I was thinking of using Photoshop or a UML program to try to attempt this but I think I may try to draw it out on paper or use PowerPoint.
PowerPoint really isn't a bad tool for this...
I'm probably just going to try to draw it out on paper and if the drawing looks rough, then make a better looking version likely using powerpoint
I have looked around for some, but most are for like comic strips or writing and not computer science. Because of this I have gone back to the old fashion pencil and paper method of drawing a storyboard.
I personally use Krita, which is a free painting/art software. It has a lot of features for customization and if you are going to make any art for your app, its a great place to do that as well! Hitting two birds with one stone.