Excessive Heat — Gregory, South Dakota

2025-07-27 to 2025-07-28 · Gregory, South Dakota

Event narrative

Dangerous heat index readings reached an afternoon peak as high as 116 degrees near Bonesteel, 112 degrees at Fort Randall Dam, 111 degrees at Fairfax, and 109 degrees at South Dakota Mesonet site GGMS2 7 miles south-southwest of Gregory.

Wider weather episode

Beneath an expansive and strong, flat ridge aloft, extreme surface-based moisture returned near a diffuse warm frontal boundary on July 26 and lasted into the afternoon of July 28. While this moisture and heat would eventually fuel the derecho during the evening hours of July 28, dangerous heat indices from 105 to 120 degrees were common.

Source: NOAA Storm Events Database, event_id 1288587. Narrative written by the NWS forecast office that issued the report.