Hi,
I will add my $.02. It is true that afternoon thunderstorms are less likely at the end of September compared to July and August. The issue that you will be contending with at that time of the year is reduced amount of daylight hours as compared to July/August. Many injuries occur as hikers are near the end of the hike due to fatigue, losing the trail in the dark, tripping on objects in the dark (even with good headlamps) etc. Therefore, one of your goals should include returning to the portal before dark or as near dark as possible. You don't want to be hiking the last few hours, downhill, in the dark as too many bad outcomes could be realized. So take your estimated round trip time, add an hour for hanging at the summit, and back your way into your start time. Sunset that day is 6:43ish and it will be full dark by 7:39. Something else to keep in mind is that a group of 9 will be as fast as the slowest member so even if you think you can round trip in 13 hours (just throwing out a number) your group will be slowed by different hiking speeds, people needing breaks at different times etc. So in summary, starting at 1am at that time of year is a good plan. Hope that helps.
Matt
