B.R.
Put her in time out as soon as she starts and make her stay there until she is done. This may sound harsh, but my friend was having a similar issue with her daughter, who is 2, and what she would do is instead of screaming, she would starting whine-crying, making a horrible high pitchd whining sound while crying. I told her to do that consistantly, and she did, and it only took a few days before her little one got tired of sitting there. The key is that you have to be absolutely consistant with it, every single time she does it she goes in time out, even if you are in a store you find a place to make her sit it out, and you have to ignore it completely while it is happening. If she gets out of time out, and is still screaming, put her back. It will be hard while you are doing it, but it has worked on my almost 3-yr old when she started that stuff about 18 mo ago (she was about 19 mo at the time), and it worked wonders for my friend's little girl. Good luck with this, and remember to be consistant!! I can't stress that enough!