An outflow boundary from earlier day convection focused a gradient of instability and storm development with additional destabilization from heating by early afternoon. Boundary interactions from sluggish storms led to training of heavy rainfall cores, as well as a couple brief tornadoes. Extensive flooding resulted in a state disaster declaration for Sioux County.

Thunderstorm Wind in PLYMOUTH, IOWA
From 05-AUG-23 17:25:00 to 05-AUG-23 17:26:00 (CST-6)
Location: LE MARS MUNI ARPT
Magnitude: 62.00 MG
Property damage: 5.00K
Reported by: Law Enforcement

A thunderstorm wind gust to 71 mph was measured by a law enforcement officer.  Some minor damage to shingles and trees was observed near the point the wind was measured.

Tornado in SIOUX, IOWA
From 05-AUG-23 13:30:00 to 05-AUG-23 13:33:00 (CST-6)
Location: MAURICE
Tornado F-scale: EF0
Reported by: Public

A highly visible tornado developed a quarter mile south of Iowa Highway 10 just to the east of Goldfinch Avenue. The slow-moving tornado was photographed by several members of the public as it tracked generally a bit south of due eastward.  Very soon after touching down, the tornado impacted a shelter belt where it did damage to several hardwood trees.  After a quarter mile, the tornado track shifted slightly toward the east-northeast across open farmland. The tornado crossed Grant Avenue and lifted shortly thereafter.  The average path width was 25 yards. The tornado was rated an EF-0 with a maximum wind speed of 75 mph.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 14:40:00 to 05-AUG-23 16:40:00 (CST-6)
Location: SIOUX CENTER
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

U.S. Highway 75 was blocked in both directions due to significant standing water between County Road B30 and County Road B46.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 16:15:00 to 05-AUG-23 18:00:00 (CST-6)
Location: BOYDEN
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Rainfall up to seven inches in around 2 hours produced significant street flooding around town.

Thunderstorm Wind in SIOUX, IOWA
From 05-AUG-23 13:30:00 to 05-AUG-23 13:31:00 (CST-6)
Location: MAURICE
Magnitude: 70.00 EG
Property damage: 50.00K
Reported by: NWS Storm Survey

A very old barn collapsed under the weight of a more modern roof. The structure fell to the southeast, most likely in relation to rear flank downdraft wind on the backside of tornado producing mesocyclone. Property damage costs are a rough estimate.

Flood in O'BRIEN, IOWA
From 05-AUG-23 11:02:00 to 05-AUG-23 12:00:00 (CST-6)
Location: SANBORN
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Several yards flooded due to a small creek overflowing.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 14:21:00 to 05-AUG-23 17:30:00 (CST-6)
Location: SIOUX CENTER
Flood cause: Heavy Rain
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Law Enforcement

A no travel advised order was put out with numerous roads under significant flowing water up to mid-vehicle depth, including 2nd Avenue SE near New Age Church and 4th Avenue SE near Children's Park. Property damage costs are rough estimates.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 14:32:00 to 05-AUG-23 17:00:00 (CST-6)
Location: ROCK VLY
Flood cause: Heavy Rain
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Significant water was ponded or flowing on Main Street by 12th Street, and also along 14th Street from 10th Avenue to 12th Avenue. Property damage costs are rough estimates.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 14:49:00 to 05-AUG-23 16:30:00 (CST-6)
Location: SIOUX CENTER
Flood cause: Heavy Rain
Property damage: 1.50M
Crop damage: 0.00K
Reported by: Law Enforcement

Water ponding in the area of Central Park overflowed roadways and entered the Sioux Center Fire Department building. Two homes suffered the collapse of basement walls from the high water. Water also inundated a transformer which cut off power to seven homes in the northwest part of town. Seven trailers were evacuated due to high water in Westbrook Village on the northwest edge of town. More than two dozen vehicles were stranded in high water. Damage costs are rough estimates.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 14:49:00 to 05-AUG-23 16:30:00 (CST-6)
Location: ROCK VLY
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Numerous roads impassible around Rock Valley due to very high water.

Flash Flood in SIOUX, IOWA
From 05-AUG-23 16:38:00 to 05-AUG-23 18:30:00 (CST-6)
Location: BOYDEN
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

U.S. Highway 18 was closed in both directions between County Road K64 and County Road L14 due to significant standing water over the highway.

Tornado in SIOUX, IOWA
From 05-AUG-23 13:42:00 to 05-AUG-23 13:42:00 (CST-6)
Location: SIOUX CENTER
Tornado F-scale: EFU
Reported by: Social Media

Photographic evidence showed a very brief tornado touchdown west of Grant Avenue north of 420th Street. There was no known damage to provide a rating.