Severe Thunderstorm Warning 112, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
133 PM MDT Tue Aug 20 2024

.SYNOPSIS...

Moist, southwest flow will maintain chances for showers and
thunderstorms across Southwest through North Central Montana
through Wednesday. A few of thunderstorms over this timeframe
could become severe and produce damaging winds and large hail.
Temperatures cool near to below normal for Wednesday and
Thursday, but warm back above normal for the day on Friday.

&&

.DISCUSSION...

Rest of today through Thursday...two more rounds of showers and
thunderstorms are expected from the afternoon through evening hours
today and then again on Wednesday as moist, southwest flow remains
in place over the Northern Rockies. A few of the thunderstorms today
and Wednesday could become strong to severe, with the primary
threats being large hail and damaging winds. For the remainder of
the day today areas along and east of a Havre, to Great Falls, to
Butte line have a Marginal Risk for severe thunderstorms, with a
Slight Risk existing east of a Hays to White Sulphur Springs line.
On Wednesday the Marginal Risk for severe thunderstorms shifts
east of a Havre to Judith Gap line, with garden variety
thunderstorms expected across the remainder of the Southwest
through North Central Montana. High temperatures and Wednesday and
Thursday will cool in wake of several weak cold front, with most
locations ranging from the upper 70s to mid-80s. Overall dry
conditions are then expected on Thursday, with increasing
southerly winds within the north-south orientated valleys of
Southwest Montana.

Friday...ensemble clusters are in strong agreement with an amplified
longwave trough digging southeast over the Western Seaboard and
Pacific Northwest/Great Basin/Northern California through the day on
Friday. Leading mode (53% of the variance) of uncertainty with this
pattern will be the exact position and/or timing of the longwave
trough as it digs southeast before eventually pivoting east over the
Western CONUS, with ~83% of the ensemble members keeping the
longwave trough axis just off of the Oregon and California Coasts
during the day on Friday while ~17% of the ensemble clusters bring
the trough axis over portions of Oregon and California by the early
evening hours. This position and/or timing difference will
primarily have impacts on when and where showers and thunderstorms
develop during the afternoon and evening hours on Friday across
Southwest Montana and along the Continental Divide. In addition to
the aforementioned precipitation, a strong warm front lifting north
within an overall southerly flow regime will help to push
temperatures into the 80s to mid-90s across the CWA, with highs
running anywhere from 5 to 10 degrees above normal across most
locations save for those areas south of the I-90 corridor where
values will be less than 5 degrees above normal. Of biggest concern
on Friday will the be threat for strong winds, most notably across
Southwest Montana where EFI values are currently ranging anywhere
from 0.7 to 0.95, with shift of tails are 0 to 1. H700 wind
anomalies through the day on Friday across these same areas are 2 to
nearly 3 standard deviations above normal, with these values
approaching the 97.5 percentile of climatology. NBM probabilities
for maximum sustained wind speeds in excess of 22kts generally range
from 75-95% along and south of the I-90 corridor on Friday, with the
chance for maximum wind gusts in excess of 48kts generally ranging
from 5-35%. While these values don't necessarily support a High Wind
scenario across Southwest Montana; the orientation of the winds,
which will be parallel to the north-south orientated valleys,
combined with deep mixing given the season are rather concerning. -
Moldan

Saturday through next Tuesday...60 percent of the ensemble model
forecast clusters are in agreement in bringing the trough onshore
and into the Western United States Saturday, weakening as it moves
northeast over the Central and Northern Rockies on Sunday and into
the northern Great Plains on Monday. This will increase chances for
additional showers and thunderstorms across the forecast area
through the weekend into early next week, as temperatures fall to
between 5 and 10 degrees below normal. However, for Tuesday, there
is discrepancy regarding weak features in a more progressive
westerly flow aloft, so the model blend result is to keep
temperatures slightly below normal with just a slight chance of
precipitation. - Coulston

&&

.AVIATION...
20/18Z TAF Period

VFR conditions will prevail during this TAF period except for during
any thunderstorm when visibilities will be briefly reduced to MVFR
levels. From now through 21/00Z there is a 20 - 40% chance of
thunderstorms at all terminals excluding the KHVR terminal. From
21/00Z through 21/06Z there is a 20 - 30% chance for thunderstorms
at all terminals excluding the KWYS terminal. From 21/06 - 21/12Z
there is 40 - 60% chance for showers and thunderstorms at KCTB,
KHVR, and KGTF terminals. From 21/12Z through the end of the TAF
period there is a 20 - 30% chance for showers and thunderstorms at
the KHVR and KCTB terminals. At the KEKS terminal there will be
wildfire smoke around beginning early tomorrow morning and
continuing through the end of the TAF period. All thunderstorms will
produce frequent lightning, gusty erratic winds, and brief heavy
rainfall. An isolated thunderstorm at all terminals excluding the
KWYS, KEKS, and KCTB terminals will produce strong to severe wind
gusts and/or large hail. Thunderstorms will cause brief periods of
mountain obscuration. -IG

