You definitely won't have to go through TSA again, at least on the outbound - just walk between your gates at JFK. On the return, everyone has to reclear TSA after going through customs/immigration.

85 mins should be fine, assuming JFK isn't having a bad day. If it's looking like weather in the New York area isn't going to be great, start keeping an eye on earlier flights. ATL-JFK is a lot less frequent than ATL-LGA, so might even be worth keeping an eye on LGA options and just taking a cab between them, depending on what things look like. Forget EWR; it's far from JFK and is a disaster right now with one runway closed for construction.

There was a line of thunderstorms that passed through the NYC area last night, which resulted in major delays at all the NYC airports - that's the type of thing in the forecast to keep an eye out for ahead of time.