OP, I wonder if your DD saw something about a hurricane or tornado on TV. Maybe heard or saw something about a car accident or someone getting swept away in a storm. There were lots of storms this past year so if she saw something maybe she can't process it. I'd take out a map and show here where you are and how far away bad storms happen. If you are in a storm area I'd take her around the house and show he how safe your home is and what to do if something bad happens. If it's none of this I'd do my best to make storms fun in some way, when I was little my grandma would cut up apples and we'd sit on the porch and listen to the rain, if I was in the car or in the house I'd look out the window and make a game of which raindrop would make it to the bottom of the glass first. When it came to my own kids we would make a big deal about putting on boots with raincoats and cute umbrellas and going puddle stomping and then have a special movie day.
This is so interesting to me, I wonder if some of the sensory pathways with migraine are similar? I get lots of migraines and it's nearly impossible to explain to someone who never had one how much my senses inflict pain, as in sounds do hurt, smells do hurt and light does hurt, even the hairs on my head can hurt and then there is the nausea. Sometimes they are disorienting and it looks like I am high or something, I can't form thoughts or think clearly, my eyes will tear and sometimes I get smell hallucinations of smoke. I get about 22 a month since I was very little but botox cuts them in half and makes the ones I have less awful. At any rate, I can't even imagine how painful life would be if I couldn't clearly and calmly articulate my experiences. I've found it best to find Dr's who themselves have had migraines, no one else can grasp the weirdness.
ANother point of view, for anyone with a child who reacts to storms people with migraine can get severe pain with the weather so as a person who can feel a storm 2 hours away I'd watch your child carefully and ask gently if she feels anything physical. Little kids often get stomach issues with migraine and have a tough time explaining, people like to think they are making it up and looking for attention. When I was very little I used to try to use construction paper to tape out the light, and I'd hide all day, I wasn't scared I was in agony, my mother didn't have migraine so she probably thought I was just off the wall. To a child the pain is very scary. Check out 21 Pilots song Migraine he does a decent job explaining what it's like.