There are many factors to consider, weather is one. There is a good amount of connecting passengers on most of the flights from the east coast . You can only know at the last minute, If you want to play on the safe side, buy a confirmed ticket now. Nowadays flights are most of of time unpredictable. As far as the departure city, it is hard to say but JFK will have the most flight to Europe all airlines combined, AF have a lot of daily flight, their website gp.net is quite accurate. By experience I have been saved by them a lot of time. I wish I could be more specific.