Prepare for anything
While snow can generally be ruled out, anything else is fair game. Last year it poured. Lunch involved standing in line sinking ankle deep in mud. Plus hurricanes tend to move north later in the season so you have that to look forward to as well. However, it can be lovely mid September. My advice, hope for sun but bring foul weather gear.
Our DS enjoyed the rain last year; meant he could leave his band instrument indoors and just watch the game and do pushups like everyone else!