September can bring dramatic temperature swings. It is usually in the 70's during the day and the 40's at night. However, there is a 50% chance of snow some time in September. If it does snow, it will usually be gone in a few hours. September is when the trees begin to change, the bears begin to eat vigorously before their hibernation, and the Elk begin their rutting season. There are many events in Estes Park during this time of the year. This includes the Elk Rutting Season, Scottish Irish Festival, etc. By the end of September many business will change to winter hours. This is often a very busy time in Estes Park and the surrounding areas. In fact, it is one of my favorite times.