Best way to ask someone out who is in another country

Last October I (M22) went to New York (I live in the uk) and met someone (F27), we have stayed in contact since sending snaps everyday and most of the time leading to a decent conversation.

Over these past few months I am pretty sure I have developed a few feelings for her, although I am unsure if she feels the same towards me. There have been a few times that might suggest she does but I am not too sure if I am looking into the snaps she sends trying to find a sign. While we have been talking she has mentioned going out with another person but it ended poorly after one date. I feel it might be a good time to say something so she doesn't try to get with someone else between now and when we see each again.

In September we are going to be meeting up while I am back over there for about two and half days. What would be the best way to go around this?

I am not too sure if expressing some of my feeling now would be a good way to go around it and maybe try to get things going in a romantic direction. Or my other strong option right now is too see how things plays out between us while we meet up and maybe say something before I say goodbye to her. I am not too sure if I send a message now either expressing my feelings or try to set up a date will make her rethink about meeting up, but at the same time I don't want to wait much longer just encase she does find someone who is more local to her.