Pax Vobiscum: I've also been lucky enough to arrive at JFK a full 1 hour early from SFO, thanks to a helpful jet stream (tail wind about 150mph
AFAIK).
In fact, before I fly I have a quick look at weather.co.uk, which includes jet stream maps (sorry, I can't get a permanent URL for the map, look under "Maps"). It's useful for guesstimating delays, and even though I always buckle up (of course!), also sometimes gives a hint of where you might get a bit of turbulence.
I expect others here will know where to get better maps than those though.
Mike