User Interface functionality modelling languages?

Question

I am looking for a UI functionality modelling language (UML-alike "thing", but for user interfaces) that is already accepted and maybe has its design patterns and handles the problem better than state or activity diagram.

This question came to mind as a result of a discovery that UML and its diagrams fails at describing complicated UI functionality with event-driven flow of execution (ie. javascript/jQuery big projects)

EDIT: I've been thinking of using BPMN but It was not created for this purpose.

Answer 1

Maybe user interface prototypes or storyboards can be helpful ... they are not part of a "modelling language" but very well proved techniques for designing GUI ...