Late August - October are the hottest, stickiest months out of the year and it's the peak of the hurricane season. Of course, it's all relative - if you're from the Florida you probably wouldn't bat an eye at the humidity levels. The heat index for Honolulu last week was in the high 90's and Aulani/Kapolei tends to be a bit hotter.
The valleys and windward sides of the Koolau mountains can see daily morning and afternoon showers, but this would not affect Aulani. The showers can blow down to the Waikiki area but it's not the type of rain you need to worry about or plan for.
The biggest weather issue facing Hawaii during these months is the seemingly constant threat of a hurricane/tropical storm. It seems like we've been dodging storms for the past 30 days. There was Hector, Lane, Norman, and now Olivia. It's been an endless cycle of prepping for a storm, watching the storm path, waiting to see if it's going to hit us, breathing a quick sigh of relief that it juuuuuust missed us, and then prepping for the next storm again. This storm season seems to be a little more active than normal but it's far from being the busiest I've seen. If I were planning a trip to Hawaii I'd be on pins and needles waiting to see if I needed to cancel or reschedule my vacation.
From a visitor's standpoint, I think it's a little misleading to say "Hurricanes rarely hit Hawaii so I'll take my chances." Even if the hurricane doesn't hit, your vacation can be impacted. Lots of local neighborhood stores and some attractions had decided on Wednesday to give their employees Thursday-Sunday off just in case the storm hit. It didn't even drizzle on Oahu yet it felt like the whole island was shut down! It was a loooooooong and boring weekend.