I took another look at the weather. I specified "Elmont, NY" instead of just NYC.
Result: it's still going to be at least 90 degrees, according to the forecast, but not as hot as either Philly or NYC are predicted to be on Saturday.
Of course, the place will be wall-to-wall people, so I might as well prepare as though it'll be a zillion degrees on Saturday. For all I know, that's what it'll feel like out there with the body heat of all the spectators to contend with. I'll be wearing light-colored garments and carrying drinking water.