Hi, Brandie!
It's my pleasure to help Guests make the most of their trips to Walt Disney World!
Your one-park-a-day plan is perfect; you'll spend more time touring than traveling, so you'll be able to squeeze in an additional attraction or other experience by cutting out park hopping. Good choice!
September in Orlando can be very warm, much like August, really, with thunderstorms remaining as a daily possibility. My son and I were there the last week of September this past year, and it was HOT most days, with the evenings still lingering in the 70s. We always pack a light jacket and at least one pair of long pants per person, no matter when we travel to WDW, just in case an evening is a little too cool typical summer attire.
While you may be able to sneak in a visit with no rain during your 5 night September stay (once in late August and once in early September for us, although very unusual) if it does rain, you'll be able to purchase a very reasonably-priced, durable poncho at just about any retail location on property. In fact, if the forecast calls for rain, you'll often see stacks of ponchos brought out at kiosks and by cash registers, making them oh so easy to grab in a hurry.
Back to your earlier question, be sure that your AK day is an early one, as the animals will seek shade in the warmer afternoon hours. You'll be done this park earlier than others, so you might want to spend that evening relaxing at the resort or enjoying dinner and a movie, bowling or shopping at
Downtown Disney.
An early night that night might be a great way to prepare for another early day the next day, too, which could be Magic Kingdom or Disney's Hollywood Studios.
Send more questions as you think of them, Brandie; I love to help!