The radar loops available on National Weather Service web sites have changed from Java-based animation to Flash-based animation. This has also changed the method for zooming in and out on the looping images.
A mouse left click zooms in on the looping image, Ctrl-left click zooms out, and Alt-left click zooms completely out to the full view.
This information is also noted just below the animated images when using the radar loops.
You will, of course, need to have the free Adobe Flash Player installed in order to use the new Flash-based radar animation loops.