O'Hare's high temperature today of 19.4°C occurred at 5am. From then until about 9am the temperature lingered around 19°C. And then the cold front came through.
Between 9am and 11am the temperature plunged 8°C. It's now 11°C and raining. Tomorrow it'll be 11°C and sunny. Overnight it'll be 3°C in the city and -2°C in the far Western suburbs.
Autumn comes to Chicago all at once. Today's the day it chose this year.