There are many different displays for HF propagation available but only one (that I can find - and the link came from a friend) that seems really usable. If you have or know of any other links that would be useful to others in ARES, please let me know W0IPL@ARRL.NET
There are more than just NVIS propagation maps here. Use what you like.
The Hourly Area Prediction (HAP) chart is a guide to frequency selection for HF communications between mobiles and a base station in a specified area. You may select Boulder Colorado or other North American Cities.