My assumption usually is, if it's raining/snowing/windy, it helps with the under if you can catch it before it drops. On the other hand, if it has rained quite a bit, but mostly passed over and gone before or right after kickoff, it favors the offense, especially wide~receivers who know exactly where they're going, and the slippery surface makes if much harder for defenders to follow/cover. Not to say that this ensures any over plays of course, but just my philosophy.....