Dave Coombs, KØEUS, sends along some valuable links for those wishing to learn more about Near Vertical Incidence Skywave Propagation, or NVIS for short. Fundamentally, this radio propagation mode involves transmitting nearly straight up, usually on a low high-frequency band such as 80 meters or 40 meters. The ionosphere reflects this back down, covering an area of a couple hundred miles or more. This makes it quite good for local communications.
I also pass along this link for near-real-time information about the maximum usable frequency. It has to be above the frequency at which you operate for NVIS to work. Here’s the link: