Extreme rainfall and severe weather occurred along a stationary frontal boundary extending across the region. Thunderstorms training over the same area brought rainfall amounts from 3 to over 13 inches causing extensive flash flooding. Many roads and cropland were flooded and damaged. Several roads had to be barricaded. There was some sandbagging at some farmsteads with several homes receiving water in their basements. The extreme rainfall also caused the dam at Lake Hiddenwood to burst sending water downstream resulting in more flash flooding. The entire lake was drained and water from the lake flowed for miles toward Sand Lake near the Walworth/Campbell county borders. The South Dakota governor requested a Presidential Disaster Declaration for Campbell, Walworth, and McPherson counties. The total estimated damage in all three counties was 3,115,000 dollars.
Some extreme rainfall amounts include, 3.96 inches at Eureka, 6.17 inches 11 miles southwest of Eureka, 6.70 inches 5 miles west of Hillsview, 8 inches 5 miles west of Long Lake, 9 inches at Long Lake, 9.5 inches 4 miles north northwest of Long Lake, and 13.15 inches 3 miles northwest of Long Lake.
Severe weather with hail up to the size of golf balls along with winds gusting to near 80 mph also brought damage to parts of central and north central South Dakota. The city of Eureka reported significant roof and property damage due to large hail.

Flash Flood in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 16:10:00 to 17-MAY-18 20:10:00 (CST-6)
Location: LONG LAKE
Flood cause: Heavy Rain
Property damage: 54.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Flash Flood in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 18:30:00 to 17-MAY-18 22:30:00 (CST-6)
Location: JAVA
Flood cause: Heavy Rain
Property damage: 3.00M
Crop damage: 0.00K
Reported by: Emergency Manager

Flash Flood in WALWORTH, SOUTH DAKOTA
From 18-MAY-18 02:00:00 to 18-MAY-18 16:00:00 (CST-6)
Location: SELBY
Flood cause: Dam / Levee Break
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Due to the very heavy rainfall earlier, the dam at Lake Hiddenwood broke in the early morning hours spreading water northwest through a creek. Several roads were flooded along with crop and pastureland. The earthen dam was built in 1927 and was 27 feet high with a concrete spillway. The bridge over the spillway was also destroyed. The lake was one of the first artificial lakes in South Dakota. Lake Hiddenwood State Park was closed indefinitely.

Flash Flood in CAMPBELL, SOUTH DAKOTA
From 17-MAY-18 18:30:00 to 17-MAY-18 22:30:00 (CST-6)
Location: MOUND CITY
Flood cause: Heavy Rain
Property damage: 34.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Heavy rains flooded several areas along Highway 271 in far southeast Campbell county along with many acres of crop and pastureland.

Flash Flood in CAMPBELL, SOUTH DAKOTA
From 18-MAY-18 06:00:00 to 18-MAY-18 18:00:00 (CST-6)
Location: MOUND CITY
Flood cause: Dam / Levee Break
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Water from the Lake Hiddenwood dam break flowed into southeast Campbell county flooding several roads and cropland.

Flood in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 22:30:00 to 18-MAY-18 17:00:00 (CST-6)
Location: GLENHAM
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Flooding continued through the night and through the next day across the region. Agricultural land along with many roads were flooded.

Flood in CAMPBELL, SOUTH DAKOTA
From 17-MAY-18 22:30:00 to 18-MAY-18 17:00:00 (CST-6)
Location: MOUND CITY
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Flooding from heavy rains continued through the rest of the night and into the next day across southeast Campbell county.

Flood in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 20:10:00 to 18-MAY-18 17:00:00 (CST-6)
Location: GREENWAY
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Extensive flooding from very heavy rains remained through the night and most of the next day for much of western and northern Mcpherson county.

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 15:00:00 to 17-MAY-18 15:00:00 (CST-6)
Location: EUREKA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 15:03:00 to 17-MAY-18 15:03:00 (CST-6)
Location: EUREKA
Magnitude: 1.75 
Reported by: Trained Spotter

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 15:17:00 to 17-MAY-18 15:17:00 (CST-6)
Location: EUREKA
Magnitude: 1.75 
Reported by: Trained Spotter

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 15:21:00 to 17-MAY-18 15:21:00 (CST-6)
Location: EUREKA
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 15:50:00 to 17-MAY-18 15:50:00 (CST-6)
Location: HILLSVIEW
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in MCPHERSON, SOUTH DAKOTA
From 17-MAY-18 16:15:00 to 17-MAY-18 16:15:00 (CST-6)
Location: HILLSVIEW
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in CORSON, SOUTH DAKOTA
From 17-MAY-18 15:40:00 to 17-MAY-18 15:40:00 (MST-7)
Location: LITTLE EAGLE
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 20:30:00 to 17-MAY-18 20:30:00 (CST-6)
Location: BLUNT
Magnitude: 52.00 EG
Crop damage: 0.00K
Reported by: Newspaper

A semi truck was blown over near the junction of Highways 14 and 83 by sixty mph winds.

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 20:44:00 to 17-MAY-18 20:44:00 (CST-6)
Location: PIERRE
Magnitude: 52.00 EG
Crop damage: 0.00K
Reported by: Public

Estimated sixty mph winds downed some tree branches and took some shingles off a roof.

Thunderstorm Wind in SULLY, SOUTH DAKOTA
From 17-MAY-18 21:12:00 to 17-MAY-18 21:12:00 (CST-6)
Location: ONIDA
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Estimated sixty mph winds took a large branch off of a tree.

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 21:20:00 to 17-MAY-18 21:20:00 (CST-6)
Location: BLUNT
Magnitude: 56.00 EG
Crop damage: 0.00K
Reported by: Trained Spotter

Some small trees were downed in Blunt by estimated sixty-five mph winds.

Thunderstorm Wind in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 16:28:00 to 17-MAY-18 16:28:00 (CST-6)
Location: JAVA
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Sixty mph winds were estimated.

Thunderstorm Wind in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 17:18:00 to 17-MAY-18 17:18:00 (CST-6)
Location: (Y26)MOBRIDGE ARPT
Magnitude: 63.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 17:35:00 to 17-MAY-18 17:35:00 (CST-6)
Location: (Y26)MOBRIDGE ARPT
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in DEWEY, SOUTH DAKOTA
From 17-MAY-18 17:04:00 to 17-MAY-18 17:04:00 (MST-7)
Location: RIDGEVIEW JEWETT ARP
Magnitude: 54.00 MG
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in EDMUNDS, SOUTH DAKOTA
From 17-MAY-18 18:49:00 to 17-MAY-18 18:49:00 (CST-6)
Location: HOSMER
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Sixty mph winds were estimated.

Thunderstorm Wind in WALWORTH, SOUTH DAKOTA
From 17-MAY-18 18:55:00 to 17-MAY-18 18:55:00 (CST-6)
Location: LOWRY
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in DEWEY, SOUTH DAKOTA
From 17-MAY-18 18:04:00 to 17-MAY-18 18:04:00 (MST-7)
Location: RIDGEVIEW JEWETT ARP
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in STANLEY, SOUTH DAKOTA
From 17-MAY-18 18:25:00 to 17-MAY-18 18:25:00 (MST-7)
Location: HAYES HAMILTON ARPT
Magnitude: 59.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in POTTER, SOUTH DAKOTA
From 17-MAY-18 19:44:00 to 17-MAY-18 19:44:00 (CST-6)
Location: GORMAN
Magnitude: 56.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in STANLEY, SOUTH DAKOTA
From 17-MAY-18 18:44:00 to 17-MAY-18 18:44:00 (MST-7)
Location: HAYES HAMILTON ARPT
Magnitude: 70.00 MG
Reported by: Mesonet

Thunderstorm Wind in SULLY, SOUTH DAKOTA
From 17-MAY-18 20:00:00 to 17-MAY-18 20:00:00 (CST-6)
Location: FIELDER
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Sixty mph winds were estimated.

Thunderstorm Wind in JONES, SOUTH DAKOTA
From 17-MAY-18 20:08:00 to 17-MAY-18 20:08:00 (CST-6)
Location: MURDO MUNI ARPT
Magnitude: 61.00 MG
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in SULLY, SOUTH DAKOTA
From 17-MAY-18 20:10:00 to 17-MAY-18 20:10:00 (CST-6)
Location: AGAR
Magnitude: 54.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 20:20:00 to 17-MAY-18 20:20:00 (CST-6)
Location: (PIR)PIERRE ARPT
Magnitude: 53.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

An apartment building roof was damaged in Pierre by winds gusting over 60 mph.

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 20:44:00 to 17-MAY-18 20:44:00 (CST-6)
Location: BLUNT
Magnitude: 55.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in HUGHES, SOUTH DAKOTA
From 17-MAY-18 20:55:00 to 17-MAY-18 20:55:00 (CST-6)
Location: BLUNT
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in LYMAN, SOUTH DAKOTA
From 17-MAY-18 21:05:00 to 17-MAY-18 21:05:00 (CST-6)
Location: MC CLURE
Magnitude: 57.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in HYDE, SOUTH DAKOTA
From 17-MAY-18 21:10:00 to 17-MAY-18 21:10:00 (CST-6)
Location: HIGHMORE ARPT
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet