Hi catlescout,
Welcome to your new family

You are very welcome.
Peoples reactions are quite unpredictable, when I came out to my parnts I ws 13 and it went down very badly, that was many yrs ago. When I finally came out to friends, family and work there has been total acceptance. I thought I would have problems at work but everyone has been fine.
In your situation you need to be prepared for the worse. As Connie said what are your options if they do react in a very negative way? Are you an only child? Parents can be more accepting if you are the only child they have as they cannot 'transfer' their love to another.
From the threads I have seen here USA people do appear to be less accepting than people from some other countries, and there does appear to be a link to conservative religious beliefs. In Australia, where I am, religious beliefs are generally far less rigid and certainly there is nothing like the community religion that the USA appears to have.
So it make it difficult to judge what will happen.
I do suggest you print out material about transgender and GID, the Wiki articles at Susan's are an excellent starting point. One thing I and many of us forget is that we are very comfortable on gender issues, we have been aware and live in a world of gender issues, so for us it is natural to talk about them. Non-TG people can find it quite alarming, even if they try to be accepting. As a case in point I was visiting my sister and brother in law this morning and we had a chat over coffee. She was asking about my hormones and what dosage I was on and chatting about breast development etc, my brother in law, who has been and is totally accepting of me was obviously having problems with the discussion. It's a bit like talking about periods to a guy, they get all weird.
So your parents may be willing to accept you but feel very uncomfortable talking about it. This is were having printed information detailing the medical condition of ->-bleeped-<- could be useful.
You could also invite them to Susan's so that they can voice their concerns to us, many of us are very comfortable talking to people about such issues, and some times people meeting us can assure them that we are not some group of satanic devil worshippers. Just normal human beings with normal jobs living normal lives.
Hugs and keep in touch
Cindy