A strong upper level wave moved across Eastern Montana during the afternoon and evening hours of the 16th. An axis of instability combined with strong and low, deep layer shear and high dewpoints resulted in a very unstable environment. This resulted in several supercell thunderstorms that produced large hail, damaging winds and one confirmed tornado across the eastern portions of the Billings Forecast Area.

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:40:00 to 21-JUN-15 19:40:00 (MST-7)
Location: MIZPAH
Magnitude: 56.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:40:00 to 21-JUN-15 19:40:00 (MST-7)
Location: MIZPAH
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Quarter size hail was also reported.

Tornado in FALLON, MONTANA
From 21-JUN-15 15:37:00 to 21-JUN-15 15:42:00 (MST-7)
Location: WILLARD
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Storm Chaser

A weak tornado was reported between Baker and Ekalaka during the late afternoon hours on the 21st. The tornado was on the ground for around a minute. It began roping out about 4 minutes later and its circulation appeared to fully cut off around 90 seconds after that.

Hail in CUSTER, MONTANA
From 21-JUN-15 14:45:00 to 21-JUN-15 14:45:00 (MST-7)
Location: ISMAY
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A spotter reported golf ball size hail with windows knocked out of the house.

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 14:55:00 to 21-JUN-15 14:55:00 (MST-7)
Location: KNOWLTON
Magnitude: 61.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A spotter reported baseball size hail that destroyed their windshield and a wind gust of 70 mph.

Hail in CUSTER, MONTANA
From 21-JUN-15 19:40:00 to 21-JUN-15 19:40:00 (MST-7)
Location: MIZPAH
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

A wind gust of 60 mph was also reported.

Thunderstorm Wind in FALLON, MONTANA
From 21-JUN-15 20:39:00 to 21-JUN-15 20:39:00 (MST-7)
Location: BAKER ARPT
Magnitude: 61.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:10:00 to 21-JUN-15 19:10:00 (MST-7)
Location: (MLS)WILEY FLD MILES
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Hail in CUSTER, MONTANA
From 21-JUN-15 14:55:00 to 21-JUN-15 14:55:00 (MST-7)
Location: KNOWLTON
Magnitude: 2.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A spotter reported baseball size hail that destroyed their windshield and a wind gust of 70 mph.

Hail in FALLON, MONTANA
From 21-JUN-15 15:00:00 to 21-JUN-15 15:00:00 (MST-7)
Location: WESTMORE
Magnitude: 2.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Golf ball to tennis ball size hail 3 to 4 inches deep was reported along the road. Trees were completely stripped bare like winter time. Roadside reflectors were completely shattered all along Highway 12 at mile marker 55.

Hail in FALLON, MONTANA
From 21-JUN-15 15:40:00 to 21-JUN-15 15:40:00 (MST-7)
Location: WILLARD
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in ROSEBUD, MONTANA
From 21-JUN-15 18:35:00 to 21-JUN-15 18:35:00 (MST-7)
Location: FORSYTH
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:02:00 to 21-JUN-15 19:02:00 (MST-7)
Location: FT KEOGH
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

A pull camper was blown over at mile marker 132.

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:10:00 to 21-JUN-15 19:10:00 (MST-7)
Location: (MLS)WILEY FLD MILES
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in CUSTER, MONTANA
From 21-JUN-15 19:14:00 to 21-JUN-15 19:14:00 (MST-7)
Location: (MLS)WILEY FLD MILES
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS