The info comes from the japanese Xbox site, as far as I'm aware weather and nighttime racing have been on top of the Forza wishlist for a long time:
As real as it gets
Ultimate car fantasy
Never race alone
http://www.xbox.com/ja-JP/games/forza-6/6_15/9d32baa8-7b67-4f7e-be5b-79cc868573bc