There is nothing to indicate that she autocasts storm in her description. It doesn't say so in the Dom3 database either. But, I've run several tests and there was a storm during battle every time. I just don't know if I can count on it reliably.
Well, she is the Queen of Storms, so it is quite possible. I don't know for certain.
The way to verify it is send her to battle alone and have her cast Storm Power as first action. If that happens several times, then she does autocast storm.
I tested it some more. She doesn't autocast for sure. But when she spends a couple of turns in a province the storm appears and stays there as long as she does. I had a storm in the battle even without her - I cloud-trapezed her away.