Drink plenty of fluids. If hiking, carry water with you - do not drink from streams, no matter how fresh and clear it appears. Afternoons are often accompanied by thunderstorms. Temperature drops combined with altitude can mean a 10-degree difference in temperature, so it would even be a good idea to have a wool sweater along. Fall By September, the days are mild and the nights cool and crisp.
Light snows can occur at higher elevations.
Winter Winters are relatively mild in the Smokies, but winter days are subject to change dramatically. One day can be spring-like, while the next might bring temperatures in the 20s with several inches of snow. The upside is that defoliated trees allow for great mountain vistas and very private hikes.