Hi Amberly!
September means back to school for most United States children, and consequently there are fewer vacationers at Walt Disney World in September than in August. The temperatures do start decreasing somewhat in September as well. But perhaps the best reason I love visiting in September is because
Mickey's Not So Scary Halloween Party begins in this month. Although the dates haven't been announced yet for 2014, the previous link will have them listed as soon as they are released and party tickets are available for purchase.
One of my favorite resources for weather forecasts is weather.com. You can view the monthly, weekly, daily, and hourly temperature and weather forecast on that website. Although I always "pack for anything", I find it helpful to begin checking the forecast about 10 days before the start of my vacation to help me finalize my clothing selections.
Rain is more abundant in central Florida in September; however this has never bothered my family. The rain is usually quite warm and we simply either don our ponchos and go about our vacation or find an indoor attraction. If a day is forecasted to be particularly rainy, we usually choose to visit Epcot because of the large number of indoor attractions scattered throughout the park.
If I had the choice of taking my vacation in August or September I personally would choose September for the above reasons. However if your family is particularly adverse to rain, you might stay with August since September historically is more rainy than August.
I hope this helps you with your planning Amberly. Please come back if you have any more vacation planning questions. We love to help!
Blessings,
Brenda ºOº