Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
213 PM MDT Mon Aug 2 2021

.SYNOPSIS...

Showers and thunderstorms over Southwestern Montana and portions of
Central Montana are expected to persist into the overnight hours.
Locally heavy rain will continue to be possible with these storms
and could produce flash flooding and debris flows, especially in
recently burn area. After some lingering showers on Tuesday, hot and
dry weather returns for Wednesday and early Thursday. Unsettled
conditions and slightly cooler temperatures can be expected for the
weekend.

&&

.DISCUSSION...

Anomalously high levels of monsoon moisture remain in place over
much of the forecast area as a weak shortwave approaches from the
west. Shower and thunderstorm development has already begun over
the southwest and this activity should expand north and eastward
later this afternoon and evening. So far the strongest storms have
occurred in Jefferson county from Boulder to to just southeast of
the Helena valley. These rainfall rates will be common this
afternoon and evening due to the combination of PWAT values being
several standard deviations higher than climatology and relatively
storm motion 10 to 15 mph. It's important to note that the
convective nature of the storms may lead to some areas receiving
very heavy rainfall while others get little to no rain. Anyone
outdoors should be be especially careful as the heavier cells may
cause dangerous mud/debris flows, particularly over any burn scars
or higher and uneven terrain. The current flash flood watch is
handling the situation reasonably well, so no changes will be made
at this time. However, Lewis and Clark County could possibly be
added to the watch area later this afternoon. Showers and storms
will begin to shift eastward and weaken by late this evening, but
expect at least scattered showers and some storms to persist
through Tuesday morning. -RCG

Rain showers may linger during the day on Tuesday, mostly for the
eastern portions of Central Montana. Then a trend toward mostly
dry and warmer conditions is expected through Thursday as an upper
level ridge rebuilds over the Northern Rockies. Unsettled weather
and cooler temperatures return Friday and continue through the
weekend as an upper trough moves from the Pacific Northwest into
the Northern Plains. mpj

&&

.AVIATION...
1152 AM MDT Mon Aug 02 2021 (02/18Z TAF period)

Monsoonal moisture will continue to bring broken/overcast mid and
upper level clouds to southwestern and north central Montana today
through Tuesday morning. Thunderstorm development is already ongoing
over the southwest and will become more pronounced by 19Z. It is
expected to be most widespread south of a KHLN to KGTF to KLWT line
with a more isolated to scattered brand of showers to the north of
the line. Some storms could produce heavy rain with gusty and
erratic winds. VFR conditions should prevail for much of the region
but precipitation will bring IFR/MVFR conditions with mountain
obscurations expected through the period. There should be some west
to east improvement by 03/04Z, but showers and some thunderstorms
will linger over into the overnight hours. -RCG

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

&&

.HYDROLOGY...

Monsoonal moisture continues to move northward through the region
this afternoon and tonight with numerous showers and thunderstorms
expected for western and southern portions of the forecast area. The
potential remains high for any shower or thunderstorm to bring heavy
rainfall given slow storm motions and anomalously high precipitable
water values. Some areas could see flash flooding, especially
in/near areas that have recently burned and in urban areas. A Flash
Flood Watch remains in effect for all of southwestern Montana and a
portion of central Montana from noon through midnight.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  62  92  57  92 /  80  10   0  10
CTB  60  89  54  87 /  40  10   0  10
HLN  60  93  61  96 /  50  10   0  10
BZN  56  88  53  91 /  80  10   0  10
WYS  46  79  40  82 /  80  10   0  10
DLN  53  85  52  88 /  70   0   0  10
HVR  64  92  59  91 /  40  20  10  10
LWT  62  86  57  88 /  50  50   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until midnight MDT tonight Beaverhead...
Broadwater...Cascade...Gallatin...Jefferson...Madison...Meagher.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
438 PM MDT Mon Aug 2 2021

MTZ012-013-050-022315-
Chouteau-Cascade-Judith Basin-
438 PM MDT Mon Aug 2 2021

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT NORTHWESTERN JUDITH
BASIN...SOUTH CENTRAL CHOUTEAU AND NORTHEASTERN CASCADE COUNTIES...

At 437 PM MDT, strong thunderstorms were clustered 9 miles west of
Geyser, or 24 miles northwest of Stanford, moving north at 15 mph.

Locations impacted include...
Fort Benton, Belt, Geraldine, Geyser, Highwood, Shonkin, Raynesford,
Armington, Monarch and Sluice Boxes State Park.

This includes the following highways...
 Highway 200 between mile markers 0 and 23.
 Highway 87 between mile markers 33 and 43.
 Highway 89 between mile markers 46 and 71, and between mile markers
72 and 73.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

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

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4700 11063 4709 11098 4789 11084 4776 11006
TIME...MOT...LOC 2237Z 197DEG 14KT 4731 11069

SC

Special Weather Statement
National Weather Service Great Falls MT
442 PM MDT Mon Aug 2 2021

MTZ015-052-053-055-022315-
Broadwater-Madison-Jefferson-Gallatin-
442 PM MDT Mon Aug 2 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT WESTERN
BROADWATER...NORTH CENTRAL MADISON...EASTERN JEFFERSON AND
NORTHWESTERN GALLATIN COUNTIES...

