-south orientated Beaverhead and Madison Valleys will lead
to terrain funneling, which will help to support the potential for
strong to potentially high winds for areas like Dillon and Ennis
during the day on Friday. Latest NBM probabilities for wind gusts in
excess of 48kts at Dillon and Ennis are a 30% chance, with a 25% and
55% chance for wind speeds to be sustained at or above 34kts at
these two locations respectively.

The threat for strong to potentially high winds then shifts further
north over the plains of Central and North Central Montana on
Saturday as the longwave trough moves over the Northern Rockies,
with the highest ECMWF EFIs of 0.7 to nearly 0.9 located along and
west of the I-15 corridor to the Rocky Mountain Front indicating the
potential for a climatologically extreme wind event. NBM
probabilities for the Rocky Mountain Front and immediate plains out
to the I-15 corridor generally range from 40-70% chance with respect
to wind gusts in excess of 48kts, with a 35-70% chance for wind
speeds to be sustained at or above 34kts in the Browning and Cut
Bank Areas respectively.

Impactful Snow from Saturday through Sunday :

Snowfall amounts and probabilities continue to trend upwards through
the upcoming weekend, with all but 12% of the ECMWF ensemble members
supporting snow as far east as Browning from Saturday evening
through Sunday morning. NBM snow levels continue to struggle this
far out, with a reasonable middle of the road solution generally
supporting snow levels as low as 5000-6000ft along and east of the
Continental Divide across Central and North Central Montana. Despite
the NBM struggling with snow levels the probabilities for 12" of
snow across the higher elevations of Glacier National Park are
around a 50% chance, with a 30% chance for 6" or more of snow as low
as the Many Glacier and Two Medicine Valleys. Give these high
probabilities and the very real likelihood that snow levels will be
much lower along the immediate eastern slopes of the East Glacier
Park Region a Winter Storm Watch has been issued given the potential
for significant impacts to operations across Glacier National Park.

Hazardous travel conditions, especially on Going-to-the-Sun and
Looking Glass Roads and over the Hudson Bay Divide on US Hwy 89 due
to slushy snow accumulations on roadways are becoming an increasing
threat. Additionally, dangerous backcountry trail conditions will
exist, especially in areas of exposure. The heavy, wet snow and
gusty winds combined with foliage may lead to downed trees and power
outages. Those looking to recreate in the backcountry   of the
Glacier National Park should be prepared for adverse trail condtions
and consider altering their plans. - Moldan

&&

.AVIATION...
19/06Z TAF Period

VFR conditions will prevail through the period.

Aside from diurnally driven winds, the primary concern this TAF
period will be isolated to scattered showers and thunderstorms
developing across Southwest and Central Montana late Thursday
afternoon into the evening. Gusty winds and lightning will be the
main concern, with hail being a secondary concern. Thunderstorm
activity is expected to taper off by 20/06Z before the next system
arrives Friday morning bringing widespread stratiform
precipitation through the weekend.

Otherwise, brief instances of sub-VFR visibility will be possible
around KHLN from wildfire activity through the TAF period.
-AM/thor

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  85  54  70 /   0  20  70  90
CTB  49  79  50  61 /   0   0  60  90
HLN  52  85  55  75 /   0  50  60  90
BZN  47  88  52  84 /   0  40  60  40
WYS  37  82  43  74 /   0  10  30  20
DLN  45  86  50  79 /   0  40  50  40
HVR  54  85  54  76 /   0   0  70  80
LWT  49  80  52  76 /   0  30  70  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday afternoon through Sunday
morning for East Glacier Park Region.

&&

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