I seem to remember that a blizzard is also attributed to Saruman at one point. What I love though is that it's not Saruman waving his arms and chanting some formula to cause the blizzard, but rather a situation where a blizzard was already possible and Saruman kind of "nudging" nature to ensure the blizzard hits in the right place and is especially violent. In a sense, it feels like the blizzard happens just because Saruman wants it to happen.

I also seem to remember that it's also implied that the ride of the Rohirim to Helms Deep should have been near impossible, but because Gandalf was with them they had the speed and stamina to make it. He doesn't explicitly do anything, but kind of "wills" them to be faster.