Seasonal precipitation anomalies associated with the continental North American monsoon system are characterized using a land-based data set derived from in situ observations across the southwestern United States and northwestern Mexico. Coherent regions of interannual continental precipitation variability are derived from principal components analysis, after defining separate "Early" and "Late" summer monsoon seasons. This seasonal separation allows examination of persistence of precipitation anomalies as an indicator of practical Late season predictability, and the possible strength of land surface feedbacks. The gravest mode of Late season interannual variability captures precipitation anomalies in the core of the continental monsoon domain. A simple spatial average is developed as an index of this core variability. Applications of this precipitation index are presented for observational and modeling studies associated with NAME, the international North American Monsoon Experiment.