Growing in August

Northeast US, my 12-12 will come late september. If i start growing outside in early august, will my plants mature enough to yield a decent harvest?

possibly if they sprout out of the ground in early august you can.

In my opinion no. Start them inside early July. After they have had a good start (at least three weeks growth) transplant outside. That will put you right around August. They will take about a week after transplanting to kick in. Then you will get about three weeks more growth before Sept. You said 12/12 came in late Sept. so this means about 9 weeks growth that is minus the sprouting time, transplant shock factor figured in.