I definitely agree with previous posters to research options now and have more than 1 alternative. I would not be as worried about the JFK-XXX flight, as the weather calls for rain (not thunderstorms) here in NY which might cause some delays but usually not cancellations. Also, international flights tend to be the last to get cancelled.

I would be more worried about your DFW connection, given the weather and connection time. I think waiting until morning of departure might be too late, especially if you are coming from the West Coast on a Saturday, as there are not many options on AA that will get you to JFK to make a 6:30ish pm departure. If possible, I might try to fly into LGA the night before.

In terms of getting an agent to make a change to your reservation, YMMV. Yes, technically, you have to wait until your flight is affected or a travel waiver is posted. But I have had an instance where the agent was willing to make a change when the weather was not looking good but there was no travel waiver yet (eventually there was). Given the agents in the last few days have probably gone through h*** and back with trying to rebook passengers, you might be able to find a sympathetic agent.