That is really tight. 7PM is major rush hour at JFK with all of the Europeon flights departing. Incoming flights are often placed into holding patterns because of so much outgoing traffic. If the weather is bad you could well be totally s...d

You will need to be in line to check in at LGA by 840PM. Traffic on Grand Central could also still be a tad slow. And the 910PM flight of course is the last out, so if you don't arrive on time you are either looking for a hotel or sleeping in the Terminal. If the event of the latter, there are some benches between Courcourse C and D but it often looks like a homeless shelter.