So here's a little Chicago snowstorm climo.
Officially, there have been 49 storms of 10"+ since records began.
If you narrow it down to 12"+ storms, there have been 26 of those.
If you narrow it down to 14"+ storms, there have been 12 of those.
So historically... when Chicago has had a 10" storm, a little over 50% of those have actually reached 12" (26 out of 49).