The key is to take away as much of the dink and dunk passing game away from the 49ers - they still have weapons on the outside, but force Purdy to have to make longer, lower percentage throws outside the hashes to move the chains....particularly with a wet football.
A soggy track negates the advantage the 49ers have had resting the past few weeks, and likely slows down that 1st step off the snap by their Dline....keeping their defensive front from wrecking the game is a necessity for the Packers to have a shot, so I say let it rain like crazy.