Hi there, Richard, and thanks for signing my Guest Book!
In answer to your question, I have to admit that I am not an expert on storm chasing. I cheated! I signed up to chase with a group called Cloud 9 Tours back in 2004, and have continued chasing with them ever since. In fact, I will be chasing with them once again for the entire month of May this year.
As one might expect, with a storm chasing group such as Cloud 9, we are mobile, so we cover a lot of territory. I believe the record for me, personally, was in 2007 when we were a half a mile from Mexico at one point, and within about a week and a half, we were about 65 miles away from Canada! (We only saw one tornado, by the way...)
Anyway, what I guess I'm trying to say is:
1. Tornadoes are elusive beasts;
2. It helps to be mobile, so you can optimize your chances of seeing a tornado if it does happen; and
3. Mother Nature simply doesn't cooperate sometimes!
I realize that that answer probably isn't terribly helpful. Sorry about that... If you have the opportunity, chasing with Cloud 9 Tours (http://www.cloud9tours.com/) or groups such as them would give you a much better chance of seeing a tornado. Otherwise, just keep doing what you're doing by keeping your eyes to the skies! If you know about the way supercells work --- which you most likely do --- then perhaps Mother Nature will cooperate!
Thad