One of the major changes from August to September is precipitation type. Not in the sense of rain to snow, too early for that change, but instead from thunderstorms to just rain. In turn, the precipitation tends to be more widespread with rain amounts also being a bit more uniformed. You will still get spot to spot variability, just not quite as severe as in the summer months. Plus, with more stratiform rain, you also will get more widespread cloud cover. This will mean instead of a mix of sun and clouds during the day, you tend to be cloudy for much longer periods of time. This impacts maximum temperatures much more than in the summer.