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  54  82  49  82 /  30  30  20  10
CTB  55  78  44  78 /  30  30  10   0
HLN  56  84  50  87 /  20  30  20   0
BZN  48  86  44  88 /  20  30  20   0
WYS  41  78  34  80 /   0   0   0   0
DLN  47  80  42  83 /  20  30  10   0
HVR  58  86  51  86 /  40  20  20   0
LWT  51  81  47  82 /  30  50  40  10

&&

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

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
909 AM MDT Tue Aug 20 2024

MTZ329-330-201545-
Ruby Mountains and Southern Beaverhead Mountains MT-
Gallatin and Madison County Mountains and Centennial Mountains MT-
909 AM MDT Tue Aug 20 2024

...STRONG THUNDERSTORMS WILL IMPACT SOUTH CENTRAL MADISON AND CENTRAL
BEAVERHEAD COUNTIES THROUGH 945 AM MDT...

At 908 AM MDT, Doppler radar was tracking a cluster of strong
thunderstorms 16 miles northeast of Lima, or 31 miles southeast of
Dillon, moving northeast at 35 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

This storm will remain over mainly rural areas of south central
Madison and central Beaverhead Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4474 11243 4484 11251 4501 11213 4481 11197
TIME...MOT...LOC 1508Z 242DEG 30KT 4481 11235

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

IG

Special Weather Statement
National Weather Service Great Falls MT
943 AM MDT Tue Aug 20 2024

MTZ325-328>330-201615-
Madison River Valley MT-
Ruby Mountains and Southern Beaverhead Mountains MT-
Gallatin and Madison County Mountains and Centennial Mountains MT-
Beaverhead and Western Madison below 6000ft MT-
943 AM MDT Tue Aug 20 2024

...STRONG THUNDERSTORMS WILL IMPACT SOUTHEASTERN MADISON AND CENTRAL
BEAVERHEAD COUNTIES THROUGH 1015 AM MDT...

At 943 AM MDT, Doppler radar was tracking a cluster of strong
thunderstorms 25 miles south of Alder, or 25 miles southwest of
Virginia City, moving northeast at 35 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

This storm will remain over mainly rural areas of southeastern
Madison and central Beaverhead Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4479 11243 4489 11252 4523 11175 4502 11157
TIME...MOT...LOC 1543Z 242DEG 29KT 4495 11214

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

IG

Preliminary Local Storm Report
National Weather Service Great Falls MT
948 AM MDT Tue Aug 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0945 PM     Tstm Wnd Gst     11 N Roy                47.49N 108.95W
08/19/2024  M57 MPH          Fergus             MT   Mesonet

            Mesonet station MTM42 Blm Roy.

&&

JTG

Preliminary Local Storm Report
National Weather Service Great Falls MT
955 AM MDT Tue Aug 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0845 PM     Tstm Wnd Gst     11 WSW Big Sandy        48.10N 110.33W
08/19/2024  M55 MPH          Chouteau           MT   Mesonet

            Mesonet station MTM90 Big Sandy Sw.

&&

JTG

Special Weather Statement
National Weather Service Great Falls MT
1007 AM MDT Tue Aug 20 2024

MTZ325-328>330-201630-
Madison River Valley MT-
Ruby Mountains and Southern Beaverhead Mountains MT-
Gallatin and Madison County Mountains and Centennial Mountains MT-
Beaverhead and Western Madison below 6000ft MT-
1007 AM MDT Tue Aug 20 2024

...A STRONG THUNDERSTORM WILL IMPACT CENTRAL MADISON AND SOUTHEASTERN
BEAVERHEAD COUNTIES THROUGH 1030 AM MDT...

At 1007 AM MDT, Doppler radar was tracking a strong thunderstorm 16
miles southwest of Virginia City, moving northeast at 45 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Virginia City, Ennis, and Cameron.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4491 11216 4503 11233 4540 11178 4519 11152
TIME...MOT...LOC 1607Z 229DEG 38KT 4508 11207

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

IG

Special Weather Statement
National Weather Service Great Falls MT
141 PM MDT Tue Aug 20 2024

MTZ327-328-202015-
Northwest Beaverhead County MT-
Beaverhead and Western Madison below 6000ft MT-
141 PM MDT Tue Aug 20 2024

...A STRONG THUNDERSTORM WILL IMPACT CENTRAL BEAVERHEAD COUNTY
THROUGH 215 PM MDT...

At 140 PM MDT, Doppler radar was tracking a strong thunderstorm 12
miles west of Dillon, moving northeast at 35 mph.

HAZARD...Wind gusts of 50 to 55 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Dillon, Bannack, and Bannack State Park.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4508 11292 4516 11303 4539 11273 4521 11248
TIME...MOT...LOC 1940Z 228DEG 29KT 4516 11288

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...55 MPH

IG