We've recently added heat maps to our quiver of trail tools. You can view min/max temps for a given month along the AT (or any trail) to get an idea of what kind of temps you'll be dealing with.
Here's an example of minimum temps for March.
You can change it by going to the settings icon->map skin
This info has long been available in the online AT data book, but the above is a more detailed view.
Enjoy!
