But it's not the weather that affects her. 
Buut she could steal draw power from that, right?
If she could draw on the surroundings' power, which would be filling the whole stage... which is a lot of that element she wants to draw on, theoretically, it would make her spells stronger with more element to draw power from, yes?
So, let's say, the default Ultimate skill is Silent Selene, which draws power from the moon (And sadly there is no "Imperishable Night/Full Moon" weather), and once there is Scorching Sun or Sunny weather, the Ultimate changes to Royal Flare?
Also, I suppose, balance the two Ultimates out, with each of them, although under 1 skill button, that changes skill with the weather. Give each ultimate a redeeming quality to use with different situations. That is what I am thinking, though the final decision still lies on Aph. I am just suggesting, that is all.