What do I say to a mother who worries excessively about tornadoes?

Just lie to her and say you are at home.

I can't do that. I usually just try to change the subject or reassure her. She would know I'm lying, anyway.

Hmm. Why does your mother think you would be safer at home than at work? As far as I know, tornadoes are equally dangerous to homes, apartment buildings, trailer parts, office complexes, etc. What provisions are there for weather emergencies where you work? Would your mom feel more comfortable about this if she knew that in the event of a warning, the loudspeaker would direct everyone to the windowless auditorium, or basement shelter, or whatever they do? (I am assuming that you are not talking about taking off for work in the middle of a tornado warning, with the town sirens blaring, but during a watch or a forecast for possible storm conditions.)

But reassuring mom with the facts may not do the trick, because this sounds a bit beyond rational concern. Is it a bit of control, too? A little guilt-tripping? Your behavior makes her worry herself to death so you should change your behavior? I don't know if that is involved or not. You probably have a sense of whether it is.

Poor Mom. It must not be fun to have an obsession like that, especially if you live in a region with dramatic weather. If you can think of ways to ease her mind, that would be a kindness. "I have to go to work, Mom. But I'll call you from the parking lot and let you know I got there safely."

If this is your mother's only annoying quirk, try to take it in stride. She could be obsessing on lots worse subject (like when are you going to give her grandchildren, or whether you are properly raising your children, or how much money you are saving for retirement). Fear of storms? Reassurance and changing the subject sound like good approaches to me.