Dogs can tend to freak out from time to time. I have a dog and he will ocasionally bark at nothing for little to no reason. That, or he may go wild. Maybe your dog just saw another dog and or animal and it startled him/her?
If you're under Tornado watch I suggest you call your dog in. Then, if you're home alone call your parents. If there is any mention of a Tornado, I suggest on moving to the basement and listen to the radio for any warnings. If not your basement, head over to your neighbours house if your parents advise you to.