A moist southeast surface flow resulting in 50 to 60 degree dewoints along with strong deep shear combined with a strong mid-level speed max were centered over eastern Montana. As a result, severe thunderstorms developed across Rosebud, Custer and Fallon Counties. These thunderstorms produced very large hail, damaging wind gusts, as well as a tornado.

Thunderstorm Wind in POWDER RIVER, MONTANA
From 11-JUN-16 17:44:00 to 11-JUN-16 17:44:00 (MST-7)
Location: MOORHEAD
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Strong winds flipped over a snowmobile trailer.

Hail in CUSTER, MONTANA
From 11-JUN-16 17:10:00 to 11-JUN-16 17:10:00 (MST-7)
Location: BONFIELD
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Storm Chaser

A storm chaser reported golf ball size hail along with 50 mph winds.

Thunderstorm Wind in CUSTER, MONTANA
From 11-JUN-16 17:00:00 to 11-JUN-16 17:00:00 (MST-7)
Location: MILES CITY
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Tree limbs 3 to 4 inches in diameter were downed. In addition, golf ball to two-inch size hail was also reported.

Hail in CUSTER, MONTANA
From 11-JUN-16 17:00:00 to 11-JUN-16 17:00:00 (MST-7)
Location: MILES CITY
Magnitude: 2.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in CUSTER, MONTANA
From 11-JUN-16 16:57:00 to 11-JUN-16 16:57:00 (MST-7)
Location: MILES CITY
Magnitude: 2.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in CUSTER, MONTANA
From 11-JUN-16 16:55:00 to 11-JUN-16 16:55:00 (MST-7)
Location: MILES CITY
Magnitude: 2.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in CUSTER, MONTANA
From 11-JUN-16 16:53:00 to 11-JUN-16 16:53:00 (MST-7)
Location: MILES CITY
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in CUSTER, MONTANA
From 11-JUN-16 16:50:00 to 11-JUN-16 16:50:00 (MST-7)
Location: MILES CITY
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Some windows were broken.

Hail in CUSTER, MONTANA
From 11-JUN-16 16:25:00 to 11-JUN-16 16:25:00 (MST-7)
Location: FT KEOGH
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in CUSTER, MONTANA
From 11-JUN-16 16:22:00 to 11-JUN-16 16:22:00 (MST-7)
Location: FT KEOGH
Magnitude: 70.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Storm Chaser

A storm chaser estimated wind speeds of 80 mph.

Thunderstorm Wind in POWDER RIVER, MONTANA
From 11-JUN-16 15:34:00 to 11-JUN-16 15:34:00 (MST-7)
Location: COALWOOD
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Branches 3 to 4 inches in diameter were broken off trees.

Tornado in FALLON, MONTANA
From 11-JUN-16 17:54:00 to 11-JUN-16 18:04:00 (MST-7)
Location: BAKER ARPT
Tornado F-scale: EF3
Property damage: 1.50M
Reported by: Public

The pre-storm environment for eastern Montana was such that severe weather was likely. Surface dew point values in the Baker area were in the lower 60s with strong southeasterly flow converging in the vicinity of a stationary surface trough. This area of strong surface convergence, which also separated moist rich air to the east and drier air to the west, played a critical role in the development of this short-lived tornado.
Although no fatalities occurred , seven injuries, some requiring hospitalization, resulted from the tornado. Two homes were totally destroyed and 40 to 50 more sustained excessive damage. The city blocks located between South 5th Street East and South 10th Street East intersected by Texas Avenue were hit the hardest and sustained the most damage. Many power poles were broken resulting in power lines down and power outages. Single tree trunks were described to look like match sticks as they were stripped of all branches and leaves.
Over 70 personnel came with boots on the ground from the surrounding tri-state area. The communities of Plevna, Ekalaka, and Wibaux, Montana, Marmarth, Rhame, Bowman and Beach, North Dakota, as well as Ludlow, South Dakota responded shortly after the tornado.
At the mesoscale level (2 to 2000 km in space), the interaction of pre-existing convergence zones or boundaries can serve as areas of storm development or enhancement. In some cases, pre-existing thunderstorms interacting with a boundary of converging surface winds can rapidly develop short-lived tornadoes with very little precursor indication from weather radar. The time scale of these events typically is on the order of a few minutes (0-10 minutes).
In the case of the thunderstorms near Baker on Saturday, June 11th, evidence of these types of boundaries was observed.

Hail in ROSEBUD, MONTANA
From 11-JUN-16 18:56:00 to 11-JUN-16 18:56:00 (MST-7)
Location: BRANDENBERG
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter