Should I email potential supervisor when I've already applied
Assuming that they are involved in your application process (which is common in some places but not others), then you should contact them now. Explain that you saw the announcement late and want to give them whatever information they might need at this time.
I wouldn't flood them with information in the first contact but be ready to supply information if they ask for it. But an introduction and expression of interest in their work won't hurt and might help.