I would have an emergency plan for them. Do you have cups/jars that you could put them in, so they can come down into the basement with you as the twister approaches? I keep the cups my bettas came in, for example, on top of thier tanks, so that I can scoop them out for quick transport in the event of an emergencey.
But please, above all, keep YOU safe! Don't waste time trying to salvage fish that would probably be impossible to save in the event of severe house damage if it would put yourself at risk. I know it is sad to have to think that way, but you need to ensure your safety and that of your family above everything else.