Awww Lynn, bless sweet Lacie. Oh Lynn, my Flakey (RIP) was petrified of storms. My Flakey bless his soul, did have several anxiety issues. Normally, he would not let me pick him up, but if a storm was on the way, oh he begged Mommy to pick him up. And I did, and would rock him in a chair. As soon as the storm was over, he wanted down. What I did was probably the wrong thing, meaning, I should have desensitized him to the storms.
Knowing, that Flakey had this problem, when I got Mia and Leo, and I knew a storm was coming, I put soft music on and played games. Neither of them are bothered by the storm.
My two are pad trained inside. All of my furbabies were.
Lynn, what I would try and do, is find out how to remove her anxiety that she suffers with when a storm comes. Perhaps when a storm comes, play soft music, at a nice volume, and distract her with happy things or ask Jackie how to go about desenitizing her from storms.
Lynn, I wouldn't force her outside while the storm is going on. More then likely she probably won't even potty outside duiring that time, she probably will just be too frightened and I think would only heighten her anxiety to the storms.
Bless her Lynn, I've seen the anxiety that my Flakey went through, he was the same way with fireworks

. Now with fireworks, what I did was, happily ask him if he wanted to go for a walk, and while we were walking, I distracted him with happy things, and believe it or not, it did help him alot. But his anxiety with storms was far worse then with fireworks.
Good luck with this and will be thinking of you and sweet Lacie.