From what I could find, the fujita scale appears to have records from as early as 1950... I am sure Jo was not meant to be over 50 by the time she had grown up.
The opening scene takes place in June of 1969, the Fujita scale would have been around then. I'm really tired of people assuming things are anachronisms. Look it up and make sure! It happened with scotch tape in The Green Mile and with velcro in Back to the Future! Both were invented at the times of the movies but were called anachronisms!
the person who submitted this is very much correct..the following is an excerpt i copied and pasted from the internet.....
Fujita Tornado Damage Scale. Developed in 1971 by T. Theodore Fujita of the University of Chicago.