An HF propagation program, HFNowcast, has been developed that takes advantage of the availability of a real-time global model of the ionosphere to specify in real time the range of frequencies that would be supported on a given HF communications circuit. HFNowcast determines the range of frequency support for a specified circuit, from the lowest usable F layer frequency (FLUF) to the highest (FMUF), together with the E layer ELUF and EMUF. The lack of real-time observations of HF absorption and HF noise at the receiver means that it is not possible to provide real-time specifications of the signal-to-noise ratio (SNR), the key parameter for HF communications. The global model (developed by Utah State University (USU)) includes particle E precipitation at high dip latitudes during the night, which often precludes the determination of foF2 and M(3000)F2 and thus the simple determination of propagation support in those regions. The FMUF is therefore derived by first calculating the vertical incidence ionogram at the control point(s) and then transforming that ionogram to the oblique incidence (OI) ionogram for the given circuit/hop length. HFNowcast has been validated favorably against several months of oblique propagation observations on midlatitude circuits, although its day-to-day variability does not match the observations very well, and it tends to underestimate the FMUFs. The systematic differences between the predictions and observations can be attributed directly to limitations of the current version of the USU model of the ionosphere.