An improved physical model for ionospheric storm propagation is presented. The model uses the Dst index to monitor storm commencement. It also employs a latitude- and storm-strength-dependent propagation speed for the pressure disturbance that propagates from the auroral source region, where energy is deposited at storm commencement, to lower latitudes. The new storm forecast code is embedded in PropMan, an ionospheric program developed at Rockwell Avionics and Communications. The predictions of the new code for maximum usable frequencies during storms are compared with radio sounding data. Improvements are reported in the capability of PropMan to predict maximum usable frequencies in the 24 hours following a storm.