Since someone seems bound and determined not to add any context...
Like much the rest of the region this weekend, downtown Chicago got something like 4 inches of rain on Saturday alone.
Record Rainfall Floods Chicago Area
Heck, across the lake they had to drain a million gallons of water out of Waldo Stadium in Kalamazoo:
Kaytie Boomer
It was simply too much water too quickly, that's all. Flash floods.