Leaving the weather out of it, the key to the game is going to be QB Aaron Rodgers. The defense has been a wreck all season and now LB Clay Matthews is out (again) with a re-broken thumb. QB Colin Kaepernick has torched them good in his last two games and the defense has gotten worse since their last meeting. Additionally, the 49ers prefer to run (417 pass attempts on the season vs. 505 rushing) and the Packers' run defense has been in shambles over the last half-season. The 49ers are going to move the ball.
Here's Rodgers's stats over his last three meetings against the 49ers, QBR from ESPN:
Now he needs to do a little better. Avoid turnovers, put up a better game than last September, and the Packers can win.