The Parkway varies in elevation from just under 650 feet at Virginia's James River to over 6,000 feet south of Mount Pisgah in North Carolina. Obviously, the weather can vary tremendously as well.
Summer along the Parkway means cooler than normal temperatures, especially at the higher elevations. It is always a good idea to carry an extra jacket or blanket and a few emergency supplies in the car. Be aware of changing weather conditions and approaching weather systems.
Check your favorite weather-related web site for the Parkway town closest to where you plan to visit.