Yeah, you probably could. I'm down 43 pounds since mid-May 2007, and hope to hit a 70-pound total loss by mid-April 2008, but my goal weight is a lower BMI than your goal, so you should be able to maintain a slightly faster pace towards the end than I'll be able to.
In general, you shouldn't lose faster than about 1% of your current weight a week, which for most people is 1-2 pounds a week. 70 pounds by September comes out to about 1.7 pounds a week.
Bear in mind that averaging 1.7 pounds a week doesn't mean that your weight per the scale will go down 1.7 pounds a week. Some weeks it'll go down 3 pounds, some weeks it won't go down at all. You need to come up with some method for keeping yourself on track without regard to whether the scale is giving you motivating feedback on any given day.