Excessive Heat in Union, South Dakota

Daily afternoon and evening heat indices ranged from 105 to 116 degrees with a maximum heat index of 116 degrees at a personal weather station near Jefferson on June 20, 109 degrees at a Road Weather Information System site in North Sioux City on June 20, and 107 degrees at the Road Weather Information System site near Beresford on June 21.

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.