An overcast day with showers can be your best friend = less crowded park. Just as you are indicating people tend to stay away and want the best weather day.
Realize this is the first week public schools are out in NJ/NY so crowds may increase...I think PA is already out.
Don't waste money on a FlashPass unless you have to. Check the lines first..and you can always go back and get one if needed.
Also stay late...lines can disappear after dinner time.