You just have to keep an eye out for severe weather reports... read them and it will tell you if the conditions are capable of producing tornados (tornado watch). Otherwise, I would keep them in if it is just severe cloud to ground lightining.
This is really good information to know! I was wondering what the best thing to do was during a storm! Now I know! We get tornado warnings multiple times during the summer, so its good to know what to do with the horses should a bad one come around (and they already have!).
Mine have a concrete block shelter with hurricane ties attaching the roof that I built myself in 96. It had doors but now it just has 2 open entrances. It can withstand a hurricane and the horses can go in or out when they choose and I have more than 50 acres that they can run on. There was a tornado alert recently but I do not know if one actually touched down somewhere nearby. The horses make the choice of where they want to be.
We don't get tornados but we do get hurricane. Our barn policy (boarding/ lesson barn with and average of about 30 horses) is that if the storm is going to be bad enought that it is unsafe for chorse to be done all horses go out. I think that it applies to any type of severe weather.