Excessive Heat in Sanborn, South Dakota

Daily afternoon and evening heat indices ranged from 105 to 110 degrees with a maximum heat index of 110 degrees at South Dakota Mesonet sites near Woonsocket and Letcher on June 21 and 109 degrees at a Road Weather Information System site near Forestburg 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.