Heat in Massac, Illinois

Heat continued into late July.

An extended heat wave across the Quad State region occurred from mid to late July. On the synoptic scale, at 500mb, a 594dam ridge was present across the Southeast U.S., with the ridge building northward to include the Ohio Valley and Great Lakes by July 22nd-23rd. The Quad State was initially on the edge of the ridge from the 17th-21st, resulting in greater coverage of showers and thunderstorms. Heat issues continued through the end of July.

High temperatures in the low to mid 90s combined with very high humidity (dew points in the mid 70s to near 80) produced heat index values routinely in the 100 to 110 range for at least portions of Southern Illinois from July 16th through the 21st, with isolated higher values. Some relief came for the northern portions of the region on the 17th-19th and 21st due to more widespread showers/storms and cloud cover.

For the entirety of the mid-late month heatwave, Carbondale, IL experienced 21 consecutive days with a 75 degree dew point, their record longest (records going back to 1996). The highest heat index in Southern Illinois for the third week of July was 113 at the Bean Ridge RAWS site in Alexander County on the 18th.