Update
As you may have guessed, the flight (5:25 PM) from ISP-PHL yesterday was cancelled. I fortunately found out at 1:30 yesterday (no thanks to US or my company's travel desk) and was able to get creative about getting home. I returned the rental car at JFK, took the subway to Manhattan, and then took Amtrak home.
Looking at today's ops, I think I made a good choice. All flights from ISP-PHL were cancelled today.