Wrong, wrong, wrong. Rain doesn't cause unders, unless it's a torrential downpour during the game. WIND is what causes unders, and I see it's not supposed to be windy.
Don't count on a sloppy field to bail you out, either. Most fields are very well kept these days. Think about it...when's the last time you saw muddy uniforms?
Everyone who bets is programmed into thinking "bad weather = underdog + under", when they should be thinking "bad weather = play the running team", which is Texas in this case (yuck). A good example is the A&M/Va Tech game, which was played in a hurricane. There, the under money was justified (very windy), but everyone played A&M just because they were the dog, when they should've been playing VT (the strong running team).
Class dismissed. Good luck!