An unusually warm and moist airmass for early September was in place across Southern Montana and Northern Wyoming. A strong upper level disturbance aloft moved across the area during the late afternoon into the evening hours. As a result, a cluster of very strong to severe thunderstorms developed. Very heavy rain, large hail and strong wind gusts accompanied the thunderstorms.

Thunderstorm Wind in FALLON, MONTANA
From 07-SEP-13 15:15:00 to 07-SEP-13 15:15:00 (MST-7)
Location: OLLIE
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Thunderstorm winds pushed 1200 pound round hay bales through two fences.

Thunderstorm Wind in SWEET GRASS, MONTANA
From 07-SEP-13 16:30:00 to 07-SEP-13 16:30:00 (MST-7)
Location: MELVILLE
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A wind gust of 60 mph was estimated 20 miles northeast of Big Timber.

Thunderstorm Wind in SWEET GRASS, MONTANA
From 07-SEP-13 17:02:00 to 07-SEP-13 17:02:00 (MST-7)
Location: MELVILLE
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

A wind gust of 70 mph was estimated 12 miles west/northwest of Rapelje.

Hail in CARTER, MONTANA
From 07-SEP-13 17:15:00 to 07-SEP-13 17:15:00 (MST-7)
Location: MILL IRON
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in SWEET GRASS, MONTANA
From 07-SEP-13 17:02:00 to 07-SEP-13 17:02:00 (MST-7)
Location: MELVILLE
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Thunderstorm Wind in STILLWATER, MONTANA
From 07-SEP-13 17:30:00 to 07-SEP-13 17:30:00 (MST-7)
Location: RAPELJE
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A wind gust of 60 mph was estimated in Rapelje.

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 19:35:00 to 07-SEP-13 19:35:00 (MST-7)
Location: RIMROCK
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 19:35:00 to 07-SEP-13 19:35:00 (MST-7)
Location: RIMROCK
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 19:47:00 to 07-SEP-13 19:47:00 (MST-7)
Location: HESPER
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 19:58:00 to 07-SEP-13 19:58:00 (MST-7)
Location: YEGEN
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 19:50:00 to 07-SEP-13 19:50:00 (MST-7)
Location: RIMROCK
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

A wind gust of 70 mph was estimated just west of Billings.

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 20:00:00 to 07-SEP-13 20:00:00 (MST-7)
Location: BILLINGS
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

A wind gust of 60 mph was estimated just southeast of Billings.

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 20:00:00 to 07-SEP-13 20:00:00 (MST-7)
Location: RIMROCK
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in YELLOWSTONE, MONTANA
From 07-SEP-13 20:05:00 to 07-SEP-13 20:05:00 (MST-7)
Location: RIMROCK
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 20:05:00 to 07-SEP-13 20:05:00 (MST-7)
Location: (BIL)LOGAN ARPT BILL
Magnitude: 63.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 20:20:00 to 07-SEP-13 20:20:00 (MST-7)
Location: LOCKWOOD
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

A wind gust of 60 mph along with very heavy rain and small hail was reported 4 miles east of Billings.

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 20:39:00 to 07-SEP-13 20:39:00 (MST-7)
Location: HUNTLEY
Magnitude: 63.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Thunderstorm Wind in YELLOWSTONE, MONTANA
From 07-SEP-13 20:50:00 to 07-SEP-13 20:50:00 (MST-7)
Location: POMPEYS PILLAR
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Power outages were reported as a result of thunderstorm wind gusts betwen 50 and 60 mph.

Flash Flood in MUSSELSHELL, MONTANA
From 07-SEP-13 20:30:00 to 07-SEP-13 21:30:00 (MST-7)
Location: ELSO
Flood cause: Heavy Rain / Burn Area
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Flash flooding was reported on the Dahl Burn Scar area with water running across numerous roads.

Flash Flood in MUSSELSHELL, MONTANA
From 07-SEP-13 20:30:00 to 07-SEP-13 21:30:00 (MST-7)
Location: DELPHIA
Flood cause: Heavy Rain / Burn Area
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Flash flooding was reported on the Delphia Fire Burn Scar area with water running across numerous roads.

Flash Flood in YELLOWSTONE, MONTANA
From 07-SEP-13 20:15:00 to 07-SEP-13 21:30:00 (MST-7)
Location: RIMROCK
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A severe thunderstorm packing high winds, hail, and torrential rain battered the city of Billings and nearby areas during the evening of the 7th. Almost 2 inches of rain fell in 30-45 minutes causing significant flooding for some residential areas, as well as in the downtown area. Winds gusted to 73 mph at the Billings Airport which resulted in power outages for thousands of people across town. Some power poles were knocked down and numerous large trees were uprooted. In addition, half-dollar size hail occurred just west of Billings.
A large rock blocked Zimmerman Trail and Highway 3 west of Zimmerman Trail had drifts of hail alongside the road. The rain that washed off the Rimrocks pushed debris, including mud and rocks, onto North 27th Street near Montana State University Billings. High water at the corner of First Avenue North and Main Street caused multiple cars to stall on the road. In addition, all of the underpasses in the downtown area were closed due to flooding.

Hail in FALLON, MONTANA
From 07-SEP-13 14:10:00 to 07-SEP-13 14:10:00 (MST-7)
Location: WESTMORE
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in FALLON, MONTANA
From 07-SEP-13 14:40:00 to 07-SEP-13 14:40:00 (MST-7)
Location: BAKER
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in FALLON, MONTANA
From 07-SEP-13 14:59:00 to 07-SEP-13 14:59:00 (MST-7)
Location: BAKER ARPT
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

The golf ball size hail was accompanied by very strong winds.

Heavy Rain in FALLON, MONTANA
From 07-SEP-13 15:00:00 to 07-SEP-13 15:00:00 (MST-7)
Location: BAKER ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Very heavy rain of 1.50 inches in 20 minutes was reported.

Thunderstorm Wind in FALLON, MONTANA
From 07-SEP-13 15:10:00 to 07-SEP-13 15:10:00 (MST-7)
Location: OLLIE
Magnitude: 56.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

The gust was measured by the Cannonball Creek RAWS.