Excessive Heat — Gregory, South Dakota

2025-06-20 to 2025-06-21 · Gregory, South Dakota

Event narrative

Daily afternoon and evening heat indices ranged from 105 to 112 degrees with a maximum heat index of 112 degrees a personal weather station near Bonesteel on June 20, 109 degrees at a personal weather station in Fairfax on June 21, 108 degrees at the South Dakota Mesonet site 16 miles north of Gregory on June 20, 107 degrees at the

Road Weather Information System site near Fort Randall Dam on June 20, and 106 degrees at the Automated Weather Observing System site at Gregory on June 20.

Wider weather episode

The upper-level pattern experienced amplification of an eastern U.S. ridge and western U.S. trough, which locked a low-level frontal boundary in place from western Nebraska through central South Dakota into the Great Lakes. This allowed for heat to spread northward as very high dew point readings pooled across the area with the recent wet conditions. This multi-day dangerous heat event featured daytime heat index readings from 100 to 120 degrees, with nighttime low temperatures remaining from the mid 70s to lower 80s.

Source: NOAA Storm Events Database, event_id 1277852. Narrative written by the NWS forecast office that issued the report.