Heat in Massac, Illinois

Broad high pressure over the middle Mississippi River Valley resulted in temperatures surging to the mid-to-upper 90s with dew points in the mid 70s, yielding heat indices around 105-110 mainly from the 16th-19th in Southern Illinois with some locally higher values. A few sites reached heat indices high enough for duration based heat criteria starting on the 14th-15th, mainly near the Ohio/Mississippi River confluence. Southeastern Illinois had more frequent dew points in the lower 70s, limiting heat advisory criteria duration. A cold front the night of the 19th-20th brought an end to the heat. The highest observed heat index was 119 on the 18th at the Crab Orchard RAWS site.