Predicting runoff producing areas and their corresponding risks is important for developing watershed management strategies for protecting water quality from nonpoint source pollution. However, the currently proposed engineering methods to do this do not account for antecedent soil wetness status, which may substantially impact risk estimates, especially where variable source area (VSA) hydrology is a dominate storm runoff process. The objective of this study is to develop a simple approach to estimate spatially-distributed risks of runoff production. By considering the development of overland flow as a bivariate process, we incorporated both rainfall and antecedent soil moisture conditions into a method for predicting VSAs based on the Natural Resource Conservation Service-Curve Number equation. We used base flow immediately preceding storm events as an index of antecedent soil wetness status. Using the data from a study hillslope near Ithaca, NY, we demonstrated that our estimates agreed with independent field-observations. We further applied the proposed approach to the Upper Susquehanna River Basin and mapped predicted saturated areas in a Geographic Information System (GIS) using a Soil Topographic Index to demonstrate large-scale applicability and identify potential issues of the approach. The proposed methodology provides a new tool to watershed planners for quantifying runoff risks across watersheds, which can be used to target water quality protection strategies.