Several severe thunderstorms moved across the Billings Forecast Area during the late afternoon into the evening hours of the 31st. The most notable severe thunderstorm moved across the Roundup area. This thunderstorm produced very large hail along with flash flooding. A damage survey was conducted which found that a majority of homes and businesses within Roundup experienced some type of hail damage. Damage was to the north and west facing aspects of homes and businesses with numerous broken windows and pitted siding and fencing. Six county buildings along with the hospital and high school were damaged.  Approximately 40 to 50 percent of the local businesses were damaged.  In addition, 100 percent of vehicles and campers left in the open were damaged with dents or windshields broken. Trees were stripped of leaves.
Flooding also occurred across town and portions of the surrounding area. Highway 87 had several areas where debris had washed over the road from floodwaters. Main Street through Roundup was similar, with low lying areas (in and around the Busy Bee Cafe) being flooded. In addition, Bender Road was washed out between Log Cabin Road and County Road #4. The peak river stage of the Musselshell River at Roundup was around 11.5 feet, a foot and a half above flood stage and the sixth highest crest on record. One home was evacuated due to being surrounded by floodwaters.

Hail in PARK, MONTANA
From 31-MAY-18 14:14:00 to 31-MAY-18 14:14:00 (MST-7)
Location: GRANNIS
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in WHEATLAND, MONTANA
From 31-MAY-18 14:20:00 to 31-MAY-18 14:20:00 (MST-7)
Location: SHAWMUT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in PARK, MONTANA
From 31-MAY-18 14:35:00 to 31-MAY-18 14:35:00 (MST-7)
Location: GRANNIS
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in WHEATLAND, MONTANA
From 31-MAY-18 14:52:00 to 31-MAY-18 14:52:00 (MST-7)
Location: SHAWMUT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

There was also pea size hail 1 inch deep on the highway.

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 16:40:00 to 31-MAY-18 16:40:00 (MST-7)
Location: ROUNDUP
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Seven windows were broken in a home.

Hail in SWEET GRASS, MONTANA
From 31-MAY-18 16:00:00 to 31-MAY-18 16:00:00 (MST-7)
Location: MELVILLE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 16:30:00 to 31-MAY-18 16:30:00 (MST-7)
Location: ROUNDUP
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Some broken windows were reported.

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 16:45:00 to 31-MAY-18 16:45:00 (MST-7)
Location: ROUNDUP
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

There was widespread golf ball to tennis ball size hail in Roundup. Please see Episode Narrative for this event.

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 16:47:00 to 31-MAY-18 16:47:00 (MST-7)
Location: ROUNDUP
Magnitude: 2.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

There was widespread golf ball to tennis ball size hail in Roundup. Please see Episode Narrative for this event.

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 16:48:00 to 31-MAY-18 16:48:00 (MST-7)
Location: ROUNDUP
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in MUSSELSHELL, MONTANA
From 31-MAY-18 16:48:00 to 31-MAY-18 16:48:00 (MST-7)
Location: ROUNDUP
Magnitude: 52.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Hail in MUSSELSHELL, MONTANA
From 31-MAY-18 17:00:00 to 31-MAY-18 17:00:00 (MST-7)
Location: GAGE
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in ROSEBUD, MONTANA
From 31-MAY-18 17:30:00 to 31-MAY-18 17:30:00 (MST-7)
Location: BIRNEY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in BIG HORN, MONTANA
From 31-MAY-18 17:38:00 to 31-MAY-18 17:38:00 (MST-7)
Location: KIRBY
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in GOLDEN VALLEY, MONTANA
From 31-MAY-18 18:12:00 to 31-MAY-18 18:12:00 (MST-7)
Location: LAVINA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in ROSEBUD, MONTANA
From 31-MAY-18 19:55:00 to 31-MAY-18 19:55:00 (MST-7)
Location: SUMATRA
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Hail in BIG HORN, MONTANA
From 31-MAY-18 21:40:00 to 31-MAY-18 21:40:00 (MST-7)
Location: HARDIN
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in GOLDEN VALLEY, MONTANA
From 31-MAY-18 15:35:00 to 31-MAY-18 15:35:00 (MST-7)
Location: FRANKLIN
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Flash Flood in MUSSELSHELL, MONTANA
From 31-MAY-18 18:30:00 to 31-MAY-18 23:00:00 (MST-7)
Location: ROUNDUP
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Please see Episode Write-up.