We don’t often get rain in NYC, but if the heavens do open then it’s likely that I will give you a chance to postpone the day (given enough advance warning). If you want to continue with the proposal come rain or shine, I do know many nice places to propose under cover from the weather.
If it’s just a light rain, sometimes that can work even better — some of my favorite shots are those that I take in the rain. [link the post in my blog.]