We provide an enhanced model of the errors induced by deviations of ionospheric delays from those estimated by the planar model used by the GPS-based Wide Area Augmentation System (WAAS). To a first approximation the nominal ionospheric spatial decorrelation of vertical equivalent signal delays, σdecorrnom, is roughly constant over the whole of the WAAS service volume. However, significant gains may be achieved by including a more detailed description of σdecorrnom as a function of various metrics such as geomagnetic latitude, time of day, as well as the quality of the planar fit as characterized by the radius, relative centroid of the fit, and the density of delay data. We take the first step in the development of this more sophisticated model by determining which of these parameters is best suited for use as a metric for determining σdecorrnom. This allows us to construct a first-order model of the ionospheric decorrelation which depends on the local density of ionospheric pierce points. Our preliminary study indicates that this first-order model will result in a better than 20% reduction in the values of broadcast grid ionospheric vertical errors (GIVEs) within the coterminous United States. We also observe a better than 50% reduction in trips of the ionospheric irregularity detector in the Alaska region, which will lead to significant improvements to continuity, although this comes at the cost of a roughly 20% increase in the median GIVE in the Alaska region.