At 442 PM MDT, radar indicated strong thunderstorms were located
along a line extending from 7 miles southwest of Montana City to 16
miles northwest of Three Forks to near Twin Bridges. Movement was
east at 15 mph.

Periods of heavy rain and winds in excess of 30 mph are possible
with these storms.

Locations impacted include...
Townsend, Three Forks, Twin Bridges, Pony, Montana City, Toston,
Harrison, Alhambra, Corbin, Elkhorn State Park, Cardwell, Winston,
Radersburg, Lewis And Clark Caverns State Park, Silos, Clancy, Willow
Creek and Jefferson City.

This includes the following highways...
 Interstate 15 between mile markers 173 and 186.
 Interstate 90 between mile markers 252 and 278.
 Highway 12 between mile markers 55 and 109.
 Highway 287 between mile markers 72 and 97.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4603 11191 4651 11213 4657 11181 4657 11165
      4661 11163 4606 11139 4547 11190 4550 11237
TIME...MOT...LOC 2242Z 265DEG 15KT 4645 11202 4604 11183 4557 11224

SC

Special Weather Statement
National Weather Service Great Falls MT
506 PM MDT Mon Aug 2 2021

MTZ012-048-049-022330-
Cascade-Southern Rocky Mountain Front-Eastern Teton-
506 PM MDT Mon Aug 2 2021

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT SOUTHEASTERN TETON
AND NORTHWESTERN CASCADE COUNTIES...

At 505 PM MDT, strong thunderstorms were clustered 7 miles northeast
of Fairfield, or 15 miles southeast of Choteau, moving northeast at
15 mph.

Periods of heavy rain and winds in excess of 40 mph are possible
with this storm.

Locations impacted include...
Fairfield, Dutton, Power, Collins, Vaughn and Sun River.

This includes the following highways...
 Interstate 15 between mile markers 290 and 324.
 Highway 200 near mile marker 139.
 Highway 89 between mile markers 9 and 39, and between mile markers 1
and 8.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4756 11146 4750 11203 4777 11220 4799 11180
      4798 11168
TIME...MOT...LOC 2305Z 238DEG 11KT 4770 11189

SC

Special Weather Statement
National Weather Service Great Falls MT
511 PM MDT Mon Aug 2 2021

MTZ012-013-049-030015-
Chouteau-Cascade-Eastern Teton-
511 PM MDT Mon Aug 2 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT EAST CENTRAL
TETON...WESTERN CHOUTEAU AND CENTRAL CASCADE COUNTIES...

At 510 PM MDT, radar indicated a gust front from decaying
thunderstorms was located along a line extending from 9 miles
northeast of The Knees to 10 miles southeast of Cascade. Movement
was east at 25 mph.

Winds in excess of 40 mph are possible with this gust front.

Locations impacted include...
Great Falls, Fort Benton, Belt, Black Eagle, The Knees, Highwood,
Carter, Floweree, Portage, Eden, Sluice Boxes State Park, Malmstrom
Afb, Stockett, Tracy, First Peoples Buffalo Jump State Park,
Centerville, Armington, Ulm and Sand Coulee.

This includes the following highways...
 Interstate 15 between mile markers 267 and 289.
 Highway 200 between mile markers 0 and 6.
 Highway 87 between mile markers 4 and 49.
 Highway 89 between mile markers 56 and 71, and between mile markers
72 and 92.

If outdoors, consider seeking shelter inside a building.

LAT...LON 4817 11044 4698 11093 4703 11111 4704 11108
      4709 11109 4708 11116 4701 11117 4700 11129
      4700 11144 4706 11170 4813 11128 4813 11083
      4819 11083
TIME...MOT...LOC 2310Z 283DEG 20KT 4814 11118 4714 11158

SC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
521 PM MDT MON AUG 2 2021

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

0503 PM     HEAVY RAIN       4 SSW POWER             47.66N 111.72W
08/02/2021  M0.06 INCH       TETON              MT   TRAINED SPOTTER

            PEAK RAIN RATE, 0.41 INCHES PER HOUR
            MEASURED AT 503 PM. RAIN FELL AT THIS RATE,
            FOR A COUPLE MINUTES.

&&

JAF

Special Weather Statement
National Weather Service Great Falls MT
522 PM MDT Mon Aug 2 2021

MTZ013-030000-
Chouteau-
522 PM MDT Mon Aug 2 2021

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT CENTRAL CHOUTEAU
COUNTY...

At 521 PM MDT, strong thunderstorms were clustered 10 miles east of
Highwood, or 13 miles south of Fort Benton, moving north at 20 mph.

Periods of heavy rain and winds in excess of 30 mph are possible
with this storm.

Locations impacted include...
Fort Benton, Highwood, Shonkin and Loma.

This includes Highway 87 between mile markers 37 and 56.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

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

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4795 11009 4745 11046 4750 11077 4753 11079
      4802 11075
TIME...MOT...LOC 2321Z 197DEG 16KT 4764 11055

SC