I dont notice too much of a difference between Midwestern weather on 4/4 as opposed to 4/11. The likelihood of garbage weather is about equal on both of those days. Really its only the upper Midwestern teams that play outdoors (CHW, CHC, CLE, MIN, PIT, DET) and maybe PIT and BOS that need to worry. The climate changes once you go about 150 miles south of Chicago... its 60 degrees almost every day in Cincinnati and StL now.
I'm sure there's data to refute me somewhere out there.