In general, rain might cause the ball to be slick, but in the absence of wind(or an absolute deluge), receivers have a huge advantage over corners(they know their routes, corners don't). Backpedaling and breaking is a lot more difficult than making a cut and running straight. To add, it's more difficult to tackle for a team who's DBs don't tackle well to begin with. The referees do a good job of changing balls and QBs will have a towel in front to dry their hands.
What will be effected is the kicking game. Will have to score TDs to win this one.
However, these are the games where Lovie's zone scheme will help. Hopefully there are a few slips and screw ups as to where our DBs get some freebies.
Sadly:
Purdue. 35
Beloved. 14