A warm front tracked from west to east across central South Dakota Tuesday afternoon, July 18th, which led to increasing temperatures into the upper 80s and low 90s as well as increasing  moisture with dewpoints into the mid to upper 60s. Warm temperatures that also existed aloft along with weak forcing to get thunderstorms started resulted in relatively low potential for thunderstorms across central South Dakota. However, a few did manage to develop near the Missouri River around 5:30pm, and they quickly grew in strength as they took advantage of a favorable environment for severe weather, and became supercells (rotating thunderstorms).

By 6:20pm, a supercell moved into Pierre and Fort Pierre, SD and began producing hail. There were numerous reports of baseball sized hail across the area, and even one larger than a softball, with an estimated diameter of 5.0". This hailstone is the largest on record in Hughes County (4.0" in Pierre 6/30/1997 and in Blunt 8/26/2018). In order for hail to get this large, it has to spend an extended amount of time lofted in the thunderstorm so it can gain layer after layer of ice. This is achieved by the thunderstorm's updraft - the stronger and larger the updraft, the larger the hail. Damage to vehicles, roofs, and other outdoor property resulted.

This storm also produced a brief tornado about 11 miles south of Fort Pierre, just west of US Highway 83, around 6:45pm CDT. These storms then moved south and east through the evening, and continued to produce large hail as well as damaging winds. A wind gust of 100 mph was recorded just west of Oacoma at 8:15pm CDT via a Mesonet at SDSTATE weather station, and damage to power poles, trees, trailers and other property resulted in and around Oacoma. Heavy rain was observed into the night across southern Lyman County as well.