The number of 90ºF readings in Illinois has dropped off dramatically since the 1930s, with many of the coolest summers coming in the last decade.
Index of /pub/data/ghcn/daily/hcn/
ALEDO ANNA 2 NNE AURORA CARLINVILLE CHARLESTON DANVILLE DECATUR WTP DIXON 1 NW DU QUOIN 4 SE GALVA HARRISBURG HILLSBORO HOOPESTON JACKSONVILLE 2E LA HARPE LINCOLN MARENGO MINONK MONMOUTH MORRISON MT CARROLL MT VERNON 3 NE OLNEY 2S OTTAWA 5SW PALESTINE PANA PARIS STP PONTIAC RUSHVILLE 4NE SPARTA 1 W URBANA WALNUT WHITE HALL 1 E WINDSOR
In my lifetime, the trend is flat, but there is a lot of variability. That matches what I’ve experienced here in IL.
I also remember more snow in the late 1970s as a kid, with a recent year being heavy too, and cold winters with Lake Michigan freezing over near the shore in the early 1990s. Not sure if my memories match the data.