Flood in Hamlin County, South Dakota

The Big Sioux River near Castlewood went into flood during the evening of July 27th, rising over three feet in four hours due to heavy rainfall from thunderstorms. The COOP observer in Castlewood recorded 5.61 inches of rain. The river crested at 11.44 feet, which is 2.44 feet above flood stage. The water inundated mainly farmland and pasture, and overtopped portions of 184th Street. The river remained in flood through the end of the month and into August.

An afternoon thunderstorm developed near the town of Henry and very quickly spun up a violent tornado. This tornado would cause significant damage to a farmstead before dissipating. The storm would go on to produce large hail, up to the size of baseballs near Lake Kampeska, with numerous additional reports of golf-ball sized hail, in and around the city of Watertown. The Watertown Automated Surface Observing System recording a peak wind gust of 71 mph.

Heavy rainfall from persistent re-development of storms over the area into the late evening hours would go on to produce flooding in the Big Sioux basin. The COOP observers in Castlewood, Rauville and Watertown would pick up 5.61, 3.68 and 2.06 inches respectively.