Showers are forecasted in NY from 12 PM until 4-5 PM tomorrow. From there it's going to be cloudy to partly cloudy, with around 22 degrees C (71 F) and 70-60% humidity.
All in all, it's likely that the final will be delayed for a few minutes, maybe more. Conditions will be humid, which means the ball will be slower (which should favour Djokovic) but the bounce will be lower (which could favour Federer more than Djokovic).
Also, unlike most of the previous days at the USO, it's not going to be a hot afternoon, which is a good thing for Djokovic as he tends to struggle a bit more in hot conditions.
All in all, I think conditions will favour Djokovic the most.
I also think those game conditions would favour Djokovic more, but I think considering Federer's age, it's better that it's not too hot.
I remember that in 2011 semifinal he was the one who suffered from the heat and got red in the 3rd and 4th sets (although Djokovic accelerated)
I think that Djokovic's suffered much less from the heat since his anti-gluten diet.
Besides the match might be delayed until the night, and Fed has played all of his matches but one by night and he loves playing by night, which would probably favour him