September and the second half of August are typically really good for crowds.
However, Labor Day weekend falls right in the middle of the period you'll be there (Labor Day is the first Monday in September). Typically these long weekends have higher crowd levels at WDW. Despite this, of all the Monday holiday weekends, in my experience Labor Day is the least crowded since parents are hesitant to take their kids out of school for even one day extra at the beginning of the school year.
In other words, I'm saying that the crowd level will be relatively good during the two weeks you'll be there (though it might surge during that weekend).
As for weather, it's still very hot and humid though not quite as unbearable as July.