Slow moving thunderstorms regenerated over the Keystone and Mount Rushmore area for several hours. These storms produced large amounts of hail to two inches in diameter and three to four inches of rain in about three hours. Flooding occurred along creeks, covering several roads in the Keystone area and Custer State Park.

Flash Flood in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 19:55:00 to 20-MAY-16 23:00:00 (MST-7)
Location: SHERIDAN LAKE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Grizzly Gulch Road was underwater and impassible at the Grizzly Bear Creek crossing. Street flooding was also reported in Keystone.

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 15:00:00 to 20-MAY-16 15:05:00 (MST-7)
Location: KEYSTONE
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Storm Chaser

Penny to quarter sized hail fell with the storm.

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 15:00:00 to 20-MAY-16 15:20:00 (MST-7)
Location: HAYWARD
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail to ping pong ball size covered the ground.

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 15:16:00 to 20-MAY-16 15:40:00 (MST-7)
Location: MT RUSHMORE
Magnitude: 1.75 
Crop damage: 0.00K
Reported by: Law Enforcement

Hail to the size of golf balls accumulated eight inches deep along Iron Mountain Road. Motorists were stranded at the Peter Norbeck Overlook until snow plows cleared the road.

Hail in CUSTER, SOUTH DAKOTA
From 20-MAY-16 16:55:00 to 20-MAY-16 17:15:00 (MST-7)
Location: HERMOSA
Magnitude: 2.00 
Crop damage: 0.00K
Reported by: Public

Hail covered the road at the corner of Playhouse Road and Iron Mountain Road.

Hail in CUSTER, SOUTH DAKOTA
From 20-MAY-16 17:00:00 to 20-MAY-16 17:00:00 (MST-7)
Location: CUSTER
Magnitude: 1.50 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail to the size of ping pong balls was reported at the Center Lake Campground in Custer State Park.

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 17:15:00 to 20-MAY-16 17:15:00 (MST-7)
Location: HAYWARD
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 17:00:00 to 20-MAY-16 17:30:00 (MST-7)
Location: KEYSTONE
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail to quarter size covered the ground several inches deep.

Hail in CUSTER, SOUTH DAKOTA
From 20-MAY-16 17:39:00 to 20-MAY-16 18:05:00 (MST-7)
Location: CUSTER
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

The ground was covered with hail.

Flood in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 23:00:00 to 21-MAY-16 01:00:00 (MST-7)
Location: KEYSTONE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Grizzly Bear Creek covered Grizzly Gulch Road, making it impassible for several hours, and caused minor flooding in Keystone.

Flash Flood in CUSTER, SOUTH DAKOTA
From 20-MAY-16 19:28:00 to 20-MAY-16 23:00:00 (MST-7)
Location: CUSTER
Flood cause: Heavy Rain
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Iron Creek overflowed its banks at Needles Highway (SD Highway 87), with three to four inches of water covering the highway. All water crossings on Iron Creek Trail 15 were either damaged or destroyed, making the trail impassible for several weeks.

Flood in CUSTER, SOUTH DAKOTA
From 20-MAY-16 23:00:00 to 21-MAY-16 11:00:00 (MST-7)
Location: CUSTER
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Iron Creek overflowed its banks upstream and downstream from Lakota Lake, causing minor flooding. Iron Creek covered the Needles Highway (SD Highway 87) with at least three to four inches of water. A road downstream from Lakota Lake was covered by water when an eight foot diameter culvert wasn't able to keep up with the flood waters.

Hail in PENNINGTON, SOUTH DAKOTA
From 20-MAY-16 15:00:00 to 20-MAY-16 15:05:00 (MST-7)
Location: KEYSTONE
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail to the size of quarters covered the ground from the Keystone Wye, the intersection of US Highways 16 and 16A, eastward along Highway 16.