Preflower is the beginning of flowering, sometimes it can take 14 weeks for a plant to finish, I live in Canada and plants havent started budding yet and it's the 45th parallel. I wouldn't worry, if there in preflower before Aug 15, your doing fine, your plant will most likey be ready to harvest 1st week of October, 1st week of november at the latest. they will finish in over the next 60day's tho prolly. Your on a lower parralel , so your photoperiod change will trigger budding a bit later, this month, the change is quick